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LM2576 3.0 A, 15 V, Step-Down Switching Regulator
The LM2576 series of regulators are monolithic integrated circuits ideally suited for easy and convenient design of a step-down switching regulator (buck converter). All circuits of this series are capable of driving a 3.0 A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3 V, 5.0 V, 12 V, 15 V, and an adjustable output version. These regulators were designed to minimize the number of external components to simplify the power supply design. Standard series of inductors optimized for use with the LM2576 are offered by several different inductor manufacturers. Since the LM2576 converter is a switch-mode power supply, its efficiency is significantly higher in comparison with popular three-terminal linear regulators, especially with higher input voltages. In many cases, the power dissipated is so low that no heatsink is required or its size could be reduced dramatically. 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. The LM2576 features include a guaranteed 4% tolerance on output voltage within specified input voltages and output load conditions, and 10% on the oscillator frequency (2% over 0C to 125C). External shutdown is included, featuring 80 mA (typical) standby current. The output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions.
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1 5
TO-220 TV SUFFIX CASE 314B
Heatsink surface connected to Pin 3
TO-220 T SUFFIX CASE 314D 1 5 Pin 1. 2. 3. 4. 5. Vin Output Ground Feedback ON/OFF
* 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions * Adjustable Version Output Voltage Range, 1.23 to 37 V 4% * * * * * * * * * * * * * * * *
Maximum Over Line and Load Conditions Guaranteed 3.0 A Output Current Wide Input Voltage Range Requires Only 4 External Components 52 kHz Fixed Frequency Internal Oscillator TTL Shutdown Capability, Low Power Standby Mode High Efficiency Uses Readily Available Standard Inductors Thermal Shutdown and Current Limit Protection Moisture Sensitivity Level (MSL) Equals 1 Pb-Free Packages are Available* Simple High-Efficiency Step-Down (Buck) Regulator Efficient Pre-Regulator for Linear Regulators On-Card Switching Regulators Positive to Negative Converter (Buck-Boost) Negative Step-Up Converters Power Supply for Battery Chargers
1 5
D2PAK D2T SUFFIX CASE 936A
Heatsink surface (shown as terminal 6 in case outline drawing) is connected to Pin 3
ORDERING INFORMATION
See detailed ordering and shipping information in the package dimensions section on page 24 of this data sheet.
DEVICE MARKING INFORMATION
See general marking information in the device marking section on page 24 of this data sheet.
Applications
*For additional information on our Pb-Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D.
(c) Semiconductor Components Industries, LLC, 2004
1
June, 2004 - Rev. 5
Publication Order Number: LM2576/D
LM2576
Typical Application (Fixed Output Voltage Versions)
Feedback 7.0 V - 40 V Unregulated DC Input +Vin Cin 100 mF 1 3 GN D 5 LM2576 4 Output 2 ON/OFF L1 100 mH D1 1N5822 Cout 1000 mF 5.0 V Regulated Output 3.0 A Load
Representative Block Diagram and Typical Application
+Vin 1 Cin 4 Feedback R2 Fixed Gain Error Amplifier Comparator Current Limit
Unregulated DC Input
3.1 V Internal Regulator
ON/OFF
ON/OFF 5
Output Voltage Versions 3.3 V 5.0 V 12 V 15 V For adjustable version R1 = open, R2 = 0 W
R2 (W) 1.7 k 3.1 k 8.84 k 11.3 k
R1 1.0 k
Driver Freq Shift 18 kHz 52 kHz Oscillator Latch Output 1.0 Amp Switch Reset Thermal Shutdown 2 GND 3 D1 L1
Regulated Output Vout Cout Load
1.235 V Band-Gap Reference
This device contains 162 active transistors.
Figure 1. Block Diagram and Typical Application
MAXIMUM RATINGS
Rating Maximum Supply Voltage ON/OFF Pin Input Voltage Output Voltage to Ground (Steady-State) Power Dissipation Case 314B and 314D (TO-220, 5-Lead) Thermal Resistance, Junction-to-Ambient Thermal Resistance, Junction-to-Case Case 936A (D2PAK) Thermal Resistance, Junction-to-Ambient Thermal Resistance, Junction-to-Case Storage Temperature Range Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 kW) Lead Temperature (Soldering, 10 seconds) Maximum Junction Temperature Symbol Vin - - PD RqJA RqJC PD RqJA RqJC Tstg - - TJ Value 45 -0.3 V V +Vin -1.0 Internally Limited 65 5.0 Internally Limited 70 5.0 -65 to +150 2.0 260 150 Unit V V V W C/W C/W W C/W C/W C kV C C
Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit values (not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied, damage may occur and reliability may be affected.
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LM2576
OPERATING RATINGS (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.) Rating Operating Junction Temperature Range Supply Voltage Symbol TJ Vin Value -40 to +125 40 Unit C V
SYSTEM PARAMETERS (Note 1 Test Circuit Figure 15)
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V for the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25C, for min/max values TJ is the operating junction temperature range that applies Note 2, unless otherwise noted.)
Characteristics LM2576-3.3 (Note 1 Test Circuit Figure 15) Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25C) Output Voltage (6.0 V Vin 40 V, 0.5 A ILoad 3.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 3.0 A) LM2576-5 (Note 1 Test Circuit Figure 15) Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25C) Output Voltage (8.0 V Vin 40 V, 0.5 A ILoad 3.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 3.0 A) LM2576-12 (Note 1 Test Circuit Figure 15) Output Voltage (Vin = 25 V, ILoad = 0.5 A, TJ = 25C) Output Voltage (15 V Vin 40 V, 0.5 A ILoad 3.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 15 V, ILoad = 3.0 A) LM2576-15 (Note 1 Test Circuit Figure 15) Output Voltage (Vin = 30 V, ILoad = 0.5 A, TJ = 25C) Output Voltage (18 V Vin 40 V, 0.5 A ILoad 3.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 18 V, ILoad = 3.0 A) LM2576 ADJUSTABLE VERSION (Note 1 Test Circuit Figure 15) Feedback Voltage (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V, TJ = 25C) Feedback Voltage (8.0 V Vin 40 V, 0.5 A ILoad 3.0 A, Vout = 5.0 V) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 3.0 A, Vout = 5.0 V) Vout Vout 1.193 1.18 - 1.23 - 77 1.267 1.28 - % 1.217 1.23 1.243 V V Vout Vout 14.4 14.25 - 15 - 88 15.6 15.75 - % 14.7 15 15.3 V V Vout Vout 11.52 11.4 - 12 - 88 12.48 12.6 - % 11.76 12 12.24 V V Vout Vout 4.8 4.75 - 5.0 - 77 5.2 5.25 - % 4.9 5.0 5.1 V V Vout Vout 3.168 3.135 - 3.3 - 75 3.432 3.465 - % 3.234 3.3 3.366 V V Symbol Min Typ Max Unit
1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2576 is used as shown in the Figure 15 test circuit, system performance will be as shown in system parameters section. 2. Tested junction temperature range for the LM2576: Tlow = -40C Thigh = +125C
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DEVICE PARAMETERS
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V for the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25C, for min/max values TJ is the operating junction temperature range that applies [Note 2], unless otherwise noted.)
