ST1CC40 datasheet(23/37 Pages) STMICROELECTRONICS

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ST1CC40

Application information

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:

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 recommended to avoid this type of capacitor for the input filter of the

device as they may be stressed by a high surge current when connected to the power

supply.

In case the selected capacitor is ceramic (so neglecting the ESR contribution), the input

voltage ripple can be calculated as:

Equation 27

7.2

Layout considerations

The layout of switching DC-DC converters is very important to minimize noise and

interference. Power-generating portions of the layout are the main cause of noise and so

high switching current loop areas should be kept as small as possible and lead lengths as

short as possible.

High impedance paths (in particular the feedback connections) are susceptible to

interference, so they should be as far as possible from the high current paths. A layout

example is provided in Figure 13.

The input and output loops are minimized to avoid radiation and high frequency resonance

problems. The feedback pin to the sensing resistor path must be designed as short as

possible to avoid pick-up noise. Another important issue is the ground plane of the board.

Since the package has an exposed pad, it is very important to connect it to an extended

ground plane in order to reduce the thermal resistance junction-to-ambient.

Table 7. List of ceramic capacitors for the ST1CC40

Manufacturer

Series

Capacitor value (µF)

Rated voltage (V)

TAIYO YUDEN

UMK325BJ106MM-T

MURATA

GRM42-2 X7R 475K 50

4.7

VIN PP

CIN fSW



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



----

–



 D





----

–





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