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| Order this document by LM2575/D LM2575 Advance Information Easy SwitcherTM 1.0 A Step-Down Voltage Regulator The LM2575 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 1.0 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 optimised for use with the LM2575 are offered by several different inductor manufacturers. Since the LM2575 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 by the LM2575 regulator is so low, that no heatsink is required or its size could be reduced dramatically. The LM2575 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 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 EASY SWITCHERTM 1.0 A STEP-DOWN VOLTAGE REGULATOR SEMICONDUCTOR TECHNICAL DATA T SUFFIX PLASTIC PACKAGE CASE 314D 1 5 Pin 1. 2. 3. 4. 5. Vin Output Ground Feedback ON/OFF TV SUFFIX PLASTIC PACKAGE CASE 314B Heatsink surface connected to Pin 3. 1 5 * * * * * * * * * * * * * * * * 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions Adjustable Version Output Voltage Range of 1.23 V to 37 V 4% Maximum Over Line and Load Conditions Guaranteed 1.0 A Output Current Wide Input Voltage Range: 4.75 V to 40 V 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 D2T SUFFIX PLASTIC PACKAGE CASE 936A (D2PAK) 1 5 Heatsink surface (shown as terminal 6 in case outline drawing) is connected to Pin 3. DEVICE TYPE/NOMINAL OUTPUT VOLTAGE LM2575-3.3 LM2575-5 LM2575-12 LM2575-15 LM2575-Adj 3.3 V 5.0 V 12 V 15 V 1.23 V to 37 V Applications Simple and High-Efficiency Step-Down (Buck) Regulators Efficient Pre-Regulator for Linear Regulators On-Card Switching Regulators Positive to Negative Converters (Buck-Boost) Negative Step-Up Converters Power Supply for Battery Chargers ORDERING INFORMATION Device LM2575T-** LM2575TV-** LM2575D2T-** Operating Temperature Range Package Straight Lead TJ = -40 to +125C Vertical Mount Surface Mount ** = Voltage Option, ie. 3.3, 5.0, 12, 15 V and ** =\Adjustable Output. This document contains information on a new product. Specifications and information herein are subject to change without notice. (c) Motorola, Inc. 1997 Rev 1 MOTOROLA ANALOG IC DEVICE DATA 1 LM2575 Figure 1. Block Diagram and Typical Application Typical Application (Fixed Output Voltage Versions) Feedback 7.0 V - 40 V Unregulated DC Input +Vin Cin 100 F 1 3 Gnd 5 LM2575 4 Output 2 ON/OFF L1 330 H D1 1N5819 5.0 V Regulated Output 1.0 A Load Cout 330 F 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 R2 () 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. ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.) 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 (Figure 34) Thermal Resistance, Junction-to-Case Storage Temperature Range Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 k) Lead Temperature (Soldering, 10 s) Maximum Junction Temperature NOTE: ESD data available upon request. Symbol Vin - - PD RJA RJC PD RJA RJC Tstg - - TJ Value 45 -0.3 V V +Vin -1.0 Internally Limited 65 5.0 Internally Limited 70 5.0 -65 to +150 3.0 260 150 Unit V V V W C/W C/W W C/W C/W C kV C C 2 MOTOROLA ANALOG IC DEVICE DATA LM2575 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 14) 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 = 200 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 LM2575-3.3 ([Note 1] Test Circuit Figure 14) Output Voltage (Vin = 12 V, ILoad = 0.2 A, TJ = 25C) Output Voltage (4.75 V Vin 40 V, 0.2 A ILoad 1.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 1.0 A) LM2575-5 ([Note 1] Test Circuit Figure 14) Output Voltage (Vin = 12 V, ILoad = 0.2 A, TJ = 25C) Output Voltage (8.0 V Vin 40 V, 0.2 A ILoad 1.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 1.0 A) LM2575-12 ([Note 1] Test Circuit Figure 14) Output Voltage (Vin = 25 V, ILoad = 0.2 A, TJ = 25C) Output Voltage (15 V Vin 40 V, 0.2 A ILoad 1.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 15V, ILoad = 1.0 A) LM2575-15 ([Note 1] Test Circuit Figure 14) Output Voltage (Vin = 30 V, ILoad = 0.2 A, TJ = 25C) Output Voltage (18 V Vin 40 V, 0.2 A ILoad 1.0 A) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 18 V, ILoad = 1.0 A) LM2575 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 14) Feedback Voltage (Vin = 12 V, ILoad = 0.2 A, Vout = 5.0 V, TJ = 25C) Feedback Voltage (8.0 V Vin 40 V, 0.2 A ILoad 1.0 A, Vout = 5.0 V) TJ = 25C TJ = -40 to +125C Efficiency (Vin = 12 V, ILoad = 1.0 A, Vout = 5.0 V) VFB VFB 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 NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2575 is used as shown in the Figure 14 test circuit, system performance will be as shown in system parameters section. 