Characteristics ALL OUTPUT VOLTAGE VERSIONS Feedback Bias Current (Vout = 5.0 V Adjustable Version Only) TJ = 25C TJ = -40 to +125C Oscillator Frequency Note 3 TJ = 25C TJ = 0 to +125C TJ = -40 to +125C Saturation Voltage (Iout = 3.0 A Note 4) TJ = 25C TJ = -40 to +125C Max Duty Cycle ("on") Note 5 Current Limit (Peak Current Notes 3 and 4) TJ = 25C TJ = -40 to +125C Output Leakage Current Notes 6 and 7, TJ = 25C Output = 0 V Output = -1.0 V Quiescent Current Note 6 TJ = 25C TJ = -40 to +125C Standby Quiescent Current (ON/OFF Pin = 5.0 V ("off")) TJ = 25C TJ = -40 to +125C ON/OFF Pin Logic Input Level (Test Circuit Figure 15) Vout = 0 V TJ = 25C TJ = -40 to +125C Vout = Nominal Output Voltage TJ = 25C TJ = -40 to +125C ON/OFF Pin Input Current (Test Circuit Figure 15) ON/OFF Pin = 5.0 V ("off"), TJ = 25C ON/OFF Pin = 0 V ("on"), TJ = 25C Ib - - fosc - 47 42 Vsat - - DC ICL 4.2 3.5 IL - - IQ - - Istby - - VIH 2.2 2.4 VIL - - IIH IIL - - 1.2 - 15 0 1.0 0.8 mA 30 5.0 1.4 - - - 80 - 200 400 V 5.0 - 9.0 11 mA 0.8 6.0 2.0 20 mA 5.8 - 6.9 7.5 mA 94 1.5 - 98 1.8 2.0 - % A 52 - - - 58 63 V 25 - 100 200 kHz nA Symbol Min Typ Max Unit
3. The oscillator frequency reduces to approximately 18 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 dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%. 4. Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin. 5. Feedback (Pin 4) removed from output and connected to 0 V. 6. Feedback (Pin 4) removed from output and connected to +12 V for the Adjustable, 3.3 V, and 5.0 V versions, and +25 V for the 12 V and 15 V versions, to force the output transistor "off". 7. Vin = 40 V.
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TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
1.0 Vout , OUTPUT VOLTAGE CHANGE (%) 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -50 Vin = 20 V ILoad = 500 mA Normalized at TJ = 25C Vout , OUTPUT VOLTAGE CHANGE (%) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 0 5.0 10 15 20 25 30 35 40 12 V and 15 V 3.3 V, 5.0 V and ADJ ILoad = 500 mA TJ = 25C
-25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (C)
Vin, INPUT VOLTAGE (V)
Figure 2. Normalized Output Voltage
Figure 3. Line Regulation
2.0 INPUT - OUTPUT DIFFERENTIAL (V) ILoad = 3.0 A I O, OUTPUT CURRENT (A) 1.5
6.5 Vin = 25 V 6.0 5.5 5.0 4.5 4.0 -50
1.0 ILoad = 500 mA 0.5 L1 = 150 mH Rind = 0.1 W 0 -50 -25 0 25 50 75 100 125
-25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (C)
TJ, JUNCTION TEMPERATURE (C)
Figure 4. Dropout Voltage
Figure 5. Current Limit
I stby , STANDBY QUIESCENT CURRENT (A)
20 I Q, QUIESCENT CURRENT (mA) 18 16 14 12 10 8.0 6.0 4.0 0 5.0 10 15 20 25 30 35 40 ILoad = 200 mA ILoad = 3.0 A Vout = 5.0 V Measured at Ground Pin TJ = 25C
200 180 160 140 120 100 80 60 40 20 0 -50 Vin = 12 V Vin = 40 V VON/OFF = 5.0 V
-25
0
25
50
75
100
125
Vin, INPUT VOLTAGE (V)
TJ, JUNCTION TEMPERATURE (C)
Figure 6. Quiescent Current
Figure 7. Standby Quiescent Current
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TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
I stby , STANDBY QUIESCENT CURRENT (A) 200 Vsat , SATURATION VOLTAGE (V) 35 180 160 140 120 100 80 60 40 20 0 TJ = 25C 1.6 1.4 1.2 1.0 0.8 25C 0.6 0.4 0.2 0 0 5 10 15 20 25 30 40 0 0.5 1.0 1.5 2.0 2.5 3.0 Vin, INPUT VOLTAGE (V) SWITCH CURRENT (A) 125C -40C
Figure 8. Standby Quiescent Current
Figure 9. Switch Saturation Voltage
8.0 NORMALIZED FREQUENCY (%) 6.0 V in, INPUT VOLTAGE (V) 4.0 2.0 0 -2.0 -4.0 -6.0 -8.0 -10 -50 -25 0 25 50 75 100 125 Vin = 12 V Normalized at 25C
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -50 -25 0 25 50 75 100 125 Vout ' 1.23 V ILoad = 500 mA Adjustable Version Only
TJ, JUNCTION TEMPERATURE (C)
TJ, JUNCTION TEMPERATURE (C)
Figure 10. Oscillator Frequency
Figure 11. Minimum Operating Voltage
100 Ib , FEEDBACK PIN CURRENT (nA) 80 60 40 20 0 -20 -40 -60 -80 -25 0 25 50 75 100 125 Adjustable Version Only
-100 -50
TJ, JUNCTION TEMPERATURE (C)
Figure 12. Feedback Pin Current
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TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
A 50 V 0 4.0 A B 2.0 A 0 4.0 A C D 2.0 A 0 5 ms/DIV 3.0 A Load 2.0 A Current 1.0 A 0 100 ms/DIV 100 mV Output 0 Voltage Change - 100 mV
Figure 13. Switching Waveforms
Vout = 15 V A: Output Pin Voltage, 10 V/DIV B: Inductor Current, 2.0 A/DIV C: Inductor Current, 2.0 A/DIV, AC-Coupled D: Output Ripple Voltage, 50 mV/dDIV, AC-Coupled Horizontal Time Base: 5.0 ms/DIV
Figure 14. Load Transient Response
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Fixed Output Voltage Versions
Feedback Vin 1 4 LM2576 Fixed Output Output 2 ON/OFF D1 MBR360 Cout 1000 mF Load L1 100 mH Vout
3 7.0 V - 40 V Unregulated DC Input Cin 100 mF
GN D
5
Cin Cout D1 L1 R1 R2
- - - - - -
100 mF, 75 V, Aluminium Electrolytic 1000 mF, 25 V, Aluminium Electrolytic Schottky, MBR360 100 mH, Pulse Eng. PE-92108 2.0 k, 0.1% 6.12 k, 0.1%
Adjustable Output Voltage Versions
Feedback Vin 1 4 LM2576 Adjustable Output 2 ON/OFF D1 MBR360 Cout 1000 mF R2 Load R1 L1 100 mH Vout 5,000 V
3 7.0 V - 40 V Unregulated DC Input Cin 100 mF
GN D
5
V out + V R2 + R1
ref
1.0 ) R2 R1 - 1.0
V out V ref
Where Vref = 1.23 V, R1 between 1.0 k and 5.0 k
Figure 15. Typical Test Circuit
PCB LAYOUT GUIDELINES As in any switching regulator, the layout of the printed circuit board is very important. Rapidly switching currents associated with wiring inductance, stray capacitance and parasitic inductance of the printed circuit board traces can generate voltage transients which can generate electromagnetic interferences (EMI) and affect the desired operation. As indicated in the Figure 15, to minimize inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. For best results, single-point grounding (as indicated) or ground plane construction should be used. On the other hand, the PCB area connected to the Pin 2 (emitter of the internal switch) of the LM2576 should be kept to a minimum in order to minimize coupling to sensitive circuitry. Another sensitive part of the circuit is the feedback. It is important to keep the sensitive feedback wiring short. To assure this, physically locate the programming resistors near to the regulator, when using the adjustable version of the LM2576 regulator.
