Datasheet

Rcomp = 1.2 x 10
5
x Rfb1[9] x Cout[13] +
Rfb1[9]
Vout[1]
'Vout = 0.4 x
8 x Fsw[5] x Cout
ESR +
1
Cin =
0.7
Fsw[5]
Rfb1 =
1.225 x Rfb2[9]
(Vout[1] ± 1.225)
Quick Guide Design Worksheet www.ti.com
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, or 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 2.5A. Assuming a 0.6V drop
across the diode, the maximum diode power dissipation can be as high as 1.5W. An SMC or SMB 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 0.75 Amp. A quality ceramic capacitor 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: Farads [12]
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 use a ceramic capacitor
(10 µF to 100 µF) An additional low ESR organic or tantalum capacitor (22 µF to 220 µF) could be added
in parallel for applications with large load transients. 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]
Step 11. Ccomp and Rcomp configure the error amplifier gain characteristics to accomplish a stable
overall loop gain. One advantage of current mode control is the ability to close the loop with only two
feedback components. Calculate the value of Rcomp:
(9)
Selected value of Rcomp: Ohms [14]
4
AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 SNVA211B–January 2007–Revised April 2013
and LM25575
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