2. Tested junction temperature range for the LM2575: Tlow = -40C Thigh = +125C MOTOROLA ANALOG IC DEVICE DATA 3 LM2575 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 = 200 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 = 1.0 A [Note 4]) TJ = 25C TJ = -40 to +125C Max Duty Cycle ("on") [Note 5] Current Limit (Peak Current [Notes 4 and 3]) 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 14) 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 14) ON/OFF Pin = 5.0 V ("off"), TJ = 25C ON/OFF Pin = 0 V ("on"), TJ = 25C Ib - - fosc - 47 42 Vsat - - DC ICL 1.7 1.4 IL - - IQ - - Istby - - VIH 2.2 2.4 VIL - - IIH IIL - - 1.2 - 15 0 1.0 0.8 A 30 5.0 1.4 - - - 80 - 200 400 V 5.0 - 9.0 11 A 0.8 6.0 2.0 20 mA 2.3 - 3.0 3.2 mA 94 1.0 - 98 1.2 1.3 - % A 52 - - - 58 63 V 25 - 100 200 kHz nA Symbol Min Typ Max Unit NOTES: 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. 4 MOTOROLA ANALOG IC DEVICE DATA LM2575 TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 14) Figure 2. Normalized Output Voltage 0.6 Vout , OUTPUT VOLTAGE CHANGE (%) Vout , OUTPUT VOLTAGE CHANGE (%) 0.4 0.2 0 -0.2 -0.4 -0.6 -50 Vin = 20 V ILoad = 200 mA Normalized at TJ = 25C 1.0 0.8 0.6 0.4 0.2 0 -0.2 12 V and 15 V ILoad = 200 mA TJ = 25C 3.3 V, 5.0 V and Adj Figure 3. Line Regulation -25 0 25 50 75 100 125 0 5.0 10 15 20 25 30 35 40 TJ, JUNCTION TEMPERATURE (C) Vin, INPUT VOLTAGE (V) Figure 4. Switch Saturation Voltage 1.2 Vsat , SATURATION VOLTAGE (V) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0 125C 25C -40C IO , OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 Figure 5. Current Limit Vin = 25 V 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 -50 -25 0 25 50 75 100 125 SWITCH CURRENT (A) TJ, JUNCTION TEMPERATURE (C) Figure 6. Dropout Voltage 2.0 INPUT-OUTPUT DIFFERENTIAL (V) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 -50 -25 0 25 50 75 100 125 ILoad = 200 mA ILoad = 1.0 A IQ , QUIESCENT CURRENT (mA) Vout = 5% Rind = 0.2 20 18 16 14 12 10 8.0 6.0 4.0 0 5.0 Figure 7. Quiescent Current Vout = 5.0 V Measured at Ground Pin TJ = 25C ILoad = 1.0 A ILoad = 200 mA 10 15 20 25 30 35 40 TJ, JUNCTION TEMPERATURE (C) Vin, INPUT VOLTAGE (V) MOTOROLA ANALOG IC DEVICE DATA 5 LM2575 Figure 8. Standby Quiescent Current Istby , STANDBY QUIESCENT CURRENT ( A) 120 100 80 60 40 20 0 0 5.0 10 15 20 25 30 35 40 Vin, INPUT VOLTAGE (V) Istby , STANDBY QUIESCENT CURRENT ( A) TJ = 25C 120 100 80 60 40 20 0 -50 Figure 9. Standby Quiescent Current Vin = 12 V VON/OFF = 5.0 V -25 0 25 50 75 100 125 TJ, JUNCTION TEMPERATURE (C) Figure 10. Oscillator Frequency 2.0 0 -2.0 -4.0 -6.0 -8.0 -10 -50 IFB , FEEDBACK PIN CURRENT (nA) NORMALIZED FREQUENCY (%) Vin = 12 V Normalized at 25C 40 Figure 11. Feedback Pin Current Adjustable Version Only 20 0 -20 -25 0 25 50 75 100 125 -40 -50 -25 0 25 50 75 100 125 TJ, JUNCTION TEMPERATURE (C) TJ, JUNCTION TEMPERATURE (C) Figure 12. Switching Waveforms OUTPUT VOLTAGE (PIN 2) I Load, LOAD CURRENT (A) Vout , OUTPUT VOLTAGE CHANGE (mV) Figure 13. Load Transient Response 100 0 10 V 0 OUTPUT 1.0 A CURRENT (PIN 2) 0 INDUCTOR CURRENT -100 1.0 A 0.5 A 1.0 0.5 0 OUTPUT 20 mV RIPPLE /DIV VOLTAGE 5.0 s/DIV 100 s/DIV 6 MOTOROLA ANALOG IC DEVICE DATA LM2575 Figure 14. Typical Test Circuit 5.0 Output Voltage Versions Feedback Vin + 1 LM2575-5 Output 3 Cin 100 F/50 V Gnd 5 2 ON/OFF D1 1N5819 Cout 330 F /16 V Load 4 L1 330 H Vout Regulated Output Vin Unregulated DC Input 8.0 V - 40 V - Adjustable Output Voltage Versions Feedback Vin + 1 4 LM2575 Adjustable Output 2 ON/OFF D1 1N5819 Cout 330 F /16 V R2 Load R1 L1 330 H Vout Regulated Output 3 Unregulated DC Input 8.0 V - 40 V Cin 100 F/50 V Gnd 5 - V out + Vref 1 ) R2 R1 V out V ref -1 R2 + R1 Where Vref = 1.23 V, R1 between 1.0 k and 5.0 k 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 14, 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 LM2575 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 LM2575 regulator. MOTOROLA ANALOG IC DEVICE DATA 7 LM2575 PIN FUNCTION DESCRIPTION Pin 1 Vin Symbol Description (Refer to Figure 1) This pin is the positive input supply for the LM2575 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 1.0 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 LM2575 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 A. The input 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 connected to ground, the regulator will be in the "on" condition. 2 Output 3 4 Gnd Feedback 5 ON/OFF DESIGN PROCEDURE Buck Converter Basics The LM2575 is a "Buck" or Step-Down Converter which is the most elementary forward-mode converter. Its basic schematic can be seen in Figure 15. 