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PIN FUNCTION DESCRIPTION
Pin 1 Symbol Vin Description (Refer to Figure 1) This pin is the positive input supply for the LM2576 step-down switching regulator. In order to minimize voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be present (Cin in Figure 1). This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.5 V. It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to minimize coupling to sensitive circuitry. Circuit ground pin. See the information about the printed circuit board layout. This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the internal resistor divider network R2, R1 and applied to the non-inverting input of the internal error amplifier. In the Adjustable version of the LM2576 switching regulator this pin is the direct input of the error amplifier and the resistor network R2, R1 is connected externally to allow programming of the output voltage. It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply current to approximately 80 mA. The threshold voltage is typically 1.4 V. Applying a voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or if this pin is left open, the regulator will be in the "on" condition.
2
Output
3 4
GND Feedback
5
ON/OFF
DESIGN PROCEDURE
Buck Converter Basics
The LM2576 is a "Buck" or Step-Down Converter which is the most elementary forward-mode converter. Its basic schematic can be seen in Figure 16. The operation of this regulator topology has two distinct time periods. The first one occurs when the series switch is on, the input voltage is connected to the input of the inductor. The output of the inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. During this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following equation:
I L(on) + V - V out t on in L
This period ends when the power switch is once again turned on. Regulation of the converter is accomplished by varying the duty cycle of the power switch. It is possible to describe the duty cycle as follows:
t d + on , where T is the period of switching. T
For the buck converter with ideal components, the duty cycle can also be described as:
V d + out V in
Figure 17 shows the buck converter, idealized waveforms of the catch diode voltage and the inductor current.
Von(SW)
Power Switch
Diode Voltage
During this "on" period, energy is stored within the core material in the form of magnetic flux. If the inductor is properly designed, there is sufficient energy stored to carry the requirements of the load during the "off" period.
L
Power Switch Off VD(FWD)
Power Switch On
Power Switch Off
Power Switch On
Vin
D
Cout
RLoad
Time
Figure 16. Basic Buck Converter
Ipk Inductor Current ILoad(AV) Imin Diode Power Switch Power Switch Time
The next period is the "off" period of the power switch. When the power switch turns off, the voltage across the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. The current now flows through the catch diode thus maintaining the load current loop. This removes the stored energy from the inductor. The inductor current during this time is:
I L(off) + V out - V L D t off
Diode
Figure 17. Buck Converter Idealized Waveforms
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LM2576
Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step-by-step design procedure and some examples are provided.
Procedure Given Parameters: Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V) Vin(max) = Maximum Input Voltage ILoad(max) = Maximum Load Current 1. Controller IC Selection According to the required input voltage, output voltage and current, select the appropriate type of the controller IC output voltage version. 2. Input Capacitor Selection (Cin) To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +Vin and ground pin GND. This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value. 3. Catch Diode Selection (D1) A. Since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design the diode should have a current rating equal to the maximum current limit of the LM2576 to be able to withstand a continuous output short B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. Inductor Selection (L1) A. According to the required working conditions, select the correct inductor value using the selection guide from Figures 18 to 22. B. From the appropriate inductor selection guide, identify the inductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code. C. Select an appropriate inductor from the several different manufacturers part numbers listed in Table 2. The designer must realize that the inductor current rating must be higher than the maximum peak current flowing through the inductor. This maximum peak current can be calculated as follows: I p(max)
+I
Example Given Parameters: Vout = 5.0 V Vin(max) = 15 V ILoad(max) = 3.0 A 1. Controller IC Selection According to the required input voltage, output voltage, current polarity and current value, use the LM2576-5 controller IC 2. Input Capacitor Selection (Cin) A 100 mF, 25 V aluminium electrolytic capacitor located near to the input and ground pins provides sufficient bypassing.
3. Catch Diode Selection (D1) A. For this example the current rating of the diode is 3.0 A.
B. Use a 20 V 1N5820 Schottky diode, or any of the suggested fast recovery diodes shown in Table 1. 4. Inductor Selection (L1) A. Use the inductor selection guide shown in Figures 19.
B. From the selection guide, the inductance area intersected by the 15 V line and 3.0 A line is L100.
C. Inductor value required is 100 mH. From Table 2, choose an inductor from any of the listed manufacturers.
Load(max)
)
V -V out t on in 2L
where ton is the "on" time of the power switch and V ton + out x 1.0 fosc V in For additional information about the inductor, see the inductor section in the "Application Hints" section of this data sheet.
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LM2576
Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step-by-step design procedure and some examples are provided.
Procedure 5. Output Capacitor Selection (Cout) A. Since the LM2576 is a forward-mode switching regulator with voltage mode control, its open loop 2-pole-1-zero frequency characteristic has the dominant pole-pair determined by the output capacitor and inductor values. For stable operation and an acceptable ripple voltage, (approximately 1% of the output voltage) a value between 680 mF and 2000 mF is recommended. B. Due to the fact that the higher voltage electrolytic capacitors generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor's voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating at least 8.0 V is appropriate, and a 10 V or 16 V rating is recommended. Example 5. Output Capacitor Selection (Cout) A. Cout = 680 mF to 2000 mF standard aluminium electrolytic.
B. Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2576-ADJ)
Procedure Given Parameters: Vout = Regulated Output Voltage Vin(max) = Maximum DC Input Voltage ILoad(max) = Maximum Load Current 1. Programming Output Voltage To select the right programming resistor R1 and R2 value (see Figure 2) use the following formula: V out + V ref 1.0 ) R2 R1 where Vref = 1.23 V R2 + R1 Given Parameters: Vout = 8.0 V Vin(max) = 25 V ILoad(max) = 2.5 A 1. Programming Output Voltage (selecting R1 and R2) Select R1 and R2: V out + 1.23 1.0 ) V out V ref R2 R1 Select R1 = 1.8 kW + 1.8 k 8.0 V * 1.0 1.23 V Example
Resistor R1 can be between 1.0 k and 5.0 kW. (For best temperature coefficient and stability with time, use 1% metal film resistors). V out R2 + R1 - 1.0 V ref 2. Input Capacitor Selection (Cin) To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +Vin and ground pin GND This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value. For additional information see input capacitor section in the "Application Hints" section of this data sheet. 3. Catch Diode Selection (D1) A. Since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design, the diode should have a current rating equal to the maximum current limit of the LM2576 to be able to withstand a continuous output short. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage.
* 1.0
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
2. Input Capacitor Selection (Cin) A 100 mF, 150 V aluminium electrolytic capacitor located near the input and ground pin provides sufficient bypassing.
