R5973D datasheet(22/39 Pages) STMICROELECTRONICS

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Application information

R5973D

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DocID026876 Rev 1

and

Equation 19

Where VF is the freewheeling diode forward voltage and VSW the voltage drop across the

internal PDMOS. Considering the range DMIN to DMAX, it is possible to determine the max.

IRMS going through the input capacitor.

Capacitors that can be considered are:

Electrolytic capacitors:

These are widely used due to their low price and their availability in a wide range of

RMS current ratings.

The only drawback is that, considering ripple current rating requirements, they are

physically larger than other capacitors.

Ceramic capacitors:

If available for the required value and voltage rating, these capacitors usually have

a higher RMS current rating for a given physical dimension (due to very low ESR).

The drawback is the considerably high cost.

Tantalum capacitors:

Very good, small tantalum capacitors with very low ESR are becoming more available.

However, they can occasionally burn if subjected to very high current during charge.

Therefore, it is better to avoid this type of capacitor for the input filter of the device.

They can, however, be subjected to high surge current when connected to the power

supply.

High dv/dt voltage spikes on the input side can be critical for DC/DC converters. A good

power layout and input voltage filtering help to minimize this issue. In addition to the above

considerations, a 1 µF/50 V ceramic capacitor as close as possible to the VCC and GND

pins is always suggested to adequately filter VCC spikes.



Output capacitor

The output capacitor is very important to meet the output voltage ripple requirement.

Using a small inductor value is useful to reduce the size of the choke but it increases the

current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required.

Nevertheless, the ESR of the output capacitor introduces a zero in the open loop gain,

which helps to increase the phase margin of the system. If the zero goes to a very high

frequency, its effect is negligible. For this reason, ceramic capacitors and very low ESR

capacitors in general should be avoided.

Tantalum and electrolytic capacitors are usually a good choice for this purpose. A list of

some tantalum capacitor manufacturers is provided in Table 7: Output capacitor selection.

Table 6. List of ceramic capacitors for the R5973D

Manufacturer

Series

Capacitor value (µF)

Rated voltage (V)

TAIYO YUDEN

UMK325BJ106MM-T

MURATA

GRM42-2 X7R 475K 50

4.7

DMIN

VOUT VF

VINMAX VSW

–

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