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: - V out t on I L(on) L 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. in Figure 15. Basic Buck Converter Diode Voltage Power Switch L Vout Power Switch Off Power Switch Off Power Switch On + V V out - V t D off L(off) 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 on d , where T is the period of switching. T For the buck converter with ideal components, the duty cycle can also be described as: V out d V in Figure 16 shows the buck converter idealized waveforms of the catch diode voltage and the inductor current. I + + + Figure 16. Buck Converter Idealized Waveforms Von(SW) Vin D1 Cout RLoad VD(FWD) Power Switch On 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 catch dioded. 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: Inductor Current Ipk ILoad(AV) Imin Diode Power Switch Diode Power Switch Time 8 MOTOROLA ANALOG IC DEVICE DATA LM2575 Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step-by-step design procedure and example is provided. Procedure Given Parameters: Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V) Vin(max) = Maximum DC 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 LM2575 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 17 to 21. 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 1 or Table 2. When using Table 2 for selecting the right inductor 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: V -V out t on in I +I p(max) Load(max) 2L Given Parameters: Vout = 5.0 V Vin(max) = 20 V ILoad(max) = 0.8 A 1. Controller IC Selection According to the required input voltage, output voltage, current polarity and current value, use the LM2575-5 controller IC 2. Input Capacitor Selection (Cin) A 47 F, 25 V aluminium electrolytic capacitor located near to the input and ground pins provides sufficient bypassing. Example 3. Catch Diode Selection (D1) A. For this example the current rating of the diode is 1.0 A. B. Use a 30 V 1N5818 Schottky diode, or any of the suggested fast recovery diodes shown in the Table 4. 4. Inductor Selection (L1) A. Use the inductor selection guide shown in Figures 17 to 21. B. From the selection guide, the inductance area intersected by the 20 V line and 0.8 A line is L330. C. Inductor value required is 330 H. From the Table 1 or Table 2, choose an inductor from any of the listed manufacturers. ) where ton is the "on" time of the power switch and V ton + out x 1 fosc V in For additional information about the inductor, see the inductor section in the "External Components" section of this data sheet. MOTOROLA ANALOG IC DEVICE DATA 9 LM2575 Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step-by-step design procedure and example is provided. Procedure 5. Output Capacitor Selection (Cout) A. Since the LM2575 is a forward-mode switching regulator with voltage mode control, its open loop 2-pole-2-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 100 F and 470 F 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 8V is appropriate, and a 10 V or 16 V rating is recommended. Example 5. Output Capacitor Selection (Cout) A. Cout = 100 F to 470 F standard aluminium electrolytic. B. Capacitor voltage rating = 16 V. Procedure (Adjustable Output Version: LM2575-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 14) use the following formula: V out Given Parameters: Vout = 8.0 V Vin(max) = 12 V ILoad(max) = 1.0 A 1. Programming Output Voltage (selecting R1 and R2) Select R1 and R2: V out where Vref = 1.23 V R2 Example + Vref 1 ) R2 R1 + + 1.23 1 ) R2 R1 V out V ref Select R1 = 1.8 k Resistor R1 can be between 1.0 k and 5.0 k. (For best temperature coefficient and stability with time, use 1% metal film resistors). V out R2 R1 -1 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 "External Components" 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 LM2575 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. + R1 8.0 * 1 + 1.8 k 1.23 VV * 1 R2 = 9.91 k, choose a 9.88 k metal film resistor. 2. Input Capacitor Selection (Cin) A 100 F 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 20 V 1N5820 or MBR320 Schottky diode or any suggested fast recovery diode in the Table 4. 10 MOTOROLA ANALOG IC DEVICE DATA LM2575 Procedure (Adjustable Output Version: LM2575-Adj) (continued) Procedure 4. Inductor Selection (L1) A. Use the following formula to calculate the inductor Volt x microsecond [V x s] constant: V out 6 ExT V - V out x 10 [V x ms] in V on F[Hz] Example 4. Inductor Selection (L1) A. Calculate E x T [V x s] constant: + ExT + (12 - 8.0) x 8.0 12 x 1000 52 + 51 [V x ms] 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 21. 