3. Catch Diode Selection (D1) A. For this example, a 3.0 A current rating is adequate.
B. Use a 30 V 1N5821 Schottky diode or any suggested fast recovery diode in the Table 1.
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Procedure (Adjustable Output Version: LM2576-ADJ) (continued)
Procedure 4. Inductor Selection (L1) A. Use the following formula to calculate the inductor Volt x microsecond [V x ms] constant: V out 6 x 10 [V x ms] E x T + V - V out in F[Hz] V in B. Match the calculated E x T value with the corresponding number on the vertical axis of the Inductor Value Selection Guide shown in Figure 22. This E x T 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. C. Next step is to identify the inductance region intersected by the E x T value and the maximum load current value on the horizontal axis shown in Figure 25. D. From the inductor code, identify the inductor value. Then select an appropriate inductor from Table 2. The inductor chosen must be rated for a switching frequency of 52 kHz and for a current rating of 1.15 x ILoad. The inductor current rating can also be determined by calculating the inductor peak current: I p(max)
+I
Example 4. Inductor Selection (L1) A. Calculate E x T [V x ms] constant: E x T + (25 - 8.0) x 8.0 x 1000 + 80 [V x ms] 52 25 B. E x T = 80 [V x ms]
C. ILoad(max) = 2.5 A Inductance Region = H150 D. Proper inductor value = 150 mH Choose the inductor from Table 2.
Load(max)
)
V
in
- V out t on 2L
where ton is the "on" time of the power switch and x 1.0 f osc in For additional information about the inductor, see the inductor section in the "External Components" section of this data sheet. t on + V 5. Output Capacitor Selection (Cout) A. Since the LM2576 is a forward-mode switching regulator with voltage mode control, its open loop 2-pole-1-zero frequency characteristic has the dominant pole-pair determined by the output capacitor and inductor values. For stable operation, the capacitor must satisfy the following requirement: V in(max) [F] Cout w 13, 300 V out x L [H] B. Capacitor values between 10 mF and 2000 mF will satisfy the loop requirements for stable operation. To achieve an acceptable output ripple voltage and transient response, the output capacitor may need to be several times larger than the above formula yields. C. Due to the fact that the higher voltage electrolytic capacitors generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor's voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating of at least 8.0 V is appropriate, and a 10 V or 16 V rating is recommended. 5. Output Capacitor Selection (Cout) A. Cout w 13, 300 x 25 + 332.5 F 8 x 150 To achieve an acceptable ripple voltage, select Cout = 680 mF electrolytic capacitor. V out
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LM2576
LM2576 Series Buck Regulator Design Procedures (continued)
Indicator Value Selection Guide (For Continuous Mode Operation)
60 40 20 15 10 8.0 7.0 6.0 60 L330 L220 L150 L100 L68 L47 5.0 0.3 MAXIMUM INPUT VOLTAGE (V) L470 40 20 15 12 10 9.0 8.0
L680
H1000 L680
H680
H470
H330
H220
H150
MAXIMUM INPUT VOLTAGE (V)
L470 L330 L220 L150 L100 L68 L47
0.4
0.5
0.6
0.8
1.0
1.5
2.0
2.5 3.0
7.0 0.3
0.4
0.5
0.6
0.8 1.0 1.2
1.5
2.0
2.5 3.0
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
Figure 18. LM2576-3.3
Figure 19. LM2576-5
60 MAXIMUM INPUT VOLTAGE (V) MAXIMUM INPUT VOLTAGE (V) 40 35 30 H1500 25 H1000 20 18 L680 16 15 14 0.3 L470 L330
60 40 35 30 25 22 20 19 18 17 0.3 L680 L470 L330 L220 L150
H1500 H1000 H680 H470
H680
H470
H330
H220
H150
H330
H220
H150
L220 L150 L100 L68
L100 L68
0.4
0.5 0.6
0.8 1.0
1.5
2.0
2.5 3.0
0.4
0.5 0.6
0.8
1.0
1.5
2.0
2.5 3.0
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
Figure 20. LM2576-12
Figure 21. LM2576-15
300 250 200 ET, VOLTAGE TIME (V s) 150 100 90 80 70 60 50 45 40 35 30 25 20 0.3
H2000 H1500 H1000 H680 H470
H330
H220
H150
L680
L470
L330
L220
L150
L100
L68 L47
0.4
0.5
0.6
0.8 1.0
1.5
2.0
2.5 3.0
IL, MAXIMUM LOAD CURRENT (A)
Figure 22. LM2576-ADJ
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LM2576
Table 1. Diode Selection Guide
Schottky 3.0 A VR 20 V Through Hole 1N5820 MBR320P SR302 1N5821 MBR330 SR303 31DQ03 1N5822 MBR340 SR304 31DQ04 MBR350 31DQ05 SR305 MBR360 DQ06 SR306 Surface Mount SK32 4.0 - 6.0 A Through Hole 1N5823 SR502 SB520 1N5824 SR503 SB530 50WQ03 MUR320 31DF1 HER302 MBRD640CT 50WQ04 (all diodes rated to at least 100 V) MURS320T3 MURD320 30WF10 (all diodes rated to at least 100 V) MUR420 HER602 MURD620CT 50WF10 Surface Mount Through Hole 3.0 A Surface Mount Fast Recovery 4.0 - 6.0 A Through Hole Surface Mount
30 V
SK33 30WQ03
40 V
SK34 30WQ04 MBRS340T3 MBRD340 SK35 30WQ05
1N5825 SR504 SB540
(all diodes rated to at least 100 V)
(all diodes rated to at least 100 V)
50 V
SB550
50WQ05
60 V
MBRS360T3 MBRD360
50SQ080
MBRD660CT
NOTE: Diodes listed in bold are available from ON Semiconductor.
Table 2. Inductor Selection by Manufacturer's Part Number
Inductor Code L47 L68 L100 L150 L220 L330 L470 L680 H150 H220 H330 H470 H680 H1000 H1500 H2200 NOTE: *Contact Manufacturer Inductor Value 47 mH 68 mH 100 mH 150 mH 220 mH 330 mH 470 mH 680 mH 150 mH 220 mH 330 mH 470 mH 680 mH 1000 mH 1500 mH 2200 mH Tech 39 77 212 77 262 77 312 77 360 77 408 77 456 * 77 506 77 362 77 412 77 462 * 77 508 77 556 * * Schott Corp. 671 26980 671 26990 671 27000 671 27010 671 27020 671 27030 671 27040 671 27050 671 27060 671 27070 671 27080 671 27090 671 27100 671 27110 671 27120 671 27130 Pulse Eng. 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 RL2442 RL2443 RL2444 RL1954 RL1953 RL1952 RL1951 RL1950 RL2445 RL2446 RL2447 RL1961 RL1960 RL1959 RL1958 RL2448
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LM2576
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Pulse Engineering, Inc. Pulse Engineering, Inc. Europe Renco Electronics, Inc. Tech 39 Schott Corporation Phone Fax Phone Fax Phone Fax Phone Fax Phone Fax + 1-619-674-8100 + 1-619-674-8262 + 353-9324-107 + 353-9324-459 + 1-516-645-5828 + 1-516-586-5562 + 33-1-4115-1681 + 33-1-4709-5051 + 1-612-475-1173 + 1-612-475-1786
EXTERNAL COMPONENTS
Input Capacitor (Cin) The Input Capacitor Should Have a Low ESR Output Capacitor (Cout)
For stable operation of the switch mode converter a low ESR (Equivalent Series Resistance) aluminium or solid tantalum bypass capacitor is needed between the input pin and the ground pin, to prevent large voltage transients from appearing at the input. It must be located near the regulator and use short leads. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower temperatures. For reliable operation in temperatures below -25C larger values of the input capacitor may be needed. Also paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures.
RMS Current Rating of Cin
The important parameter of the input capacitor is the RMS current rating. Capacitors that are physically large and have large surface area will typically have higher RMS current ratings. For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS current rating. The consequence of operating an electrolytic capacitor beyond the RMS current rating is a shortened operating life. In order to assure maximum capacitor operating lifetime, the capacitor's RMS ripple current rating should be:
Irms > 1.2 x d x ILoad
For low output ripple voltage and good stability, low ESR output capacitors are recommended. An output capacitor has two main functions: it filters the output and provides regulator loop stability. The ESR of the output capacitor and the peak-to-peak value of the inductor ripple current are the main factors contributing to the output ripple voltage value. Standard aluminium electrolytics could be adequate for some applications but for quality design, low ESR types are recommended. An aluminium electrolytic capacitor's ESR value is related to many factors such as the capacitance value, the voltage rating, the physical size and the type of construction. In most cases, the higher voltage electrolytic capacitors have lower ESR value. Often capacitors with much higher voltage ratings may be needed to provide low ESR values that, are required for low output ripple voltage.
