LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator

August 2004

General Description
The LM2576 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving 3A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, 12V, 15V, and an adjustable output version. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation and a fixed-frequency oscillator. The LM2576 series offers a high-efficiency replacement for popular three-terminal linear regulators. It substantially reduces the size of the heat sink, and in some cases no heat sink is required. A standard series of inductors optimized for use with the LM2576 are available from several different manufacturers. This feature greatly simplifies the design of switch-mode power supplies. Other features include a guaranteed 4% tolerance on output voltage within specified input voltages and output load conditions, and 10% on the oscillator frequency. External shutdown is included, featuring 50 A (typical) standby current. The output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions.

Features

n 3.3V, 5V, 12V, 15V, and adjustable output versions n Adjustable version output voltage range, 1.23V to 37V (57V for HV version) 4% max over line and load conditions n Guaranteed 3A output current n Wide input voltage range, 40V up to 60V for HV version n Requires only 4 external components n 52 kHz fixed frequency internal oscillator n TTL shutdown capability, low power standby mode n High efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection n P+ Product Enhancement tested

Applications

n n n n Simple high-efficiency step-down (buck) regulator Efficient pre-regulator for linear regulators On-card switching regulators Positive to negative converter (Buck-Boost)

Typical Application

Versions)

(Fixed Output Voltage

01147601

FIGURE 1.

SIMPLE SWITCHER is a registered trademark of National Semiconductor Corporation.
2004 National Semiconductor Corporation

DS011476

www.national.com

LM2576/LM2576HV

Block Diagram

01147602

3.3V R2 = 1.7k 5V, R2 = 3.1k 12V, R2 = 8.84k 15V, R2 = 11.3k For ADJ. Version R1 = Open, R2 = 0 Patent Pending

Ordering Information

Temperature Range Output Voltage 3.3 5.0 LM2576HVS-5.0 LM2576S-5.0 LM2576SX-5.0 LM2576HVT-5.0 LM2576T-5.0 LM2576HVT-5.0 Flow LB03 LM2576T-5.0 Flow LBLM2576S-12 LM2576SX-12 LM2576T-12 Flow LB03 LM2576T-12 Flow LBLM2576S-15 LM2576SX-15 LM2576T-15 Flow LB03 LM2576T-15 Flow LB03 ADJ LM2576S-ADJ LM2576SX-ADJ LM2576T-ADJ T05D Flow LB03 LM2576T-ADJ Flow LB03 TS5B Tape & Reel T05A TO-220 LM2576HVS-12 LM2576HVS-15 LM2576HVS-ADJ NS Package Package Type Number TS5B TO-263
40C TA LM2576HVS-3.3 125C LM2576S-3.3 LM2576SX-3.3 LM2576HVT-3.3 LM2576T-3.3 LM2576HVT-3.3 Flow LB03 LM2576T-3.3 Flow LB03
LM2576HVSX-3.3 LM2576HVSX-5.0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Supply Voltage LM2576 LM2576HV ON /OFF Pin Input Voltage Output Voltage to Ground (Steady State) Power Dissipation Storage Temperature Range Maximum Junction Temperature 1V Internally Limited 65C to +150C 150C 45V 63V 0.3V V +VIN

Minimum ESD Rating (C = 100 pF, R = 1.5 k) Lead Temperature (Soldering, 10 Seconds) 260C 2 kV

Operating Ratings

Temperature Range LM2576/LM2576HV Supply Voltage LM2576 LM2576HV 40V 60V 40C TJ +125C
LM2576-3.3, LM2576HV-3.3 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 12V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 6V VIN 40V, 0.5A ILOAD 3A Circuit of Figure 2 6V VIN 60V, 0.5A ILOAD 3A Circuit of Figure 2 VIN = 12V, ILOAD = 3A 75 3.3 3.168/3.135 3.450/3.482 3.3 3.168/3.135 3.432/3.465 3.3 3.234 3.366 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-3.3 LM2576HV-3.3 Limit (Note 2) Units (Limits)
LM2576-5.0, LM2576HV-5.0 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with Figure 2 boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 12V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV 0.5A ILOAD 3A, 8V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 8V VIN 60V Circuit of Figure 2 5.0 4.800/4.750 5.225/5.275 5.0 4.800/4.750 5.200/5.250 5.0 4.900 5.100 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) LM2576-5.0 LM2576HV-5.0 Limit (Note 2) Units (Limits)
LM2576-5.0, LM2576HV-5.0 Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25C, and those with Figure 2 boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 Efficiency VIN = 12V, ILOAD = 3A 77 % LM2576-5.0 LM2576HV-5.0 Limit (Note 2) Units (Limits)
LM2576-12, LM2576HV-12 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 25V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 15V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 15V VIN 60V Circuit of Figure 2 VIN = 15V, ILOAD = 3A 11.52/11.40 12.54/12.11.52/11.40 12.48/12.11.76 12.24 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-12 LM2576HV-12 Limit (Note 2) Units (Limits)
LM2576-15, LM2576HV-15 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 25V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 18V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 18V VIN 60V Circuit of Figure 2 VIN = 18V, ILOAD = 3A 14.40/14.25 15.68/15.14.40/14.25 15.60/15.14.70 15.30 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-15 LM2576HV-15 Limit (Note 2) Units (Limits)

