PTH04040W

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SLTS238C – SEPTEMBER 2005 – REVISED JUNE 2010

CAPACITOR TABLE

Table 1 identifies the characteristics of capacitors from a number of vendors with acceptable ESR and ripple

current (rms) ratings. The recommended number of capacitors required at both the input and output buses is

identified for each capacitor type.

Note: This is not an extensive capacitor list. Capacitors from other vendors are available with comparable

specifications. Those listed are for guidance. The RMS ripple current rating and ESR (at 100kHz) are critical

parameters necessary to insure both optimum regulator performance and long capacitor life.

DESIGNING FOR VERY FAST LOAD TRANSIENTS

The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of

1 A/ms. The typical voltage deviation for this load transient is given in the data sheet specification table using the

minimum required value of output capacitance. As the di/dt of a transient is increased, the response of a

converter’s regulation circuit ultimately depends on its output capacitor decoupling network. This is an inherent

limitation with any dc/dc converter once the speed of the transient exceeds its bandwidth capability. If the target

application specifies a higher di/dt or lower voltage deviation, the requirement can only be met with additional

output capacitor decoupling. In these cases special attention must be paid to the type, value and ESR of the

capacitors selected. Additional input capacitance may be required to insure the stability of the input bus during

higher current transient di/dt.

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