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 21. D. From the inductor code, identify the inductor value. Then select an appropriate inductor from the Table 1 or Table 2. The inductor chosen must be rated for a switching frequency of 52 kHz and for a current rating of 1.15 x IIoad. The inductor current rating can also be determined by calculating the inductor peak current: I p(max) B. E x T = 51 [V x s] C. ILoad(max) = 1.0 A Inductance Region = L220 D. Proper inductor value = 220 H Choose the inductor from the Table 1 or Table 2. + ILoad(max) ) t on V in - V out ton 2L where ton is the "on" time of the power switch and osc in For additional information about the inductor, see the inductor section in the "External Components" section of this data sheet. 5. Output Capacitor Selection (Cout) A. Since the LM2575 is a forward-mode switching regulator with voltage mode control, its open loop 2-pole-2-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) Cout 7.785 [F] V out x L [H] 5. Output Capacitor Selection (Cout) A. 53 F Cout 7.785 12 8.220 + VVout x f 1 w + To achieve an acceptable ripple voltage, select Cout = 100 F electrolytic capacitor. w B. Capacitor values between 10 F and 2000 F 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 8V is appropriate, and a 10 V or 16 V rating is recommended. MOTOROLA ANALOG IC DEVICE DATA 11 LM2575 INDUCTOR VALUE SELECTION GUIDE Figure 17. LM2575-3.3 60 Vin , MAXIMUM INPUT VOLTAGE (V) 20 15 10 8.0 7.0 6.0 Vin , MAXIMUM INPUT VOLTAGE (V) H1000 L680 L470 L330 L220 L150 L100 5.0 0.2 60 40 25 20 15 12 10 9.0 L220 8.0 L150 7.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 H1500 H1000 L680 L470 L330 Figure 18. LM2575-5.0 0.3 0.4 0.5 0.6 0.8 1.0 IL, MAXIMUM LOAD CURRENT (A) IL, MAXIMUM LOAD CURRENT (A) Figure 19. LM2575-12 60 Vin , MAXIMUM INPUT VOLTAGE (V) 40 30 25 20 18 17 16 15 14 0.2 L680 L470 L330 L220 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 H2200 Vin , MAXIMUM INPUT VOLTAGE (V) H1500 H1000 H680 H470 60 40 35 30 25 22 20 19 18 17 0.2 H2200 Figure 20. LM2575-15 H1500 H1000 H680 H470 L680 L470 L330 L220 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 IL, MAXIMUM LOAD CURRENT (A) IL, MAXIMUM LOAD CURRENT (A) Figure 21. LM2575-Adj 200 150 125 100 80 70 60 50 40 30 20 0.2 H2200 H1500 H1000 H680 H470 ET, VOLTAGE TIME (V s) L680 L470 L330 L220 L150 L100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 IL, MAXIMUM LOAD CURRENT (A) NOTE: This Inductor Value Selection Guide is applicable for continuous mode only. 12 MOTOROLA ANALOG IC DEVICE DATA LM2575 Table 1. Inductor Selection Guide Inductor Code L100 L150 L220 L330 L470 L680 H150 H220 H330 H470 H680 H1000 H1500 H2200 Inductor Value 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 Pulse Eng 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 RL2444 RL1954 RL1953 RL1952 RL1951 RL1950 RL2445 RL2446 RL2447 RL1961 RL1960 RL1959 RL1958 RL2448 AIE 415-0930 415-0953 415-0922 415-0926 415-0927 415-0928 415-0936 430-0636 430-0635 430-0634 415-0935 415-0934 415-0933 415-0945 Tech 39 77 308 BV 77 358 BV 77 408 BV 77 458 BV - 77 508 BV 77 368 BV 77 410 BV 77 460 BV - 77 510 BV 77 558 BV - 77 610 BV Table 2. Inductor Selection Guide Inductance (H) Current (A) 0.32 0.58 68 0.99 1.78 0.48 100 0.82 1.47 0.39 150 0.66 1.20 0.32 220 0.55 1.00 0.42 330 0.80 NOTE: Schott THT 67143940 67143990 67144070 67144140 67143980 67144060 67144130 - 67144050 67144120 67143960 67144040 67144110 67144030 67144100 SMT 67144310 67144360 67144450 67144520 67144350 67144440 67144510 67144340 67144430 67144500 67144330 67144420 67144490 67144410 67144480 THT Renco SMT RL1500-68 RL1500-68 RL1500-68 - RL1500-100 RL1500-100 - RL1500-150 RL1500-150 - RL1500-220 RL1500-220 - RL1500-330 - Pulse Engineering THT PE-53804 PE-53812 PE-53821 PE-53830 PE-53811 PE-53820 PE-53829 PE-53810 PE-53819 PE-53828 PE-53809 PE-53818 PE-53827 PE-53817 PE-53826 SMT PE-53804-S PE-53812-S PE-53821-S PE-53830-S PE-53811-S PE-53820-S PE-53829-S PE-53810-S PE-53819-S PE-53828-S PE-53809-S PE-53818-S PE-53827-S PE-53817-S PE-53826-S Coilcraft SMT DO1608-68 DO3308-683 DO3316-683 DO5022P-683 DO3308-104 DO3316-104 DO5022P-104 DO3308-154 DO3316-154 DO5022P-154 DO3308-224 DO3316-224 DO5022P-224 DO3316-334 DO5022P-334 RL-1284-68-43 RL-5470-6 RL-5471-5 RL-5471-5 RL-5470-5 RL-5471-4 RL-5471-4 RL-5470-4 RL-5471-3 RL-5471-3 RL-5470-3 RL-5471-2 RL-5471-2 RL-5471-1 RL-5471-1 Table 1 and Table 2 of this Indicator Selection Guide shows some examples of different manufacturer products suitable for design with the LM2575. MOTOROLA ANALOG IC DEVICE DATA 13 LM2575 Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers Pulse Engineering Inc. Pulse Engineering Inc. Europe Renco Electronics Inc. AIE Magnetics Coilcraft Inc. Coilcraft Inc., Europe Tech 39 Schott Corp. Phone Fax Phone Fax Phone Fax Phone Fax Phone Fax Phone Fax Phone Fax Phone Fax + 1-619-674-8100 + 1-619-674-8262 + 353 93 24 107 + 353 93 24 459 + 1-516-645-5828 + 1-516-586-5562 + 1-813-347-2181 + 1-708-322-2645 + 1-708-639-1469 + 44 1236 730 595 + 44 1236 730 627 + 33 8425 2626 + 33 8425 2610 + 1-612-475-1173 + 1-612-475-1786 Table 4. Diode Selection Guide gives an overview about both surface-mount and through-hole diodes for an effective design. Device listed in bold are available from Motorola. Schottky 1.0 A VR 20 V SMT SK12 THT 1N5817 SR102 1N5818 SR103 11DQ03 1N5819 SR104 11DQ04 MBR150 SR105 11DQ05 SMT SK32 MBRD320 SK33 MBRD330 3.0 A THT 1N5820 MBR320 SR302 1N5821 MBR330 SR303 31DQ03 1N5822 MBR340 SR304 31DQ04 MBR350 SR305 11DQ05 MURS320T3 MURS120T3 MUR120 11DF1 HER102 MURD320 MUR320 30WF10 MUR420 SMT 1.0 A THT SMT Ultra-Fast Recovery 3.0 A THT 30 V MBRS130LT3 SK13 40 V MBRS140T3 SK14 10BQ040 10MQ040 MBRS150 10BQ050 MBRS340T3 MBRD340 30WQ04 SK34 MBRD350 SK35 30WQ05 10BF10 50 V 31DF1 HER302 14 MOTOROLA ANALOG IC DEVICE DATA LM2575 EXTERNAL COMPONENTS Input Capacitor (Cin) The Input Capacitor Should Have a Low ESR 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. At Low Temperatures, Put in Parallel Aluminium Electrolytic Capacitors with Tantalum Capacitors 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. Catch Diode Locate the Catch Diode Close to the LM2575 The LM2575 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 LM2575 using short leads and short printed circuit traces to avoid EMI problems. 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 above 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 where d is the duty cycle, for a buck regulator V out t on d T V in |V out| t on and d for a buck T |V out| V in ++ ++ ) *boost regulator. Output Capacitor (Cout) 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. Use a Schottky or a Soft Switching Ultra-Fast Recovery Diode Since the rectifier diodes are very significant source 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 a quality, low noise design requirements. Table 4 provides a list of suitable diodes for the LM2575 regulator. Standard 50/60 Hz rectifier diodes such as the 1N4001 series or 1N5400 series are NOT suitable. Inductor 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. 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 ), 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. MOTOROLA ANALOG IC DEVICE DATA Continuous and Discontinuous Mode of Operation The LM2575 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 15 LM2575 conditions, the circuit will be forced to the discontinuous mode when inductor current falls to zero for certain period of time (see Figure 22 and Figure 23). 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 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 LM2575 regulator was added to this data sheet (Figures 17 through 21). 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 200 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. Figure 22. Continuous Mode Switching Current Waveforms POWER SWITCH CURRENT (A) 1.0 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 completely 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 completely contained within the core. 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 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 LM2575 internal switch into cycle-by-cycle current limit, thus reducing the dc output load current. This can also result in overheating of the inductor and/or the LM2575. Different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. Figure 23. Discontinuous Mode Switching Current Waveforms POWER SWITCH CURRENT (A) INDUCTOR CURRENT (A) 0 INDUCTOR CURRENT (A) 1.0 0 HORTIZONTAL TIME BASE: 5.0 s/DIV 0.1 0 Selecting the Right Inductor Style 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, 0.1 0 HORTIZONTAL TIME BASE: 5.0 s/DIV 16 MOTOROLA ANALOG IC DEVICE DATA LM2575 GENERAL RECOMMENDATIONS Output Voltage Ripple and Transients Source of the Output Ripple Since the LM2575 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. a heatsink for ambient temperatures up to approximately 50C (depending on the output voltage and load current). Higher ambient temperatures require some heatsinking, either to the printed circuit (PC) board or an external heatsink. 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 24). 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 minimise 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. Figure 24. Output Ripple Voltage Waveforms Voltage spikes caused by switching action of the output switch and the parasitic inductance of the output capacitor The Surface Mount Package D 2PAK and its Heatsinking 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 100 mm2) and ideally should have 2 or more square inches (1300 mm2) of 0.0028 inch copper. Additional increasing of copper area beyond approximately 3.0 in2 (2000 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. Thermal Analysis and Design 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. maximum ambient temperature in the 2. TA(max) application. maximum allowed junction temperature 3. TJ(max) (125C for the LM2575). 