L7987 datasheet(20/37 Pages) STMICROELECTRONICS

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Application information

L7987

20/37

DocID025589 Rev 3

Application information

5.1

Input capacitor selection

The input capacitor must be rated for the maximum input operating voltage and the

maximum RMS input current.

Since the step-down converters input current is a sequence of pulses from 0 A to IOUT, the

input capacitor must absorb the equivalent RMS current which can be up to the load current

divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these

capacitors must be very high to minimize the power dissipation generated by the internal

ESR, thereby improving system reliability and efficiency.

The RMS input current (flowing through the input capacitor) is roughly estimated by:

Equation 7

Actual DC/DC conversion duty cycle, D = VOUT/VIN, is influenced by a few parameters:

Equation 8

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

internal high-side MOSFET. Considering the range DMIN to DMAX it is possible to determine

the maximum ICIN,RMS flowing through the input capacitor.

The input capacitor value must be dimensioned to safely handle the input RMS current and

to limit the VIN and VCC ramp-up slew-rate to 0.5 V/

s maximum, in order to avoid the

device active ESD protections turn-on.

Different capacitors can be considered:



Electrolytic capacitors

These are the most commonly used due to their low cost and wide range of operative

voltage. 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 the very low ESR).

The drawback is their high cost.



Tantalum capacitors

Small, good quality tantalum capacitors with very low ESR are becoming more

available. However, they can occasionally burn if subjected to very high current, for

example when they are connected to the power supply.

The amount of the input voltage ripple can be roughly overestimated by Equation 9.

ICIN RMS



IOUT

D1 D

–







DMAX

VOUT VF

VIN MIN



VSW MAX



–

----------------------------------------------------

DMIN

VOUT VF

VIN MAX



VSW MIN



–

----------------------------------------------------

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