LM2576-ADJ, LM2576HV-ADJ Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Feedback Voltage VIN = 12V, ILOAD = 0.5A VOUT = 5V, Circuit of Figure 2 VOUT Feedback Voltage LM2576 VOUT Feedback Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 8V VIN 40V VOUT = 5V, Circuit of Figure 2 0.5A ILOAD 3A, 8V VIN 60V VOUT = 5V, Circuit of Figure 2 VIN = 12V, ILOAD = 3A, VOUT = 5V 77 1.230 1.193/1.180 1.273/1.286 1.230 1.193/1.180 1.267/1.280 1.230 1.217 1.243 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-ADJ LM2576HV-ADJ Limit (Note 2) Units (Limits)
All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN = 30V for the 15V version. ILOAD = 500 mA. Symbol Parameter Conditions LM2576-XX LM2576HV-XX Typ DEVICE PARAMETERS Ib fO Feedback Bias Current Oscillator Frequency VOUT = 5V (Adjustable Version Only) (Note 11) 47/42 58/63 VSAT DC ICL Saturation Voltage Max Duty Cycle (ON) Current Limit IOUT = 3A (Note 4) (Note 5) (Notes 4, 11) 1.4 1.8/2.93 5.8 4.2/3.5 6.9/7.5 IL Output Leakage Current (Notes 6, 7): Output = 0V Output = 1V Output = 1V IQ ISTBY Quiescent Current Standby Quiescent Current (Note 6) ON /OFF Pin = 5V (OFF) 7.2 100/500 nA kHz kHz (Min) kHz (Max) V V(Max) % %(Min) A A(Min) A(Max) mA(Max) mA mA(Max) mA mA(Max) A A(Max) Limit (Note 2) Units (Limits)
All Output Voltage Versions Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN = 30V for the 15V version. ILOAD = 500 mA. Symbol Parameter Conditions LM2576-XX LM2576HV-XX Typ DEVICE PARAMETERS JA JA JC JA VIH VIL IIH IIL ON /OFF Pin Logic Input Level ON /OFF Pin Input Current ON /OFF Pin = 0V (ON) Thermal Resistance T Package, Junction to Ambient (Note 8) T Package, Junction to Ambient (Note 9) T Package, Junction to Case S Package, Junction to Ambient (Note 10) VOUT = 0V VOUT = Nominal Output Voltage ON /OFF Pin = 5V (OFF) 1.4 1.30 2.2/2.4 1.0/0.8 V(Min) V(Max) A A(Max) A A(Max) C/W Limit (Note 2) Units (Limits)

ON /OFF CONTROL Test Circuit Figure 2
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2576/LM2576HV is used as shown in the Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics. Note 4: Output pin sourcing current. No diode, inductor or capacitor connected to output. Note 5: Feedback pin removed from output and connected to 0V. Note 6: Feedback pin removed from output and connected to +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to force the output transistor OFF. Note 7: VIN = 40V (60V for high voltage version). Note 8: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 12 inch leads in a socket, or on a PC board with minimum copper area. Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 14 inch leads soldered to a PC board containing approximately 4 square inches of copper area surrounding the leads. Note 10: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using 0.5 square inches of copper area, JA is 50C/W, with 1 square inch of copper area, JA is 37C/W, and with 1.6 or more square inches of copper area, JA is 32C/W. Note 11: The oscillator frequency reduces to approximately 11 kHz in the event of an output short or an overload which causes the regulated output voltage to drop approximately 40% from the nominal output voltage. This self protection feature lowers the average power dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%.
Typical Performance Characteristics
(Circuit of Figure 2) Normalized Output Voltage Line Regulation

01147627

01147628
Typical Performance Characteristics (Circuit of Figure 2)

Dropout Voltage

(Continued) Current Limit

01147629

01147630

Quiescent Current

Standby Quiescent Current

01147631

01147632

Oscillator Frequency

Switch Saturation Voltage

01147633

01147634

Efficiency

(Continued)
Minimum Operating Voltage

01147635

01147636
Quiescent Current vs Duty Cycle
Feedback Voltage vs Duty Cycle

01147637

01147638

Feedback Pin Current

01147604
Maximum Power Dissipation (TO-263) (See Note 10)

Switching Waveforms

01147624

01147606

VOUT = 15V A: Output Pin Voltage, 50V/div B: Output Pin Current, 2A/div C: Inductor Current, 2A/div D: Output Ripple Voltage, 50 mV/div, AC-Coupled Horizontal Time Base: 5 s/div

Load Transient Response

01147605
Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance generate voltage transients which can cause problems. For minimal inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible.
Single-point grounding (as indicated) or ground plane construction should be used for best results. When using the Adjustable version, physically locate the programming resistors near the regulator, to keep the sensitive feedback wiring short.
Fixed Output Voltage Versions

01147607

CIN 100 F, 75V, Aluminum Electrolytic COUT 1000 F, 25V, Aluminum Electrolytic D1 Schottky, MBR360 LH, Pulse Eng. PE-92108 R1 2k, 0.1% R2 6.12k, 0.1%
Adjustable Output Voltage Version

01147608

where VREF = 1.23V, R1 between 1k and 5k.

FIGURE 2.

LM2576 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) Given: VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V) VIN(Max) = Maximum Input Voltage ILOAD(Max) = Maximum Load Current 1. Inductor Selection (L1) A. Select the correct Inductor value selection guide from Figures 3, 4, 5 or Figure 6. (Output voltages of 3.3V, 5V, 12V or 15V respectively). For other output voltages, see the design procedure for the adjustable version. B. From the inductor value selection guide, identify the inductance region intersected by VIN(Max) and ILOAD(Max), and note the inductor code for that region. C. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Figure 3. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2576 switching frequency (52 kHz) and for a current rating of 1.15 x ILOAD. For additional inductor information, see the inductor section in the Application Hints section of this data sheet. 2. Output Capacitor Selection (COUT) A. The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation and an acceptable output ripple voltage, (approximately 1% of the output voltage) a value between 100 F and 470 F is recommended. B. The capacitors voltage rating should be at least 1.5 times greater than the output voltage. For a 5V regulator, a rating of at least 8V is appropriate, and a 10V or 15V rating is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rated for a higher voltage than would normally be needed. 3. Catch Diode Selection (D1) A.The catch-diode current rating must be at least 1.2 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2576. The most stressful condition for this diode is an overload or shorted output condition. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. EXAMPLE (Fixed Output Voltage Versions) Given: VOUT = 5V VIN(Max) = 15V ILOAD(Max) = 3A

