LSP5523

Document number: DS44073 Rev. 7 - 2

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LSP5523

I

nput Capacitor

The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the

converter. A low ESR Electrolytic (EC) capacitor is highly recommended. Since large current flows in and

out of this capacitor during switching, its ESR also affects efficiency.

When EC cap is used, the input capacitance needs to be equal to or higher than 100uF.The RMS ripple current

rating needs to be higher than 50% of the output current. The input capacitor should be placed close to the

VIN and GND pins of the IC, with the shortest traces possible. The input capacitor can be placed a little bit

away if a small parallel 0.1uF ceramic capacitor is placed right next to the IC.

When Vin is >15V, pure ceramic Cin (* no EC cap) is not recommended. This is because the ESR of a

ceramic cap is often too small, Pure ceramic Cin will work with the parasite inductance of the input trace and

forms a Vin resonant tank. When Vin is hot plug in/out, this resonant tank will boost the Vin spike to a very high

voltage and damage the IC.

Output Capacitor

The output capacitor also needs to have low ESR to keep low output voltage ripple. In the case of ceramic

output capacitors, RESR is very small and does not contribute to the ripple. Therefore, a lower capacitance

value can be used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is

dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have

sufficiently low ESR.

For ceramic output capacitors, typically choose two capacitors of about 22uF. For tantalum or electrolytic

capacitors, choose a capacitor with less than 50mΩ ESR.

Optional Schottky Diode

During the transition between high-side switch and low-side switch, the body diode of the low side power

MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky

diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 3 lists example

Schottky diodes and their Manufacturers.

Table 3－Diode Selection Guide

Vin max

Part Number

Voltage/Current

Rating

Vendor

<20V

B130

30V, 1A

Diodes Incorporated

<20V

SK13

30V, 1A

Diodes Incorporated

>20V

B140

40V, 1A

Diodes Incorporated

>20V

SK14

40V, 1A

Diodes Incorporated