TYPICAL PERFORMANCE CHARACTERISTICS (continued)
(Circuit Figure 25 and Figure 26)
Feedback Pin Current
Figure 21.
Switching Waveforms
V OUT = 5V
A: Output Pin Voltage, 10V/div
B: Output Pin Current, 1A/div
C: Inductor Current, 0.5A/div
D: Output Ripple Voltage, 20 mV/div,
AC-Coupled
Horizontal Time Base: 5 μs/div
Figure 23.
Maximum Power Dissipation
(TO-263) (See (1))
Figure 22.
Load Transient Response
Figure 24
If the DDPAK/TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package: Using 0.5 square inches of copper area, θJA is 50°C/W; with 1 square inch of copper area, θJA is 37°C/W; and with 1.6 or more square inches of copper area, θJA is 32°C/W.
TEST CIRCUIT AND LAYOUT GUIDELINES
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance generate voltage transients which can cause problems. For minimal inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. Single-point grounding (as indicated) or ground plane construction should be used for best results. When using the Adjustable version, physically locate the programming resistors near the regulator, to keep the sensitive feedback wiring short.
CIN — 100 μF, 75V, Aluminum Electrolytic
COUT —
330 μF, 25V, Aluminum Electrolytic
D1 — Schottky, 11DQ06
L1 — 330 μH, PE-52627 (for 5V in, 3.3V out, use 100 μH, PE-92108)
Figure 25. Fixed Output Voltage Versions
where
VREF = 1.23V, R1 between 1k and 5k. R1 — 2k, 0.1% R2 — 6.12k, 0.1%
Pin numbers are for the TO-220 package.
Figure 26. Adjustable Output Voltage Version
LM2575 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions) Given: VOUT = 5V Given: VIN(Max) = Maximum Input Voltage V = Regulated Output Voltage (3.3V, 5V, 12V, or 15V) OUTILOAD(Max) = Maximum Load Current VIN(Max) = 20V ILOAD(Max) = 0.8A 1. Inductor Selection (L1) A. Use the selection guide shown in Figure 28. B. From the selection guide, the inductance area intersected by the 20V line and 0.8A line is L330. C. Inductor value required is 330 μH. From the table in Table 2, choose AIE 415-0926, Pulse Engineering PE-52627, or RL1952. 1. Inductor Selection (L1) A. Select the correct Inductor value selection guide from Figure 27, Figure 28, Figure 29 and Figure 30 (Output voltages of 3.3V, 5V, 12V or 15V respectively). For other output voltages, see the design procedure of Figure 26 . B. From the inductor value selection guide, identify the inductance region intersected by VIN(Max) and ILOAD(Max), and note theinductor code for that region. C. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Table 2. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2575 switching frequency (52 kHz) and for a current rating of 1.15 × ILOAD. For additional inductor information, see INDUCTOR SELECTION. 2. Output Capacitor Selection (COUT) A. The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation and an acceptable output ripple voltage, (approximately 1% of the output voltage) a value between 100 μF and 470 μF is recommended. B. The capacitor'svoltage rating should be at least 1.5 times greater than the output voltage. For a 5V regulator, a rating of at least 8V is appropriate, and a 10V or 15V rating is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rated for a higher voltage than would normally be needed. 2. Output Capacitor Selection (COUT) A. COUT = 100 μF to 470 μF standard aluminum electrolytic. B. Capacitor voltage rating = 20V. 3. Catch Diode Selection (D1) A. The catch-diode current rating must be at least 1.2 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2575. The most stressful condition for this diode is an overload or shorted output condition. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 3. Catch Diode Selection (D1) A. For this example, a 1A current rating is adequate. B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the suggested fast-recovery diodes shown in Table 1.
4. Input Capacitor (CIN) A 47 μF, 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing.
Inductor Value Selection Guides
(For Continuous Mode Operation)
Figure 27. LM2575(HV)-3.3 Figure 28. LM2575(HV)-5.0
Figure 29. LM2575(HV)-12 Figure 30. LM2575(HV)-15
Figure 31. LM2575(HV)-ADJ
LM2575HVSX-12 TI高压三端稳压器 - 图文