1. Inductor Selection (L1) A. Use the selection guide shown in Figure 4. B. From the selection guide, the inductance area intersected by the 15V line and 3A line is L100. C. Inductor value required is 100 H. From the table in Figure 3. Choose AIE 415-0930, Pulse Engineering PE92108, or Renco RL2444.
2. Output Capacitor Selection (COUT) A. COUT = 680 F to 2000 F standard aluminum electrolytic. B.Capacitor voltage rating = 20V.
3. Catch Diode Selection (D1) A.For this example, a 3A current rating is adequate. B. Use a 20V 1N5823 or SR302 Schottky diode, or any of the suggested fast-recovery diodes shown in Figure 8.
4. Input Capacitor (CIN) A 100 F, 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing.
LM2576 Series Buck Regulator Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

01147611

FIGURE 5. LM2576(HV)-12

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FIGURE 3. LM2576(HV)-3.3

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01147610

FIGURE 6. LM2576(HV)-15

FIGURE 4. LM2576(HV)-5.0

01147613

FIGURE 7. LM2576(HV)-ADJ
PROCEDURE (Adjustable Output Voltage Versions) Given: VOUT = Regulated Output Voltage VIN(Max) = Maximum Input Voltage ILOAD(Max) = Maximum Load Current F = Switching Frequency (Fixed at 52 kHz) 1. Programming Output Voltage (Selecting R1 and R2, as shown in Figure 2) Use the following formula to select the appropriate resistor values.
EXAMPLE (Adjustable Output Voltage Versions) Given: VOUT = 10V VIN(Max) = 25V ILOAD(Max) = 3A F = 52 kHz 1. Programming Output Voltage (Selecting R1 and R2)
R1 can be between 1k and 5k. (For best temperature coefficient and stability with time, use 1% metal film resistors)
R2 = 1k (8.13 1) = 7.13k, closest 1% value is 7.15k
PROCEDURE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate the inductor Volt microsecond constant, E T (V s), from the following formula:
EXAMPLE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate E T (V s)
B. Use the E T value from the previous formula and match it with the E T number on the vertical axis of the Inductor Value Selection Guide shown in Figure 7. C. On the horizontal axis, select the maximum load current. D. Identify the inductance region intersected by the E T value and the maximum load current value, and note the inductor code for that region. E. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Figure 9. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2576 switching frequency (52 kHz) and for a current rating of 1.15 x ILOAD. For additional inductor information, see the inductor section in the application hints section of this data sheet. 3. Output Capacitor Selection (COUT) A. The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation, the capacitor must satisfy the following requirement:

B. E T = 115 V s C. ILOAD(Max) = 3A D. Inductance Region = H150 E. Inductor Value = 150 H Choose from AIE part #415-0936 Pulse Engineering part #PE-531115, or Renco part #RL2445.
3. Output Capacitor Selection (COUT)
However, for acceptable output ripple voltage select COUT 680 F COUT = 680 F electrolytic capacitor
The above formula yields capacitor values between 10 F and 2200 F that will satisfy the loop requirements for stable operation. But to achieve an acceptable output ripple voltage, (approximately 1% of the output voltage) and transient response, the output capacitor may need to be several times larger than the above formula yields. B. The capacitors voltage rating should be at last 1.5 times greater than the output voltage. For a 10V regulator, a rating of at least 15V or more is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rate for a higher voltage than would normally be needed. 4. Catch Diode Selection (D1) A. The catch-diode current rating must be at least 1.2 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2576. The most stressful condition for this diode is an overload or shorted output. See diode selection guide in Figure 8. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 5. Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. To further simplify the buck regulator design procedure, National Semiconductor is making available computer design software to be used with the SIMPLE SWITCHER line of 4. Catch Diode Selection (D1) A. For this example, a 3.3A current rating is adequate. B. Use a 30V 31DQ03 Schottky diode, or any of the suggested fast-recovery diodes in Figure 8.
5. Input Capacitor (CIN) A 100 F aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. switching regulators. Switchers Made Simple (Version 3.3) is available on a (312") diskette for IBM compatible computers from a National Semiconductor sales office in your area.
VR 3A 20V 1N5820 MBR320P SR302 30V 1N5821 MBR330 31DQ03 SR303 40V 1N5822 MBR340 31DQ04 SR304 50V MBR350 31DQ05 SR305 60V MBR360 DQ06 SR306

Schottky 4A6A 1N5823 3A

Fast Recovery 4A6A
50WQ03 1N5824 The following diodes are all rated to 100V 31DF1 HER302 The following diodes are all rated to 100V 50WF10 MUR410 HER602
MBR340 50WQ04 1N5825 50WQ05

50WR06 50SQ060

FIGURE 8. Diode Selection Guide
Inductor Code L47 L68 L100 L150 L220 L330 L470 L680 H150 H220 H330 H470 H680 H1000 H1500 H2200
Inductor Value 47 H 68 H 100 H 150 H 220 H 330 H 470 H 680 H 150 H 220 H 330 H 470 H 680 H 1000 H 1500 H 2200 H
Schott (Note 12) 671 27130
Pulse Eng. (Note 13) PE-53112 PE-92114 PE-92108 PE-53113 PE-52626 PE-52627 PE-53114 PE-52629 PE-53115 PE-53116 PE-53117 PE-53118 PE-53119 PE-53120 PE-53121 PE-53122
Renco (Note 14) RL2442 RL2443 RL2444 RL1954 RL1953 RL1952 RL1951 RL1950 RL2445 RL2446 RL2447 RL1961 RL1960 RL1959 RL1958 RL2448
Note 12: Schott Corporation, (612) 475-1173, 1000 Parkers Lake Road, Wayzata, MN 55391. Note 13: Pulse Engineering, (619) 674-8100, P.O. Box 12235, San Diego, CA 92112. Note 14: Renco Electronics Incorporated, (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.
FIGURE 9. Inductor Selection by Manufacturers Part Number