The Output Capacitor Requires an ESR Value That Has an Upper and Lower Limit
As mentioned above, a low ESR value is needed for low output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor's ESR is extremely low (below 0.05 W), there is a possibility of an unstable feedback loop, resulting in oscillation at the output. This situation can occur when a tantalum capacitor, that can have a very low ESR, is used as the only output capacitor.
At Low Temperatures, Put in Parallel Aluminium Electrolytic Capacitors with Tantalum Capacitors
where d is the duty cycle, for a buck regulator
V t d + on + out V T in |V out| t on and d + + for a buck boost regulator. * |V out| ) V T in
Electrolytic capacitors are not recommended for temperatures below -25C. The ESR rises dramatically at cold temperatures and typically rises 3 times at -25C and as much as 10 times at -40C. Solid tantalum capacitors have much better ESR spec at cold temperatures and are recommended for temperatures below -25C. They can be also used in parallel with aluminium electrolytics. The value of the tantalum capacitor should be about 10% or 20% of the total capacitance. The output capacitor should have at least 50% higher RMS ripple current rating at 52 kHz than the peak-to-peak inductor ripple current.
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LM2576
Catch Diode Locate the Catch Diode Close to the LM2576
The LM2576 is a step-down buck converter; it requires a fast diode to provide a return path for the inductor current when the switch turns off. This diode must be located close to the LM2576 using short leads and short printed circuit traces to avoid EMI problems.
Use a Schottky or a Soft Switching Ultra-Fast Recovery Diode
Inductor
2.0 A Inductor Current Waveform 0A 2.0 A Power Switch Current Waveform 0A HORIZONTAL TIME BASE: 5.0 ms/DIV
The magnetic components are the cornerstone of all switching power supply designs. The style of the core and the winding technique used in the magnetic component's design has a great influence on the reliability of the overall power supply. Using an improper or poorly designed inductor can cause high voltage spikes generated by the rate of transitions in current within the switching power supply, and the possibility of core saturation can arise during an abnormal operational mode. Voltage spikes can cause the semiconductors to enter avalanche breakdown and the part can instantly fail if enough energy is applied. It can also cause significant RFI (Radio Frequency Interference) and EMI (Electro-Magnetic Interference) problems.
Continuous and Discontinuous Mode of Operation
Figure 23. Continuous Mode Switching Current Waveforms
Selecting the Right Inductor Style
The LM2576 step-down converter can operate in both the continuous and the discontinuous modes of operation. The regulator works in the continuous mode when loads are relatively heavy, the current flows through the inductor continuously and never falls to zero. Under light load conditions, the circuit will be forced to the discontinuous mode when inductor current falls to zero for certain period of time (see Figure 23 and Figure 24). Each mode has distinctively different operating characteristics, which can affect the regulator performance and requirements. In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower peak currents in the switch, inductor and diode, and can have a lower output
Some important considerations when selecting a core type are core material, cost, the output power of the power supply, the physical volume the inductor must fit within, and the amount of EMI (Electro-Magnetic Interference) shielding that the core must provide. The inductor selection guide covers different styles of inductors, such as pot core, E-core, toroid and bobbin core, as well as different core materials such as ferrites and powdered iron from different manufacturers. For high quality design regulators the toroid core seems to be the best choice. Since the magnetic flux is contained within the core, it generates less EMI, reducing noise problems in sensitive circuits. The least expensive is the bobbin core type, which consists of wire wound on a ferrite rod core. This type of inductor generates more EMI due to the fact that its core is open, and the magnetic flux is not contained within the core.
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VERTRICAL RESOLUTION 1.0 A/DIV
Since the rectifier diodes are very significant sources of losses within switching power supplies, choosing the rectifier that best fits into the converter design is an important process. Schottky diodes provide the best performance because of their fast switching speed and low forward voltage drop. They provide the best efficiency especially in low output voltage applications (5.0 V and lower). Another choice could be Fast-Recovery, or Ultra-Fast Recovery diodes. It has to be noted, that some types of these diodes with an abrupt turnoff characteristic may cause instability or EMI troubles. A fast-recovery diode with soft recovery characteristics can better fulfill some quality, low noise design requirements. Table 1 provides a list of suitable diodes for the LM2576 regulator. Standard 50/60 Hz rectifier diodes, such as the 1N4001 series or 1N5400 series are NOT suitable.
ripple voltage. On the other hand it does require larger inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or high input voltages. To simplify the inductor selection process, an inductor selection guide for the LM2576 regulator was added to this data sheet (Figures 18 through 22). This guide assumes that the regulator is operating in the continuous mode, and selects an inductor that will allow a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. This percentage is allowed to change as different design load currents are selected. For light loads (less than approximately 300 mA) it may be desirable to operate the regulator in the discontinuous mode, because the inductor value and size can be kept relatively low. Consequently, the percentage of inductor peak-to-peak current increases. This discontinuous mode of operation is perfectly acceptable for this type of switching converter. Any buck regulator will be forced to enter discontinuous mode if the load current is light enough.
LM2576
When multiple switching regulators are located on the same printed circuit board, open core magnetics can cause interference between two or more of the regulator circuits, especially at high currents due to mutual coupling. A toroid, pot core or E-core (closed magnetic structure) should be used in such applications.
Do Not Operate an Inductor Beyond its Maximum Rated Current
inductor and/or the LM2576. Different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor.
Exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire losses, or the core may saturate. Core saturation occurs when the flux density is too high and consequently the cross sectional area of the core can no longer support additional lines of magnetic flux. This causes the permeability of the core to drop, the inductance value decreases rapidly and the inductor begins to look mainly resistive. It has only the DC resistance of the winding. This can cause the switch current to rise very rapidly and force the LM2576 internal switch into cycle-by-cycle current limit, thus reducing the DC output load current. This can also result in overheating of the
0.4 A Inductor Current Waveform 0A 0.4 A Power Switch Current Waveform 0A HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 24. Discontinuous Mode Switching Current Waveforms
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients Source of the Output Ripple
Minimizing the Output Ripple
Since the LM2576 is a switch mode power supply regulator, its output voltage, if left unfiltered, will contain a sawtooth ripple voltage at the switching frequency. The output ripple voltage value ranges from 0.5% to 3% of the output voltage. It is caused mainly by the inductor sawtooth ripple current multiplied by the ESR of the output capacitor.
Short Voltage Spikes and How to Reduce Them
The regulator output voltage may also contain short voltage spikes at the peaks of the sawtooth waveform (see Figure 25). These voltage spikes are present because of the fast switching action of the output switch, and the parasitic inductance of the output filter capacitor. There are some other important factors such as wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all these contribute to the amplitude of these spikes. To minimize these voltage spikes, low inductance capacitors should be used, and their lead lengths must be kept short. The importance of quality printed circuit board layout design should also be highlighted.
Voltage spikes caused by switching action of the output switch and the parasitic inductance of the output capacitor
In order to minimize the output ripple voltage it is possible to enlarge the inductance value of the inductor L1 and/or to use a larger value output capacitor. There is also another way to smooth the output by means of an additional LC filter (20 mH, 100 mF), that can be added to the output (see Figure 34) to further reduce the amount of output ripple and transients. With such a filter it is possible to reduce the output ripple voltage transients 10 times or more. Figure 25 shows the difference between filtered and unfiltered output waveforms of the regulator shown in Figure 34. The lower waveform is from the normal unfiltered output of the converter, while the upper waveform shows the output ripple voltage filtered by an additional LC filter.