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. package thermal resistance junction-case. 4. RJC 5. RJA package thermal resistance junction-ambient. (Refer to Absolute Maximum Ratings in this data sheet or RJC and RJA values). The following formula is to calculate the total power dissipated by the LM2575: PD = (Vin x IQ) + d x ILoad x Vsat where d is the duty cycle and for buck converter V t on O, d T V in IQ (quiescent current) and Vsat can be found in the LM2575 data sheet, Vin is minimum input voltage applied, VO is the regulator output voltage, ILoad is the load current. UNFILITERED OUTPUT VOLTAGE VERTICAL RESOLUTION: 20 mV/DIV FILITERED OUTPUT VOLTAGE HORTIZONTAL TIME BASE: 10 s/DIV Minimizing the Output Ripple In order to minimise 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 H, 100 F), that can be added to the output (see Figure 33) 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 24 shows the difference between filtered and unfiltered output waveforms of the regulator shown in Figure 33. The upper waveform is from the normal unfiltered output of the converter, while the lower waveform shows the output ripple voltage filtered by an additional LC filter. Heatsinking and Thermal Considerations The Through-Hole Package TO-220 The LM2575 is available in two packages, a 5-pin TO-220(T, TV) and a 5-pin surface mount D2PAK(D2T). There are many applications that require no heatsink to keep the LM2575 junction temperature within the allowed operating range. The TO-220 package can be used without MOTOROLA ANALOG IC DEVICE DATA ++ 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) For a free-standing application when no heatsink is used, the junction temperature can be determined by the following expression: TJ = (RJA) (PD) + TA where (RJA)(PD) represents the junction temperature rise caused by the dissipated power and TA is the maximum ambient temperature. 17 LM2575 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 (RJA + RCS + RSA) + TA where RJC is the thermal resistance junction-case, RCS is the thermal resistance case-heatsink, RSA is the thermal resistance heatsink-ambient. 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. This circuit configuration is able to deliver approximately 0.35 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 startup input current, even for light loads. This may overload an input power source with a current limit less than 1.5 A. Such an amount of input startup 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 startup currents required by this inverting regulator topology, the use of a delayed startup or an undervoltage lockout circuit is recommended. Using a delayed startup 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 startup is now partially supplied by the input capacitor Cin. 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 colour of the traces. The size, quantity and spacing of other components on the board can also influence its effectiveness to dissipate the heat. Figure 25. Inverting Buck-Boost Regulator Using the LM2575-12 Develops -12 V @ 0.35 A Unregulated DC Input 12 V to 25 V Cin 100 F /50 V Feedback +Vin 1 3 Gnd 5 4 LM2575-12 Output 2 ON/OFF L1 100 H D1 1N5819 Cout 1800 F /16 V Regulated Output -12 V @ 0.35 A ADDITIONAL APPLICATIONS Inverting Regulator An inverting buck-boost regulator using the LM2575-12 is shown in Figure 25. 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 LM2575-12 is used to generate a -12 V output. The maximum input voltage in this case Design Recommendations: 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 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 F). The recommended range of inductor values for the inverting converter design is between 68 H and 220 H. 