Application Hints

INPUT CAPACITOR (CIN) To maintain stability, the regulator input pin must be bypassed with at least a 100 F electrolytic capacitor. The capacitors leads must be kept short, and located near the regulator.
If the operating temperature range includes temperatures below 25C, the input capacitor value may need to be larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower temperatures and age. Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures. For maximum capacitor operating lifetime, the capacitors RMS ripple current rating should be greater than

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rapidly. Different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. The inductor manufacturers data sheets include current and energy limits to avoid inductor saturation. INDUCTOR RIPPLE CURRENT When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage, the peak-topeak amplitude of this inductor current waveform remains constant. As the load current rises or falls, the entire sawtooth current waveform also rises or falls. The average DC value of this waveform is equal to the DC load current (in the buck regulator configuration). If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation. Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough. OUTPUT CAPACITOR An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor should be located near the LM2576 using short pc board traces. Standard aluminum electrolytics are usually adequate, but low ESR types are recommended for low output ripple voltage and good stability. The ESR of a capacitor depends on many factors, some which are: the value, the voltage rating, physical size and the type of construction. In general, low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR numbers. The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output capacitor and the amplitude of the inductor ripple current (IIND). See the section on inductor ripple current in Application Hints. The lower capacitor values (220 F1000 F) will allow typically 50 mV to 150 mV of output ripple voltage, while larger-value capacitors will reduce the ripple to approximately 20 mV to 50 mV. Output Ripple Voltage = (IIND) (ESR of COUT) To further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a higher-grade capacitor may be used. Such capacitors are often called high-frequency, low-inductance, or low-ESR. These will reduce the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode, reducing the ESR below 0.03 can cause instability in the regulator. Tantalum capacitors can have a very low ESR, and should be carefully evaluated if it is the only output capacitor. Because of their good low temperature characteristics, a tantalum can be used in parallel with aluminum electrolytics, with the tantalum making up 10% or 20% of the total capacitance. The capacitors ripple current rating at 52 kHz should be at least 50% higher than the peak-to-peak inductor ripple current.

INDUCTOR SELECTION All switching regulators have two basic modes of operation: continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulator performance and requirements. The LM2576 (or any of the SIMPLE SWITCHER family) can be used for both continuous and discontinuous modes of operation. The inductor value selection guides in Figure 3 through Figure 7 were designed for buck regulator designs of the continuous inductor current type. When using inductor values shown in the inductor selection guide, the peak-to-peak inductor ripple current will be approximately 20% to 30% of the maximum DC current. With relatively heavy load currents, the circuit operates in the continuous mode (inductor current always flowing), but under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a period of time). This discontinuous mode of operation is perfectly acceptable. For light loads (less than approximately 300 mA) it may be desirable to operate the regulator in the discontinuous mode, primarily because of the lower inductor values required for the discontinuous mode. The selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value chosen is prohibitively high, the designer should investigate the possibility of discontinuous operation. The computer design software Switchers Made Simple will provide all component values for discontinuous (as well as continuous) mode of operation. Inductors are available in different styles such as pot core, toriod, E-frame, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire wrapped on a ferrite rod core. This type of construction makes for an inexpensive inductor, but since the magnetic flux is not completely contained within the core, it generates more electromagnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe. The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for Pulse Engineering, and ferrite bobbin core for Renco. An inductor should not be operated beyond its maximum rated current because it may saturate. When an inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance of the winding). This will cause the switch current to rise very

CATCH DIODE

Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode should be located close to the LM2576 using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best efficiency, especially in low output voltage switching regulators (less than 5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery diodes are also suitable, but some types with an abrupt turn-off characteristic may cause instability and EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or 1N5400, etc.) are also not suitable. See Figure 8 for Schottky and soft fast-recovery diode selection guide. OUTPUT VOLTAGE RIPPLE AND TRANSIENTS The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency, typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth waveform. The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output capacitor. (See the inductor selection in the application hints.) The voltage spikes are present because of the the fast switching action of the output switch, and the parasitic inductance of the output filter capacitor. To minimize these voltage spikes, special low inductance capacitors can be used, and their lead lengths must be kept short. Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute to the amplitude of these spikes. An additional small LC filter (20 H & 100 F) can be added to the output (as shown in Figure 15) to further reduce the amount of output ripple and transients. A 10 x reduction in output ripple voltage and transients is possible with this filter. FEEDBACK CONNECTION The LM2576 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching power supply. When using the adjustable version, physically locate both output voltage programming resistors near the LM2576 to avoid picking up unwanted noise. Avoid using resistors greater than 100 k because of the increased chance of noise pickup. ON /OFF INPUT For normal operation, the ON /OFF pin should be grounded or driven with a low-level TTL voltage (typically below 1.6V). To put the regulator into standby mode, drive this pin with a high-level TTL or CMOS signal. The ON /OFF pin can be safely pulled up to +VIN without a resistor in series with it. The ON /OFF pin should not be left open. GROUNDING To maintain output voltage stability, the power ground connections must be low-impedance (see Figure 2). For the 5-lead TO-220 and TO-263 style package, both the tab and pin 3 are ground and either connection may be used, as they are both part of the same copper lead frame.

