Output Capacitors

The output capacitor selection is based on the output voltage ripple requirements. This ripple is related to the

current ripple through the inductor and is almost entirely due to the ESR of the output capacitor. Therefore,

the goal is to achieve an ESR lower than a certain value, regardless of the actual capacitance value.

The maximum current ripple of the +5.1V section is:

considering the values obtained in the paragraph "+5.1 V Inductor".

As for the +3.3V, the maximum ripple is given by:

3

4

⋅

3.3

Anyhow, the maximum ESR will be:

≤

where the subscript x refers to either section.

In pulse-skipping operation, the capacitive component of the output ripple is comparable to the resistive

one, thus both should be considered:

⋅

1

2

⋅ 



1

1







comply with it as well.

Further constraints on the minimum output capacitance can arise from specifications regarding the maxi-

mum undershoot,

>

L

2

in min

>

L

2

out

the +3.3V or +5.1V section.

High performance capacitors should be employed to reduce the capacitance needed for a given ESR, to

avoid paralleling several parts with a considerable waste of space. Although excellent electrolytic capaci-

tors are available, OS-CON or tantalums may be preferred especially if very compact design is required,

or in case of surface mount assemblies. Multilayer ceramic capacitors with extremely low ESR are now

available, but they have a large spread of the capacitance value, so they should be paralleled with an-

other more stable, high-ESR capacitor.

Miscellaneous components

The feedback loop has virtually unlimited bandwidth, thus a filter is necessary to make the system insen-

sitive to the switching frequency ripple and, in general, to prevent noise from disturbing the correct op-

eration of the error summing comparator. Anyway, the cut-off frequency of this filter can be very high, so

that line and load transient response is extremely fast. This filter is a simple R-C type where resistance

and capacitance can be chosen for a typical 3dB cut-off frequency of 60 kHz.

As to the bootstrap diodes, even though small signal p-n diodes might be effectively used, it is preferable

to employ low-power Schottky rectifiers, since that increase slightly the gate drive voltage, in favour of ef-

ficiency. The bootstrap capacitor can be a 100nF film capacitor.

The soft-start capacitors determine the time during which the current limitation circuit moves gradually the

setpoint from zero up to 100 mV in order to limit the current inflow at start-up. This ramp lasts approximately

1 ms per nF of soft-start capacitance (10 to 100 nF typical values), but the actual time necessary to the out-

put voltage to reach the steady-state value depends on the load current and the output filter capacitance.

There are some critical points of the IC that may require by-pass capacitors to prevent noise from dis-

DESIGN PROCEDURE (continued)

L4992

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