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: I (V |V |) V x t on Load in O in I peak V 2L 1 in |V | O where t on x 1 , and f osc 52 kHz. V |V | f osc in O Under normal continuous inductor current operating conditions, the worst case occurs when Vin is minimal. Note that the voltage appearing across the regulator is the absolute sum of the input and output voltage, and must not exceed 40 V. [ ) ) + ) + 18 MOTOROLA ANALOG IC DEVICE DATA LM2575 Figure 26. Inverting Buck-Boost Regulator with Delayed Startup Unregulated DC Input 12 V to 25 V Cin C1 100 F /50 V 0.1 F Feedback +Vin 1 5 R1 47 k ON/OFF 3 LM2575-12 L1 4 100 H Output 2 Gnd D1 1N5819 R2 47 k Cout 1800 F /16 V +Vin Cin 100 F Q1 2N3906 5 Figure 28. Inverting Buck-Boost Regulator Shut Down Circuit Using a PNP Transistor +V 0 On R2 5.6 k +Vin 1 LM2575-XX Off Shutdown Input Regulated Output -12 V @ 0.35 A ON/OFF 3 R1 12 k Gnd It has been already mentioned above, that in some situations, the delayed startup or the undervoltage lockout features could be very useful. A delayed startup circuit applied to a buck-boost converter is shown in Figure 26. Figure 32 in the "Undervoltage Lockout" section describes an undervoltage lockout feature for the same converter topology. Figure 27. Inverting Buck-Boost Regulator Shut Down Circuit Using an Optocoupler +Vin +Vin 1 Cin R1 100 F 47 k LM2575-XX -Vout NOTE: This picture does not show the complete circuit. Negative Boost Regulator This example is a variation of the buck-boost topology and is called a 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 29 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. Figure 29. Negative Boost Regulator 5.0 V 0 On Shutdown Input Off R3 470 5 ON/OFF 3 Gnd R2 47 k -Vout MOC8101 Cin 100 F /50 V +Vin 1 3 Gnd 5 LM2575-12 4 Feedback Output 2 ON/OFF D1 1N5817 Cout 1000 F /16 V NOTE: This picture does not show the complete circuit. 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 (1.4 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 27 and 28. Regulated Output Vout = -12 V L1 Unregulated DC Input -Vin = -5.0 V to -12 V 150 H Load Current from 200 mA for Vin = -5.2 V to 500 mA for Vin = -7.0 V MOTOROLA ANALOG IC DEVICE DATA 19 LM2575 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 standard buck converter. Low input voltages or high output currents require a large value output capacitor (in the range of thousands of F). The recommended range of inductor values for the negative boost regulator is the same as for inverting converter design. 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 Startup There are some applications, like the inverting regulator already mentioned above, which require a higher amount of startup current. In such cases, if the input power source is limited, this delayed startup feature becomes very useful. To provide a time delay between the time the input voltage is applied and the time when the output voltage comes up, the circuit in Figure 30 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 1.4 V, the regulator starts up. Resistor R1 is included to limit the maximum voltage applied to the ON/OFF pin, 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. Figure 30. Delayed Startup Circuitry shown in Figure 32. Resistor R3 pulls the ON/OFF pin high and keeps the regulator off until the input voltage reaches a predetermined threshold level, which is determined by the following expression: (Q1) V V 1 R2 V th Z1 BE R1 [ )) Figure 31. Undervoltage Lockout Circuit for Buck Converter +Vin +Vin 1 LM2575-5.0 R2 10 k R3 47 k Cin 100 F 5 ON/OFF 3 Gnd Z1 1N5242B Q1 2N3904 R1 10 k Vth 13 V NOTE: This picture does not show the complete circuit. Figure 32. Undervoltage Lockout Circuit for Buck-Boost Converter +Vin +Vin 1 LM2575-5.0 R2 15 k R3 68 k Cin 100 F 5 ON/OFF 3 Gnd +Vin +Vin 1 C1 0.1 F LM2575-XX Z1 1N5242B Gnd R1 15 k R2 47 k Q1 2N3904 Vth 13 V 5 ON/OFF 3 Cin 100 F R1 47 k Vout = -5.0 V This picture does not show the complete circuit. NOTE: NOTE: This picture does not show the complete circuit. Undervoltage Lockout Some applications require the regulator to remain off until the input voltage reaches a certain threshold level. Figure 31 shows an undervoltage lockout circuit applied to a buck regulator. A version of this circuit for buck-boost converter is Adjustable Output, Low-Ripple Power Supply A 1.0 A output current capability power supply that features an adjustable output voltage is shown in Figure 33. This regulator delivers 1.0 A into 1.2 V to 35 V output. The input voltage ranges from roughly 8.0 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. 