Where fosc = 52 kHz. Under normal continuous inductor current operating conditions, the minimum VIN represents the worst case. Select an inductor that is rated for the peak current anticipated.

01147614

FIGURE 10. Inverting Buck-Boost Develops 12V Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage. For a 12V output, the maximum input voltage for the LM2576 is +28V, or +48V for the LM2576HV. The Switchers Made Simple (version 3.0) design software can be used to determine the feasibility of regulator designs using different topologies, different input-output parameters, different components, etc.
Note: Complete circuit not shown.

01147616

NEGATIVE BOOST REGULATOR Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 11 accepts an input voltage ranging from 5V to 12V and provides a regulated 12V output. Input voltages greater than 12V will cause the output to rise above 12V, but will not damage the regulator.
FIGURE 12. Undervoltage Lockout for Buck Circuit
ing. Increasing the RC time constant can provide longer delay times. But excessively large RC time constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple into the ON /OFF pin. ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY A 3A power supply that features an adjustable output voltage is shown in Figure 15. An additional L-C filter that reduces the output ripple by a factor of 10 or more is included in this circuit.

01147617

Note: Complete circuit not shown (see Figure 10).
FIGURE 13. Undervoltage Lockout for Buck-Boost Circuit

01147618

DELAYED STARTUP The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 14. With an input voltage of 20V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit begins switch-
FIGURE 14. Delayed Startup

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FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple

Definition of Terms

BUCK REGULATOR A switching regulator topology in which a higher voltage is converted to a lower voltage. Also known as a step-down switching regulator. BUCK-BOOST REGULATOR A switching regulator topology in which a positive voltage is converted to a negative voltage without a transformer. DUTY CYCLE (D) Ratio of the output switchs on-time to the oscillator period. CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current when the LM2576 switch is OFF. EFFICIENCY () The proportion of input power actually delivered to the load.

OPERATING VOLT MICROSECOND CONSTANT (E Top) The product (in VoIt s) of the voltage applied to the inductor and the time the voltage is applied. This E Top constant is a measure of the energy handling capability of an inductor and is dependent upon the type of core, the core area, the number of turns, and the duty cycle.
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitors impedance (see Figure 16). It causes power loss resulting in capacitor heating, which directly affects the capacitors operating lifetime. When used as a switching regulator output filter, higher ESR values result in higher output ripple voltages.
Connection Diagrams (Note 15)
Straight Leads 5-Lead TO-220 (T) Top View

01147620

FIGURE 16. Simple Model of a Real Capacitor Most standard aluminum electrolytic capacitors in the 100 F1000 F range have 0.5 to 0.1 ESR. Highergrade capacitors (low-ESR, high-frequency, or lowinductance) in the 100 F1000 F range generally have ESR of less than 0.15. EQUIVALENT SERIES INDUCTANCE (ESL) The pure inductance component of a capacitor (see Figure 16). The amount of inductance is determined to a large extent on the capacitors construction. In a buck regulator, this unwanted inductance causes voltage spikes to appear on the output. OUTPUT RIPPLE VOLTAGE The AC component of the switching regulators output voltage. It is usually dominated by the output capacitors ESR multiplied by the inductors ripple current (IIND). The peakto-peak value of this sawtooth ripple current can be determined by reading the Inductor Ripple Current section of the Application hints. CAPACITOR RIPPLE CURRENT RMS value of the maximum allowable alternating current at which a capacitor can be operated continuously at a specified temperature. STANDBY QUIESCENT CURRENT (ISTBY) Supply current required by the LM2576 when in the standby mode (ON /OFF pin is driven to TTL-high voltage, thus turning the output switch OFF). INDUCTOR RIPPLE CURRENT (IIND) The peak-to-peak value of the inductor current waveform, typically a sawtooth waveform when the regulator is operating in the continuous mode (vs. discontinuous mode). CONTINUOUS/DISCONTINUOUS MODE OPERATION Relates to the inductor current. In the continuous mode, the inductor current is always flowing and never drops to zero, vs. the discontinuous mode, where the inductor current drops to zero for a period of time in the normal switching cycle. INDUCTOR SATURATION The condition which exists when an inductor cannot hold any more magnetic flux. When an inductor saturates, the inductor appears less inductive and the resistive component dominates. Inductor current is then limited only by the DC resistance of the wire and the available source current.

www.national.com 20

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LM2576T-XX or LM2576HVT-XX NS Package Number T05A TO-263 (S) 5-Lead Surface-Mount Package Top View

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LM2576S-XX or LM2576HVS-XX NS Package Number TS5B LM2576SX-XX or LM2576HVSX-XX NS Package Number TS5B, Tape and Reel Bent, Staggered Leads 5-Lead TO-220 (T) Top View

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LM2576T-XX Flow LB03 or LM2576HVT-XX Flow LB03 NS Package Number T05D
Note 15: (XX indicates output voltage option. See ordering information table for complete part number.)

