Datasheet

'Vout = 0.8 x
8 x Fsw[5] x Cout
ESR +
1
Cin =
1.5
Fsw[5]
Rfb1 =
1.225 x Rfb2[9]
(Vout[1] ± 1.225)
Quick Guide Design Worksheet www.ti.com
Step 6. Calculate the value of C
RAMP
:
C
RAMP
= L1[7] x 10
-5
(5)
Selected value of C
RAMP
: Farads [8]
Step 7. Set Rfb2 to 5kOhms if Vout[1] is less than or equal to 5 Volts. If Vout[1] is greater than 5V set
Rfb2 to 10K Ohms.
Selected value of Rfb2: Ohms [9]
Calculate the value of Rfb1:
(6)
Selected value of Rfb1: Ohms [10]
Step 8. Select the re-circulating diode, D1. A Schottky type diode is required for all applications. Ultra-fast
diodes are not recommended and may result in damage to the IC due to reverse recovery current
transients. The reverse breakdown rating should be greater than the Input Voltage Max Spec[3], plus
some safety margin. For worst case design, assume a short circuit load condition. In this case the diode
will carry the output current almost continuously. This current can be as high as 5.1A. Assuming a 0.6V
drop across the diode, the maximum diode power dissipation can be as high as 3W. A DPAK or SMC
case is recommended.
Selected diode part number: [11]
Step 9. A good quality input capacitor(s) is necessary to limit the ripple voltage at the VIN pin while
supplying most of the switch current during the on-time. The minimum RMS ripple current rating for the
input capacitor(s) is 1.5Amp. A quality ceramic capacitor(s) with a low ESR is recommended. The input
capacitor voltage rating should be greater than the Input Voltage Max Spec [3], plus some safety margin.
A guide to select the input capacitor(s) value in proportion to the operating frequency is:
(7)
Selected value for Cin: [12]
Farads:
Step 10. The output capacitor(s) smooth the inductor ripple current and provide a source of charge for
transient loading conditions. A good starting point for the output capacitance is to parallel a ceramic
capacitor (10 µF to 47 µF) and a low ESR organic or tantalum capacitor (22 µF to 220 µF). The ceramic
capacitor provides ultra low ESR to reduce the output ripple voltage and noise spikes, while the larger bulk
capacitor provides a source of charge for transient loading conditions. The output capacitor voltage rating
should be greater than the Output Voltage Spec [1], plus some safety margin. An approximation for the
output ripple voltage is:
(8)
Selected value for Cout: Farads [13]
4
AN-1572 Quick Start Guide for a 3 Amp Buck Regulator Using the LM5576 SNVA212B–January 2007–Revised April 2013
and LM25576
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