Heatsinking and Thermal Considerations The Through-Hole Package TO-220
Filtered Output Voltage
The LM2576 is available in two packages, a 5-pin TO-220(T, TV) and a 5-pin surface mount D2PAK(D2T). Although the TO-220(T) package needs a heatsink under most conditions, there are some applications that require no heatsink to keep the LM2576 junction temperature within the allowed operating range. Higher ambient temperatures require some heat sinking, either to the printed circuit (PC) board or an external heatsink.
The Surface Mount Package D 2PAK and its Heatsinking
Unfiltered Output Voltage HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 25. Output Ripple Voltage Waveforms
The other type of package, the surface mount D2PAK, is designed to be soldered to the copper on the PC board. The copper and the board are the heatsink for this package and the other heat producing components, such as the catch diode and inductor. The PC board copper area that the package is soldered to should be at least 0.4 in2 (or 260 mm2) and ideally should have 2 or more square inches (1300 mm2) of 0.0028 inch copper. Additional increases of copper area
VERTRICAL RESOLUTION 20 mV/DIV
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VERTICAL RESOLUTION 200 mA/DIV
LM2576
beyond approximately 6.0 in2 (4000 mm2) will not improve heat dissipation significantly. If further thermal improvements are needed, double sided or multilayer PC boards with large copper areas should be considered. In order to achieve the best thermal performance, it is highly recommended to use wide copper traces as well as large areas of copper in the printed circuit board layout. The only exception to this is the OUTPUT (switch) pin, which should not have large areas of copper (see page 8 `PCB Layout Guideline').
Thermal Analysis and Design Packages on a Heatsink
If the actual operating junction temperature is greater than the selected safe operating junction temperature determined in step 3, than a heatsink is required. The junction temperature will be calculated as follows:
TJ = PD (RqJA + RqCS + RqSA) + TA
where
RqJC is the thermal resistance junction-case, RqCS is the thermal resistance case-heatsink, RqSA is the thermal resistance heatsink-ambient.
The following procedure must be performed to determine whether or not a heatsink will be required. First determine: 1. PD(max) maximum regulator power dissipation in the application. 2. TA(max) maximum ambient temperature in the application. 3. TJ(max) maximum allowed junction temperature (125C for the LM2576). For a conservative design, the maximum junction temperature should not exceed 110C to assure safe operation. For every additional +10C temperature rise that the junction must withstand, the estimated operating lifetime of the component is halved. 4. RqJC package thermal resistance junction-case. 5. RqJA package thermal resistance junction-ambient. (Refer to Maximum Ratings on page 2 of this data sheet or RqJC and RqJA values). The following formula is to calculate the approximate total power dissipated by the LM2576:
PD = (Vin x IQ) + d x ILoad x Vsat
If the actual operating temperature is greater than the selected safe operating junction temperature, then a larger heatsink is required.
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and the junction temperature rise numbers are all approximate, and there are many factors that will affect these numbers, such as PC board size, shape, thickness, physical position, location, board temperature, as well as whether the surrounding air is moving or still. Other factors are trace width, total printed circuit copper area, copper thickness, single- or double-sided, multilayer board, the amount of solder on the board or even color of the traces. The size, quantity and spacing of other components on the board can also influence its effectiveness to dissipate the heat.
12 to 40 V Unregulated DC Input Cin 100 mF Feedback +Vin 1 3 GN D 5 4 LM2576-12 Output 2 ON/OFF L1 68 mH D1 1N5822 Cout 2200 mF -12 V @ 0.7 A Regulated Output
where d is the duty cycle and for buck converter
V t d + on + O , V T in
IQ
(quiescent current) and Vsat can be found in the LM2576 data sheet, Vin is minimum input voltage applied, VO is the regulator output voltage, ILoad is the load current. The dynamic switching losses during turn-on and turn-off can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free-Standing)
Figure 26. Inverting Buck-Boost Develops -12 V
ADDITIONAL APPLICATIONS
Inverting Regulator
For a free-standing application when no heatsink is used, the junction temperature can be determined by the following expression:
TJ = (RqJA) (PD) + TA
where (RqJA)(PD) represents the junction temperature rise caused by the dissipated power and TA is the maximum ambient temperature.
An inverting buck-boost regulator using the LM2576-12 is shown in Figure 26. This circuit converts a positive input voltage to a negative output voltage with a common ground by bootstrapping the regulators ground to the negative output voltage. By grounding the feedback pin, the regulator senses the inverted output voltage and regulates it. In this example the LM2576-12 is used to generate a -12 V output. The maximum input voltage in this case cannot exceed +28 V because the maximum voltage appearing across the regulator is the absolute sum of the input and output voltages and this must be limited to a maximum of 40 V.
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LM2576
This circuit configuration is able to deliver approximately 0.7 A to the output when the input voltage is 12 V or higher. At lighter loads the minimum input voltage required drops to approximately 4.7 V, because the buck-boost regulator topology can produce an output voltage that, in its absolute value, is either greater or less than the input voltage. Since the switch currents in this buck-boost configuration are higher than in the standard buck converter topology, the available output current is lower. This type of buck-boost inverting regulator can also require a larger amount of start-up input current, even for light loads. This may overload an input power source with a current limit less than 5.0 A. Such an amount of input start-up current is needed for at least 2.0 ms or more. The actual time depends on the output voltage and size of the output capacitor. Because of the relatively high start-up currents required by this inverting regulator topology, the use of a delayed start-up or an undervoltage lockout circuit is recommended. Using a delayed start-up arrangement, the input capacitor can charge up to a higher voltage before the switch-mode regulator begins to operate. The high input current needed for start-up is now partially supplied by the input capacitor Cin. It has been already mentioned above, that in some situations, the delayed start-up or the undervoltage lockout features could be very useful. A delayed start-up circuit applied to a buck-boost converter is shown in Figure 27, Figure 33 in the "Undervoltage Lockout" section describes an undervoltage lockout feature for the same converter topology.
Design Recommendations:
I (V ) |V |) O ) V in x t on [ Load in V 2L 1 in |V | O where t on + x 1.0 , and f osc + 52 kHz. V ) |V | f osc in O I peak
Under normal continuous inductor current operating conditions, the worst case occurs when Vin is minimal.
12 V to 25 V Unregulated DC Input Cin 100 mF /50 V C1 0.1 mF R1 47 k Feedback +Vin 1 5 ON/OFF 3 LM2576-12 4 Output 2 GN D L1 68 mH
D1 1N5822
R2 47 k
Cout 2200 mF /16 V
-12 V @ 700 m A Regulated Output
Figure 27. Inverting Buck-Boost Regulator with Delayed start-up
+Vin
+Vin 1 Cin R1 100 mF 47 k LM2576-XX
5.0 V 0 On
Shutdown Input Off R3 470
5
ON/OFF 3
GN D
The inverting regulator operates in a different manner than the buck converter and so a different design procedure has to be used to select the inductor L1 or the output capacitor Cout. The output capacitor values must be larger than what is normally required for buck converter designs. Low input voltages or high output currents require a large value output capacitor (in the range of thousands of mF). The recommended range of inductor values for the inverting converter design is between 68 mH and 220 mH. To select an inductor with an appropriate current rating, the inductor peak current has to be calculated. The following formula is used to obtain the peak inductor current:
R2 47 k -Vout MOC8101
NOTE: This picture does not show the complete circuit.