Physical Dimensions

unless otherwise noted

inches (millimeters)

5-Lead TO-220 (T) Order Number LM2576T-3.3, LM2576HVT-3.3, LM2576T-5.0, LM2576HVT-5.0, LM2576T-12, LM2576HVT-12, LM2576T-15, LM2576HVT-15, LM2576T-ADJ or LM2576HVT-ADJ NS Package Number T05A
inches (millimeters) unless otherwise noted (Continued)
Bent, Staggered 5-Lead TO-220 (T) Order Number LM2576T-3.3 Flow LB03, LM2576T-XX Flow LB03, LM2576HVT-3.3 Flow LB03, LM2576T-5.0 Flow LB03, LM2576HVT-5.0 Flow LB03, LM2576T-12 Flow LB03, LM2576HVT-12 Flow LB03, LM2576T-15 Flow LB03, LM2576HVT-15 Flow LB03, LM2576T-ADJ Flow LB03 or LM2576HVT-ADJ Flow LB03 NS Package Number T05D
5-Lead TO-263 (S) Order Number LM2576S-3.3, LM2576S-5.0, LM2576S-12,LM2576S-15, LM2576S-ADJ, LM2576HVS-3.3, LM2576HVS-5.0, LM2576HVS-12, LM2576HVS-15, or LM2576HVS-ADJ NS Package Number TS5B 5-Lead TO-263 in Tape & Reel (SX) Order Number LM2576SX-3.3, LM2576SX-5.0, LM2576SX-12, LM2576SX-15, LM2576SX-ADJ, LM2576HVSX-3.3, LM2576HVSX-5.0, LM2576HVSX-12, LM2576HVSX-15, or LM2576HVSX-ADJ NS Package Number TS5B
LIFE SUPPORT POLICY NATIONALS PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no Banned Substances as defined in CSP-9-111S2.
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High-Efficiency 3A Battery Chargers Use LM2576 Regulators
This paper describes two LM2576-based designs which provide up to 3A of current for battery charging.
National Semiconductor Application Note 946 Chester Simpson May 1994
CIRCUIT CONCEPT This battery charger shown in Figure 1 was developed specifically for applications using either Nickel-Cadmium (Ni-Cd) or Nickel-Metal Hydride (Ni-MH) batteries that will accept a 3A fast-charge rate, and provides automatic shutoff of the high-current charge when the battery is full. After shutoff, a continuous (low level) charge current is used to trickle charge the battery which keeps it topped off and prevents charge loss due to any internal leakage. The trickle charge rate used must always be low enough that the amount of gas developed within the cell is small enough that it can recombine, thus preventing pressure build-up and venting (opening of the cells internal vent to release pressure). The maximum safe trickle charging rate is determined by the size and type of battery (this is covered later in the paper).
3A Battery Charger Has Built-In Overcharge Protection
This design is a 3A battery charger intended for use with 5-cell Ni-Cd or Ni-MH battery packs (but can be modified to suit other numbers of cells). The circuit includes automatic shutoff that occurs when the battery temperature rises 10C above ambient.

01205401

Notes (Unless Otherwise Specified): All capacitance values are in F. All resistors are 5%, 14W. U1 requires small heatsink (RTH < 15C/W). C1 and C4 must be low ESR aluminum electrolytic. U5D is not used.

AN-946

FIGURE 1. 3A Charger with Overcharge Protection
2002 National Semiconductor Corporation

AN012054

www.national.com
3A Battery Charger Has Built-In Overcharge Protection (Continued)
The critical specification for a battery is its Amp-hour (A-hr) rating, which is numerically equal to the maximum amount of current the battery can supply to a load for one hour before the cell reaches its end-of-life voltage (usually taken as 1.0V/cell for Ni-Cd and Ni-MH batteries. When a battery is charged or discharged at a current that is equal to its A-hr rating, this is known as the c rate. Most Ni-Cd and Ni-MH batteries can be safely charged at a 1c rate, as long as they are not overcharged. However, the battery temperature must be within a range of about 15C to 45C (the reasons are detailed later in this paper). OVERCHARGING: THE SILENT KILLER The nemesis of all rechargeable batteries is overcharge although some battery types tolerate it better than others, the results of overcharge range from minor damage to catastrophic failure. In the case of Ni-Cd, which is the most popular rechargeable battery type presently in use, sustained overcharge causes increasing pressure within the battery that eventually causes the cells vent to open and release oxygen. This has a detrimental effect on the battery, although it may still retain some useful capacity. If Ni-MH batteries are overcharged, they will also build up pressure and release gas: however, the gas released will be hydrogen, which is extremely explosive near spark or flame. One battery manufacturer created an interesting euphemism for some of the unfortunate accidents in cases where Ni-MH batteries were overcharged: Rapid Spontaneous Disassembly. DETECTING END-OF-CHARGE There are several ways to detect end-of-charge for Ni-Cd or Ni-MH batteries, but one way that is both simple and reliable is called a T detector. It measures both the ambient temperature and the battery temperature and cuts off the high current charger when the battery rises a pre-set amount above ambient. This design uses a 10C rise as the cutoff point (which is recommended by most battery makers), but can be easily adjusted by changing resistor values. Ni-Cd cells are perfectly suited for T cutoff techniques, because their charge process is endothermic (they get slightly cooler when a discharged battery is being recharged). Even at fast charge rates, the battery will not begin to heat until it is nearly fully recharged. At that point, the battery is no longer converting the electrical current into a chemical reaction, so it must be dissipated as heat. The resulting increase in temperature provides a very accurate indicator that it is time to stop charging. The Ni-MH battery is not quite as accommodating: the recharge cycle is exothermic (the battery gets slightly warmer during recharge) but still shows a fairly well defined increase in temperature when the battery is fully charged. Using a 10C T detection point will give good results in most cases, and is recommended by the battery makers. NOTE: WARNINGS ABOUT FAST CHARGING NI-MH AND NI-CD BATTERIES Since the Ni-MH battery normally gives off heat during recharge, the 10C window may have to be adjusted to suit the characteristics of the specific cell: The window must be

