STPD01 datasheet(33/43 Pages) STMICROELECTRONICS

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Figure 44. Thermal aspects

9.2

Selecting components for the application

This section provides information to assist in the selection of the most appropriate components for the intended

application.

Output capacitor

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

value is useful to reduce the size of the coil, but this increases the current ripple. Therefore, to reduce the output

voltage ripple a low ESR capacitor is required.

The output voltage ripple (VOUT_RIPPLE), in continuous mode, is:

VOUT_RIPPLE=∆I× COUT_ESR+ 1

8×COUT×Fsw

(3)

where ΔI is the ripple current and FSW is the switching frequency.

Inductor

The inductor value is very important because it fixes the ripple current flowing through the output capacitor. The

ripple current is usually fixed at 20-40% of Iout_max, which is 0.6-1.2 A with Iout_max = 3 A. The approximate

inductor value is obtained using the following formula:

L= VIN−VOUT

∆I ×TON

(4)

where TON is the ON time of the internal switch, given by D · T.

For example, with VOUT = 5 V, VIN = 24 V, FSW = 500 kHz and ΔIOUT = 0.8 A, the inductor value is around 10

μH.

The peak current thought the inductor is given by:

IPK=IOUT+∆I2

(5)

It can be observed that if the inductor value decreases, the peak current (which must be lower than the current

limit of the device) increases. So, for fixed peak current protection, a higher value of the inductor allows a higher

value for the output current.

Thermal considerations

The dissipated power of the device is given by three separate factors:

•

Switching losses due to the RDS(on). These are equal to:

PHS=RDSon IOUT2D

(6)

and

STPD01

Selecting components for the application

DS13473 - Rev 1

page 33/43

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