AN3352

Component selection guidelines

Doc ID 018526 Rev 3

17/26

2.5

DC-DC output capacitors (C3, C4, C6) and ferrite bead

An electrolytic low cost capacitor is needed on the DC-DC converter output stage (C3 in

Figure 6

). Moreover, two ceramic capacitors are recommended to reduce the high

frequency switching noise. The switching noise is due to the voltage spikes of the fast

switching action of the output switch, and to the parasitic inductance of the output

capacitors. To minimize these voltage spikes, special low-inductance ceramic capacitors

can be used, and their lead lengths must be kept short and as close as possible to the IC

pins (C4 and C6 in Figure 9). In the case of high switching noise, it is possible to increase

the C6 capacitor up to 4.7 µF, 2.2 µF is a good compromise to reduce the switching noise.

The most important parameter for the DC-DC output electrolytic capacitors is the effective

series resistance (ESR). The DC-DC converter control loop circuit has been designed to

work properly with low-cost electrolytic capacitors which have ESR in the range of 200 m

Ω.

A 220 µF with ESR between 100 m

Ω and 350 mΩ is a good choice in most application

conditions. If it is requested to further reduce the switching noise, a ferrite bead with a

current rating of at least 2 A and impedance higher than 60

Ω at 100 MHz may be used.

In this case, it is recommended to use two electrolytic capacitors of 100 µF (see C3 and C3A

in Figure 9) with ESR between 150 m

Ω and 350 mΩ adding the ferrite bead in accordance

to Figure 9.

The DC-DC capacitor's voltage rating must be at least 25 V, but higher voltage capacitors

are recommended.

2.6

Input capacitors (C1)

An electrolytic bypass capacitor (C1 in Figure 6) between 100 µF and 470 µF, located close

to the LNBH24L, is needed for stable operation. In any case, a ceramic capacitor (C2 in

Figure 6

) between 100 nF and 470 nF is recommended to reduce the switching noise at the

Figure 9.

DC-DC converter output stage with ferrite bead