20 MOTOROLA ANALOG IC DEVICE DATA LM2575 Figure 33. Adjustable Power Supply with Low Ripple Voltage Feedback Unregulated DC Input + +Vin 1 LM2575-Adj Output 3 Cin 100 F /50 V Gnd 5 2 ON/OFF 4 L1 150 H R2 50 k D1 1N5819 Cout 2200 F C1 100 F R1 1.1 k L2 20 H Regulated Output Voltage 1.2 V to 35 V @1.0 A Optional Output Ripple Filter Figure 34. D2PAK Thermal Resistance and Maximum Power Dissipation versus P.C.B. Copper Length PD(max) for TA = 50C 70 60 50 40 RJA 30 0 5.0 10 15 20 L, LENGTH OF COPPER (mm) Free Air Mounted Vertically 3.0 2.0 oz. Copper L Minimum Size Pad L 2.5 2.0 1.5 1.0 PD, MAXIMUM POWER DISSIPATION (W) 80 R JA, THERMAL RESISTANCE JUNCTION-TO-AIR (C/W) 3.5 MOTOROLA ANALOG IC DEVICE DATA IIII IIII IIII 25 30 21 LM2575 THE LM2575-5.0 STEP-DOWN VOLTAGE REGULATOR WITH 5.0 V @ 1.0 A OUTPUT POWER CAPABILITY. TYPICAL APPLICATION WITH THROUGH-HOLE PC BOARD LAYOUT Figure 35. Schematic Diagram of the LM2575-5.0 Step-Down Converter Feedback Unregulated DC Input +Vin = +7.0 V to +40 V +Vin LM2575-5.0 1 3 C1 100 F /50 V Gnd 5 Output 2 ON/OFF 4 L1 330 H Regulated Output +Vout1 = 5.0 V @ 1.0 A J1 D1 1N5819 Cout 330 F /16 V Gndout Gndin C1 C2 D1 L1 - - - - 100 F, 50 V, Aluminium Electrolytic 330 F, 16 V, Aluminium Electrolytic 1.0 A, 40 V, Schottky Rectifier, 1N5819 330 H, Tech 39: 77 458 BV, Toroid Core, Through-Hole, Pin 3 = Start, Pin 7 = Finish Figure 36. Printed Circuit Board Component Side Gndin C1 U1 LM2575 Gndout Figure 37. Printed Circuit Board Copper Side J1 L1 D1 C2 DC-DC Converter +Vin NOTE: Not to scale. +Vout1 NOTE: Not to scale. 22 MOTOROLA ANALOG IC DEVICE DATA LM2575 THE LM2575-ADJ STEP-DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER CAPABILITY. TYPICAL APPLICATION WITH THROUGH-HOLE PC BOARD LAYOUT Figure 38. Schematic Diagram of the 8.0 V @ 1.0 V Step-Down Converter Using the LM2575-Adj (An additional LC filter is included to achieve low output ripple voltage) Regulated Output Unfiltered Vout1 = 8.0 V @1.0 A 4 Unregulated DC Input +Vin = +10 V to + 40 V +Vin 1 LM2575-Adj Output 3 C1 100 F /50 V Gnd 5 2 ON/OFF Feedback L1 330 H L2 25 H Regulated Output Filtered Vout2 = 8.0 V @1.0 A R2 10 k D1 1N5819 C2 330 F /16 V R1 1.8 k C3 100 F /16 V V C1 C2 C3 D1 L1 L2 R1 R2 - - - - - - - - out + Vref ) 1 ) R2 R1 100 F, 50 V, Aluminium Electrolytic R1 is between 1.0 k and 5.0 k 330 F, 16 V, Aluminium Electrolytic 100 F, 16 V, Aluminium Electrolytic 1.0 A, 40 V, Schottky Rectifier, 1N5819 330 H, Tech 39: 77 458 BV, Toroid Core, Through-Hole, Pin 3 = Start, Pin 7 = Finish 25 H, TDK: SFT52501, Toroid Core, Through-Hole 1.8 k 10 k Vref = 1.23 V Figure 39. PC Board Component Side Gndin C1 L1 D1 J1 U1 LM2575 C2 Gndout C3 Figure 40. PC Board Copper Side L2 +Vin R2 R1 NOTE: Not to scale. +Vout2 +Vout1 MOTOROLA NOTE: Not to scale. References * * * * National Semiconductor LM2575 Data Sheet and Application Note National Semiconductor LM2595 Data Sheet and Application Note Marty Brown "Pratical Switching Power Supply Design", Academic Press, Inc., San Diego 1990 Ray Ridley "High Frequency Magnetics Design", Ridley Engineering, Inc. 1995 MOTOROLA ANALOG IC DEVICE DATA 23 LM2575 OUTLINE DIMENSIONS T SUFFIX PLASTIC PACKAGE CASE 314D-03 ISSUE D -T- -Q- U A L 12345 SEATING PLANE C B E 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. 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 1.020 1.065 0.320 0.365 0.140 0.153 0.105 0.117 0.543 0.582 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 25.908 27.051 8.128 9.271 3.556 3.886 2.667 2.972 13.792 14.783 K S G D 5 PL J H M DIM A B C D E G H J K L Q U S 0.356 (0.014) M TQ TV SUFFIX PLASTIC PACKAGE CASE 314B-05 ISSUE J Q B -P- C OPTIONAL CHAMFER E 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 U K F A S L W V 5X J T H N -T- SEATING PLANE G 5X 0.24 (0.610) M D M 0.10 (0.254) TP M 24 MOTOROLA ANALOG IC DEVICE DATA LM2575 OUTLINE DIMENSIONS D2T SUFFIX PLASTIC PACKAGE CASE 936A-02 (D2PAK) ISSUE A 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 -T- A K B 12345 OPTIONAL CHAMFER TERMINAL 6 E U V S H M L 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 MOTOROLA ANALOG IC DEVICE DATA 25 LM2575 Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. "Typical" parameters which may be provided in Motorola data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be validated for each customer application by customer's technical experts. Motorola does not convey any license under its patent rights nor the rights of others. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury or death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorola and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that Motorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal Opportunity/Affirmative Action Employer. Mfax is a trademark of Motorola, Inc. How to reach us: USA / EUROPE / Locations Not Listed: Motorola Literature Distribution; P.O. Box 5405, Denver, Colorado 80217. 303-675-2140 or 1-800-441-2447 JAPAN: Nippon Motorola Ltd.: SPD, Strategic Planning Office, 4-32-1, Nishi-Gotanda, Shinagawa-ku, Tokyo 141, Japan. 81-3-5487-8488 MfaxTM: RMFAX0@email.sps.mot.com - TOUCHTONE 602-244-6609 ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park, - US & Canada ONLY 1-800-774-1848 51 Ting Kok Road, Tai Po, N.T., Hong Kong. 852-26629298 INTERNET: http://motorola.com/sps 26 MOTOROLA ANALOG IC DEVICE DATA LM2575/D |
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