ALED6000 datasheet(19/45 Pages) STMICROELECTRONICS

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Application information

6.1

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) in step-down conversion is roughly estimated by:

ICIN,RMS≅IOUT⋅ D⋅ 1−D

(7)

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

DMAX= VOUT+VF

VIN,MIN−VSW,MAX

DMIN= VOUT+VF

VIN,MAX−VSW,MIN

(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/µs 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 capacitor

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 the following equation:.

VIN,PP=D⋅ 1−D ⋅IOUT

CIN⋅FSW +RES,IN⋅IOUT

(9)

In case of MLCC ceramic input capacitors, the equivalent series resistance (RES,IN) is negligible.

In addition to the above considerations, a ceramic capacitor with an appropriate voltage

rating and with a value 1 µF or higher should always be placed across VIN and power

ground and across VCC and the IC GND pins, as close as possible to the ALED6000 device. This solution is

necessary for spike filtering purposes.

6.2

Output capacitor and inductor selection

The output capacitor is very important in order to satisfy the output voltage ripple requirement. Using a small

inductor value is useful to reduce the size of the choke but increases the current ripple. So, to reduce the output

voltage ripple, a low ESR capacitor is required.

ALED6000

Application information

DS13018 - Rev 4

page 19/45

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