wide enough to prevent premature cutoff from normal heating, but narrow enough to detect the temperature rise which occurs at full charge (and execute appropriate charge termination). Any new design that uses Ni-MH batteries should be carefully evaluated to verify accurate end-of-charge termination because of the potential for battery explosion if hydrogen is released. IMPORTANT: With Ni-Cd or Ni-MH cells, the 1c (fast) charge rate can only be safely used if the battery temperature is in the range of about 15C to 45C. At low temperatures, gas recombination within NiCd and NiMH batteries does not occur as easily, which limits the amount of charging current that can be safely used before venting will occur. If low-temperature ( < 15C) recharging is required, consult the battery maker for safe charging current levels. A battery that is recharged at elevated temperature will retain substantially less energy than a battery recharged at 25C. At high temperatures ( > 35C) gas generation within the cell occurs at a much lower state of charge, meaning that the cell will not accept as much charge (compared to 25C) for a given amount of cell temperature rise. The poor charging efficiency seen at high battery temperatures means that extremely long recharge times (at low charging currents) are required to deliver full (25C) capacity of charge to a hot battery. TRICKLE CHARGE CURRENT All batteries lose charge internally due to self-discharge, usually occurring due to leakage paths through the battery separators (insulators). The amount of leakage is dependent primarily on battery age and usage, with leakage increasing dramatically in batteries that are old or have completed many cycles of charge and discharge. Trickle charging is a continuous low-level charging current that tops off the total charge in the battery, and prevents any energy loss that would occur due to leakage. The maximum safe trickle charging current for a typical Ni-Cd cell is about 0.1c, this being the maximum charge rate at which all of the gas developed internally is able to recombine (so there is no internal pressure buildup that would cause venting). For Ni-MH batteries, the maximum (safe) trickle charge rate is lower (one manufacturer specifies c/40). This is an important difference between Ni-Cd and Ni-MH batteries, and must not be exceeded for continuous charging. In this design, the trickle charge current is provided by the resistor labeled RTR (see Figure 1). This current flows any time VIN is present, regardless of operation of the high-current charger. When the high-current charger is operating, the total charging current is the sum of the trickle current and the current provided by U1. Once the input voltage VIN and the desired trickle charge current ITR are known, the value for RTR is found using Ohms Law: RTR = (VIN 7 0.7)/ITR The maximum power dissipation in RTR must also be calculated (when selecting a resistor, make sure the power rating is greater than the value calculated below): PMAX (RTR) = (VIN 4 0.7)2/RTR

Note that the power dissipation in the resistor is dependent on the battery voltage. As the battery voltage increases, the voltage drop across RTR decreases (causing the power dissipation to decrease). In the above equation, a battery voltage of 4V is assumed as a worst-case minimum value for battery operating voltage for a five-cell battery pack (which would provide the maximum power dissipation for RTR). A good 5-cell Ni-Cd or Ni-MH battery which is being trickle charged (after being fully recharged) will read about 7V, which will produce the minimum power dissipation in resistor RTR. DETAILS OF CIRCUIT FUNCTION (REFER TO Figure 1) The 3A of charging current provided by the fast-charger is obtained from an LM2576, which is a buck regulator that switches at 52 kHz. Because it is a switcher, it allows the user the option of using a wider input voltage range and still retaining high power conversion efficiency (about 80% @3A with VIN in the 10V14V range). The LM2576 IC (U1) is used to provide a charging current that is independent of the battery voltage. Whenever the ON/OFF pin is held low, U1 will source current into the battery through D3. A current-control feedback loop is established using U5B, R12, and associated components. R12 is used as a current shunt, and it provides a voltage to the input of U5B that is proportional to the charging current. U5B functions as an amplifier with a gain of 8.5, which causes the output of U5B to be 1.23V when the current through R12 is about 2.9A. The 1.23V signal on the feedback pin of U1 will lock the loop at this value of charging current. A fast-charge current value other than 2.9A can be set by adjusting the values of R7, R9, or R12. These values (which set the overall gain of the stage) should be adjusted so that the output of U5B is 1.23V at the desired amount of fast-charge current. AUTOMATIC SHUTDOWN AT FULL CHARGE The crucial part of fast charging a battery (especially if it is Ni-MH) is knowing when to stop. This design uses a T detector that measures both the battery temperature and the ambient temperature, and shuts down the fast-charge current source when the battery is +10C above ambient. This method is superior to techniques which sense only battery temperature. Single-ended temperature sensing may not accurately measure charge: a cold battery will have to heat up too much before the detection point is reached (overcharging it), while a hot battery will terminate charge long before full charge has been delivered to the battery (because its temperature starts out too near the detection level). Two LM35 temperature sensors (U3 and U4) provide output voltages of 10 mV/C (proportional to their temperature). U3 is used to measure the ambient, while U4 measures the battery temperature. Note: U4 must be in contact with the metal case of the battery to accurately measure its temperature. The plastic sleeve around the battery may have to be opened up to allow flush contact. Best results are obtained if the sensor is located between two batteries (touching both).

