AN2620 datasheet(4/20 Pages) STMICROELECTRONICS

4 / 20 page

Application information component selection

AN2620

4/20

Doc ID 13883 Rev 3

Application information component selection

1.1

Input capacitor

The ST1S10 features two VIN pins: VIN_SW for the power supply input voltage where the

switching peak current is drawn, and VIN_A to supply the ST1S10 internal circuitry and

drivers. The VIN_SW input capacitor reduces the current peaks drawn from the input power

supply and reduces switching noise in the IC. High power supply source impedance requires

larger input capacitance.

For the VIN_SW input capacitor the RMS current rating is a critical parameter that must be

higher than the RMS input current. The maximum RMS input current can be calculated

using the following equation:

Equation 1

where

η is the expected system efficiency, D is the duty cycle and IO the output DC current.

This function reaches its maximum value at D = 0.5 and the equivalent RMS current is equal

to IO divided by 2 (considering

η= 1).

The maximum and minimum duty cycles are:

Equation 2

Equation 3

where VF is the voltage drop across the internal NMOS and VSW the voltage drop across the

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

IRMS following through the input capacitor.

A minimum value of 4.7 µF for the VIN_SW and a 0.1 µF ceramic capacitor for the VIN_A are

suitable in most application conditions. A 10 µF or higher ceramic capacitor for the VIN_SW

and a 1 µF (VIN_A) are advisable in case of higher power supply source impedance or where

it is needed to have long wires between the power supply source and the VIN pins. The

above suggested higher input capacitors values are also advisable in case of high output

capacitive load which can impact the switching peak current drawn from the input capacitor

during the startup transient.

It is also advisable to use ceramic capacitors with a voltage rating in the range of 1.5 times

the maximum input voltage. The input capacitors should be located as close as possible to

the VIN pins.

Different capacitors can be considered:

●

Electrolytic capacitors. These are the most commonly used because they are the least

expensive and are available with a wide range of RMS current ratings. The only

RMS

⋅

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

–

-------

⋅

MAX

out

inMIN

–

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

MIN

out

inMAX

–

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

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