Figure 28. Inverting Buck-Boost Regulator Shutdown Circuit Using an Optocoupler
With the inverting configuration, the use of the ON/OFF pin requires some level shifting techniques. This is caused by the fact, that the ground pin of the converter IC is no longer at ground. Now, the ON/OFF pin threshold voltage (1.3 V approximately) has to be related to the negative output voltage level. There are many different possible shut down methods, two of them are shown in Figures 28 and 29.
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LM2576
+V 0 On R2 5.6 k +Vin Cin 100 mF Q1 2N3906 5 +Vin 1 LM2576-XX Off Shutdown Input
Another important point is that these negative boost converters cannot provide current limiting load protection in the event of a short in the output so some other means, such as a fuse, may be necessary to provide the load protection.
Delayed Start-up
ON/OFF 3 R1 12 k
GN D -Vout
NOTE: This picture does not show the complete circuit.
Figure 29. Inverting Buck-Boost Regulator Shutdown Circuit Using a PNP Transistor Negative Boost Regulator
This example is a variation of the buck-boost topology and it is called negative boost regulator. This regulator experiences relatively high switch current, especially at low input voltages. The internal switch current limiting results in lower output load current capability. The circuit in Figure 30 shows the negative boost configuration. The input voltage in this application ranges from -5.0 V to -12 V and provides a regulated -12 V output. If the input voltage is greater than -12 V, the output will rise above -12 V accordingly, but will not damage the regulator.
There are some applications, like the inverting regulator already mentioned above, which require a higher amount of start-up current. In such cases, if the input power source is limited, this delayed start-up feature becomes very useful. To provide a time delay between the time when the input voltage is applied and the time when the output voltage comes up, the circuit in Figure 31 can be used. As the input voltage is applied, the capacitor C1 charges up, and the voltage across the resistor R2 falls down. When the voltage on the ON/OFF pin falls below the threshold value 1.3 V, the regulator starts up. Resistor R1 is included to limit the maximum voltage applied to the ON/OFF pin. It reduces the power supply noise sensitivity, and also limits the capacitor C1 discharge current, but its use is not mandatory. When a high 50 Hz or 60 Hz (100 Hz or 120 Hz respectively) ripple voltage exists, a long delay time can cause some problems by coupling the ripple into the ON/OFF pin, the regulator could be switched periodically on and off with the line (or double) frequency.
+Vin
+Vin 1 C1 0.1 mF LM2576-XX
5
ON/OFF 3
4 Vin 1 Cin 100 mF 3 GND 5 LM2576-12 Feedback Output 2 ON/OFF 1N5820
Cout 2200 mF Low Esr
Cin 100 mF
GN D
R1 47 k
R2 47 k
NOTE: This picture does not show the complete circuit. Vout = -12 V
Figure 31. Delayed Start-up Circuitry Undervoltage Lockout
Vin -5.0 V to -12 V
100 mH
Typical Load Current 400 mA for Vin = -5.2 V 750 mA for Vin = -7.0 V
Figure 30. Negative Boost Regulator
Design Recommendations:
The same design rules as for the previous inverting buck-boost converter can be applied. The output capacitor Cout must be chosen larger than would be required for a what standard buck converter. Low input voltages or high output currents require a large value output capacitor (in the range of thousands of mF). The recommended range of inductor values for the negative boost regulator is the same as for inverting converter design.
Some applications require the regulator to remain off until the input voltage reaches a certain threshold level. Figure 32 shows an undervoltage lockout circuit applied to a buck regulator. A version of this circuit for buck-boost converter is shown in Figure 33. Resistor R3 pulls the ON/OFF pin high and keeps the regulator off until the input voltage reaches a predetermined threshold level with respect to the ground Pin 3, which is determined by the following expression:
V th [V Z1 (Q1) ) 1.0 ) R2 V R1 BE
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LM2576
Under normal continuous inductor current operating conditions, the worst case occurs when Vin is minimal.
LM2576-XX
+Vin
+Vin 1
R2 10 k
R3 47 k
Cin 100 mF 5
ON/OFF 3
GN D
+Vin
+Vin 1
LM2576-XX
Z1 1N5242B Q1 2N3904 R1 10 k Vth 13 V
R2 15 k
R3 47 k
Cin 100 mF 5
ON/OFF 3
GN D
Z1 1N5242B Q1 2N3904 R1 15 k
Vth 13 V
NOTE: This picture does not show the complete circuit.
Figure 32. Undervoltage Lockout Circuit for Buck Converter
Vout
NOTE: This picture does not show the complete circuit.
The following formula is used to obtain the peak inductor current:
I (V ) |V |) O ) V in x t on [ Load in peak 2L 1 V in |V | O where t on + x 1.0 , and f osc + 52 kHz. V ) |V | f osc in O I
Figure 33. Undervoltage Lockout Circuit for Buck-Boost Converter Adjustable Output, Low-Ripple Power Supply
A 3.0 A output current capability power supply that features an adjustable output voltage is shown in Figure 34. This regulator delivers 3.0 A into 1.2 V to 35 V output. The input voltage ranges from roughly 3.0 V to 40 V. In order to achieve a 10 or more times reduction of output ripple, an additional L-C filter is included in this circuit.
40 V Max Unregulated DC Input
Feedback +Vin 1 LM2574-Adj Output 3 GN D 5 2 ON/OFF D1 1N5822 Cout 2200 mF R1 1.21 k 4 L1 150 mH R2 50 k C1 100 mF L2 20 mH Output Voltage 1.2 to 35 V @ 3.0 A
Cin 100 mF
Optional Output Ripple Filter
Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple
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LM2576
THE LM2576-5 STEP-DOWN VOLTAGE REGULATOR WITH 5.0 V @ 3.0 A OUTPUT POWER CAPABILITY. TYPICAL APPLICATION WITH THROUGH-HOLE PC BOARD LAYOUT
Feedback Unregulated DC Input +Vin = 7.0 to 40 V +Vin LM2576-5 1 3 C1 100 mF /50 V GN D 5 Output 2 ON/OFF 4 L1 150 mH Regulated Output Vout1 = 5.0 V @ 3.0 A
ON/OFF
D1 1N5822
Cout 1000 mF /16 V GNDout
GNDin
C1 C2 D1 L1
- - - -
100 mF, 50 V, Aluminium Electrolytic 1000 mF, 16 V, Aluminium Electrolytic 3.0 A, 40 V, Schottky Rectifier, 1N5822 150 mH, RL2444, Renco Electronics
Figure 35. Schematic Diagram of the LM2576-5 Step-Down Converter
U1
D1 + C2 C1 + Vou
t
ON/OFF +Vin L1
GNDi
n
GNDout NOTE: Not to scale. NOTE: Not to scale.
Figure 36. Printed Circuit Board Layout Component Side
00060_00
LM2576
Figure 37. Printed Circuit Board Layout Copper Side
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LM2576
THE LM2576-ADJ STEP-DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER CAPABILITY. TYPICAL APPLICATION WITH THROUGH-HOLE PC BOARD LAYOUT
4 Unregulated DC Input +Vin = 10 V to 40 V +Vin 1 LM2576-ADJ Output 3 C1 100 mF /50 V ON/OFF GN D 5 2 ON/OFF Feedback
L1 150 mH R2 10 k C2 1000 mF /16 V
Regulated Output Filtered Vout2 = 8.0 V @ 3.0 A
D1 1N5822
R1 1.8 k
V
C1 C2 D1 L1 R1 R2 - - - - - - 100 mF, 50 V, Aluminium Electrolytic 1000 mF, 16 V, Aluminium Electrolytic 3.0 A, 40 V, Schottky Rectifier, 1N5822 150 mH, RL2444, Renco Electronics 1.8 kW, 0.25 W 10 kW, 0.25 W
R2 out + V ref ) 1.0 ) R1
Vref = 1.23 V R1 is between 1.0 k and 5.0 k
Figure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step-Down Converter Using the LM2576-ADJ
LM2576
U1 D1 R1 R2 ON/OFF C1 + C2 Vout +Vin L1 +
GNDin NOTE: Not to scale.