Monitoring more than one battery virtually eliminates the possibility that the sensor happens to be reading a bad (shorted) cell which will not heat up and provide charge termination. In some laptops, multiple sensors are used so that all battery cells are monitored, with charge termination occurring when any cell temperature reaches the trip level. The 78L05 regulator (U2) is used to provide a 5V source to power the LM35 sensors and also acts as a reference point for resistive divider R2 and R3. Resistors R1 and R11 are used to sink current (since the LM35 can not). CONTROLLING THE FAST-CHARGE CURRENT SOURCE U5C acts as a comparator which controls the on/off pin of the high-current charging source (U1). When the output of U5C is low, the 3A current source is turned on. When the output of U5C is high, U1 is turned off and LED1 is lit which indicates that the charger has completed the high-current charge phase and is now trickle charging. Hysteresis is built into U5C (see R13), which effectively latches the output of U5C high after it completes the fast-charge portion of the cycle (it stays latched until the input power is cycled on and off). Without hysteresis, the charger would again turn on the 3A charger after the fully-charged battery had cooled during trickle charging. DETECTING AN END-OF-CHARGE CONDITION The signals that are sent to U5C are derived from the temperature sensors. They cannot be compared directly, since detection must occur when the signal coming from U4 (the battery sensor) is 100 mV above the signal coming from U3 (the ambient sensor). In this design, the signal from U3 is DC level shifted up about 0.1V by U5A and its associated components. R2 and R3 set a 0.1V reference point for U5A, whose output voltage is the voltage at the output of U3 added to the 0.1V reference. With the signal from U3 level shifted by an amount that is equal to 10C, U5C can be used to compare the level-shifted signal from U3 to the signal from U4. When these two are equal, the temperature sensed by U4 (the battery) will be 10C above the temperature sensed by U3 (the ambient). This is the point where shutdown of the 3A charger occurs, and trickle charging continues.
3A Battery Charger has Logic-Level Current Controls
This design is a 3A battery charger with logic-level controls, allowing a logic controller to adjust the battery charging current to any one of four rates. The circuit was designed to implement P-based charging control in a system that operates from Ni-Cd or Ni-MH batteries. GENERAL DESCRIPTION The circuit shown in Figure 1 is a 3A (maximum) battery charger that uses a 52 kHz switching converter to step down the input DC voltage and regulate the charging current flowing into the battery. The switching regulator maintains good efficiency over a wide input voltage range, which allows the use of a cheap, poorly regulated DC wall adaptor for the input source. The key feature of this circuit is that it allows the P controller inside the PC to select from one of four different charging currents by changing the logic levels at two bits. The various

(Continued) charge levels are necessary to accommodate both Ni-Cd and Ni-MH type batteries, as they require slightly different charge methods. Both Ni-Cd and Ni-MH batteries can be charged at the high-current c rate up until the end-of-charge limit is reached, but the two batteries must be trickle-charged differently (trickle charging is a continuous, low-current charging rate that keeps the battery topped off after the high-current charge cycle has delivered about 95% of the batterys total charge capacity). The recommended trickle-charge rate for a Ni-Cd is about c/10, but for Ni-MH most manufacturers recommend that the charge rate not exceed c/40. If a continuous charge rate greater than c/40 is applied to a Ni-MH battery, the internal pressure can build up to the point where the battery will vent hydrogen gas. This is detrimental to the life of the Ni-MH battery and potentially dangerous for the user (hydrogen gas is easily ignited). The circuit shown in Figure 2 was designed to charge a 3A-hr Ni-Cd or Ni-MH battery with high efficiency, using logic-level signals to control the charging current. The four selectable charge rates are 3A, 0.75A, 0.3A, and 0.075A which correspond to charge rates of c, c/4, c/10, and c/40 for the 3A-hr battery used in this application. CIRCUIT OPERATION (REFER TO Figure 2) The unregulated DC input voltage is stepped down using an LM2576 3A buck regulator, providing up to 3A of current to charge the battery. In order to regulate the amount of charging current flowing into the battery, a current control loop is implemented using op-amp U2. The voltage drop across the sense resistor R8 provides a voltage to U2 that is proportional to the charging current. Note: The 0.05 value for R8 was specified by the customer in this application to minimize the power dissipated in this
resistor. If a higher Ohmic value is used (more resistance), a larger sense voltage is developed and a less precise (cheaper) op-amp can be used at U2, since the input offset voltage would not be as critical (of course, increasing the value of R8 also increases its power dissipation). When the current-control loop is operating, the voltage at the feedback pin of U1 is held at 1.23V. The battery charging current that corresponds to this voltage is dependent on the overall gain of U2 and the attenuators made up of Q1, Q2 and the resistors R10, R11, R2 and R3. Turning Q1 on (by putting a 1 on logic input A) provides an increase of 4:1 in load current. The load current is higher with Q1 on because R2 and R3 divide down the output of U2 by 4:1, requiring U2 to output a higher voltage to get the 1.23V on the feedback line of U1. Higher voltage at the output of U2 means that more charging current is flowing through R8 (also the battery). The operation of Q2 is similar to Q1: when Q2 is turned on by putting a logic 1 on input B, the load current is increased by a factor of 10:1. This is because when Q2 is on, the sense voltage coming from R8 is divided down by R10 and R11, requiring ten times as much signal voltage across R8 to get the same voltage at the non-inverting input of U2. Although both attenuating dividers could have been placed on the input side of U2, putting the 4:1 divider at the output improves the accuracy and noise immunity of the amplifier U2 (because the voltage applied to the input of U2 is larger, this reduces the input-offset voltage error and switching noise degradation). R5, R6, and D2 are included to provide a voltage-control loop in the case where the battery is disconnected. These components prevent the voltage at the cathode side of D3 from rising above about 8V when there is no path for the charging current to return (and the current control loop would not be operational). Capacitor C2 is included to filter some of the 52 kHz noise present on the control line coming from U2. Adding this component improved the accuracy of the measured charging current on the breadboard (compared to the predicted design values).

(Continued)

01205402
Notes (Unless Otherwise Specified): Note 1: All resistors are in , 5% tolerance, 14W Note 2: All capacitors are in F Note 3: Q1 and Q2 are made by SUPERTEX Note 4: For 3A current, U1 requires small heatsink (RTH 15C/W)
BENCH TEST DATA Logic Input A Logic Input B 3.0 0.75 0.30 0.075 Nominal Battery Charging Current (A) (C RATE) (C/4 RATE) (C/10 RATE) (C/40 RATE) Measured Battery Charging Current (A) with VIN = 10V 3.06 0.78 0.30 0.077 Power Conversion Efficiency (%) with VIN = 10V 77 79
FIGURE 2. 3A Battery Charger With Logic-Level Current Controls
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.