GNDout NOTE: Not to scale.
Figure 39. Printed Circuit Board Layout Component Side References
00059_00
Figure 40. Printed Circuit Board Layout Copper Side
* * * *
National Semiconductor LM2576 Data Sheet and Application Note National Semiconductor LM2595 Data Sheet and Application Note Marty Brown "Practical Switching Power Supply Design", Academic Press, Inc., San Diego 1990 Ray Ridley "High Frequency Magnetics Design", Ridley Engineering, Inc. 1995
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23
LM2576
ORDERING INFORMATION
Device LM2576TV-ADJ LM2576TV-ADJG LM2576T-ADJ LM2576D2T-ADJ LM2576D2T-ADJR4 LM2576TV-3.3 LM2576T-3.3 LM2576D2T-3.3 LM2576D2TR4-3.3 LM2576D2TR4-3.3G LM2576TV-5 LM2576T-5 LM2576D2T-5 LM2576D2T-5G LM2576D2TR4-5 LM2576TV-12 LM2576TV-12G LM2576T-12 LM2576D2T-12 LM2576D2TR4-12 LM2576TV-15 LM2576T-15 LM2576D2T-15 15 V TJ = -40 to +125C 12 V TJ = -40 to +125C 40 t 125C 5.0 V TJ = -40 to +125C 3.3 V TJ = -40 to +125C 1.23 t 1 23 V to 37 V TJ = -40 to +125C 40 t 125C Nominal Output Voltage Operating Temperature Range Package TO-220 (Vertical Mount) TO-220 (Vertical Mount) (Pb-Free) TO-220 (Straight Lead) D2PAK (Surface Mount) D2PAK (Surface Mount) TO-220 (Vertical Mount) TO-220 (Straight Lead) D2PAK (Surface Mount) D2PAK (Surface Mount) D2PAK (Surface Mount) (Pb-Free) 2500 Tape & Reel 2500 Tape & Reel 50 Units/Rail 50 Units/Rail 2500 Tape & Reel Shipping 50 Units/Rail 50 Units/Rail
TO-220 (Vertical Mount) TO-220 (Straight Lead) D2PAK (Surface Mount) D2PAK (Surface Mount) (Pb-Free) D2PAK (Surface Mount) TO-220 (Vertical Mount) TO-220 (Vertical Mount) (Pb-Free) TO-220 (Straight Lead) D2PAK (Surface Mount) D2PAK (Surface Mount) TO-220 (Vertical Mount) TO-220 (Straight Lead) D2PAK (Surface Mount) 50 Units/Rail 50 Units/Rail 2500 Tape & Reel 50 Units/Rail 2500 Tape & Reel 50 Units/Rail 50 Units/Rail 50 Units/Rail
For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D.
MARKING DIAGRAMS
TO-220 TV SUFFIX CASE 314B TO-220 T SUFFIX CASE 314D D2PAK D2T SUFFIX CASE 936A
LM 2576T-xxx AWLYWW
LM 2576T-xxx AWLYWW
LM 2576-xxx AWLYWW
LM 2576D2T-xxx AWLYWW
1 1 5 1 5
5
1
5 xxx A WL Y WW = 3.3, 5.0, 12, 15, or ADJ = Assembly Location = Wafer Lot = Year = Work Week
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24
LM2576
PACKAGE DIMENSIONS
TO-220 TV SUFFIX CASE 314B-05 ISSUE J
Q
B -P-
C
OPTIONAL CHAMFER
E
U K F
A S L W V
NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: INCH. 3. DIMENSION D DOES NOT INCLUDE INTERCONNECT BAR (DAMBAR) PROTRUSION. DIMENSION D INCLUDING PROTRUSION SHALL NOT EXCEED 0.043 (1.092) MAXIMUM. DIM A B C D E F G H J K L N Q S U V W INCHES MIN MAX 0.572 0.613 0.390 0.415 0.170 0.180 0.025 0.038 0.048 0.055 0.850 0.935 0.067 BSC 0.166 BSC 0.015 0.025 0.900 1.100 0.320 0.365 0.320 BSC 0.140 0.153 --- 0.620 0.468 0.505 --- 0.735 0.090 0.110 MILLIMETERS MIN MAX 14.529 15.570 9.906 10.541 4.318 4.572 0.635 0.965 1.219 1.397 21.590 23.749 1.702 BSC 4.216 BSC 0.381 0.635 22.860 27.940 8.128 9.271 8.128 BSC 3.556 3.886 --- 15.748 11.888 12.827 --- 18.669 2.286 2.794
5X
J T H N -T-
SEATING PLANE
G
5X
0.24 (0.610)
M
D
M
0.10 (0.254)
TP
M
TO-220 T SUFFIX CASE 314D-04 ISSUE E
-T- -Q- B C E
SEATING PLANE
NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: INCH. 3. DIMENSION D DOES NOT INCLUDE INTERCONNECT BAR (DAMBAR) PROTRUSION. DIMENSION D INCLUDING PROTRUSION SHALL NOT EXCEED 10.92 (0.043) MAXIMUM. DIM A B C D E G H J K L Q U INCHES MIN MAX 0.572 0.613 0.390 0.415 0.170 0.180 0.025 0.038 0.048 0.055 0.067 BSC 0.087 0.112 0.015 0.025 0.990 1.045 0.320 0.365 0.140 0.153 0.105 0.117 MILLIMETERS MIN MAX 14.529 15.570 9.906 10.541 4.318 4.572 0.635 0.965 1.219 1.397 1.702 BSC 2.210 2.845 0.381 0.635 25.146 26.543 8.128 9.271 3.556 3.886 2.667 2.972
U K
12345
A L
G D
5 PL
J H
M
0.356 (0.014)
M
TQ
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25
LM2576
PACKAGE DIMENSIONS
D2PAK D2T SUFFIX CASE 936A-02 ISSUE B
-T- A K B
12345 OPTIONAL CHAMFER
TERMINAL 6
E
U V
S H M L
NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: INCH. 3. TAB CONTOUR OPTIONAL WITHIN DIMENSIONS A AND K. 4. DIMENSIONS U AND V ESTABLISH A MINIMUM MOUNTING SURFACE FOR TERMINAL 6. 5. DIMENSIONS A AND B DO NOT INCLUDE MOLD FLASH OR GATE PROTRUSIONS. MOLD FLASH AND GATE PROTRUSIONS NOT TO EXCEED 0.025 (0.635) MAXIMUM. INCHES MIN MAX 0.386 0.403 0.356 0.368 0.170 0.180 0.026 0.036 0.045 0.055 0.067 BSC 0.539 0.579 0.050 REF 0.000 0.010 0.088 0.102 0.018 0.026 0.058 0.078 5 _ REF 0.116 REF 0.200 MIN 0.250 MIN MILLIMETERS MIN MAX 9.804 10.236 9.042 9.347 4.318 4.572 0.660 0.914 1.143 1.397 1.702 BSC 13.691 14.707 1.270 REF 0.000 0.254 2.235 2.591 0.457 0.660 1.473 1.981 5 _ REF 2.946 REF 5.080 MIN 6.350 MIN
D 0.010 (0.254)
M
T
N G R
P
C
DIM A B C D E G H K L M N P R S U V
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LM2576/D

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