PM6681ATR datasheet(31/47 Pages) STMICROELECTRONICS

31 / 47 page

PM6681A

Design guidelines

31/47

Equation 15

If hard saturation inductors are used, the inductor saturation current should be much greater

than the maximum inductor peak current Ipeak:

Equation 16

Using soft saturation inductors it's possible to choose inductors with saturation current limit

nearly to Ipeak. Below there is a list of some inductor manufacturers.

9.3

Output capacitor

The selection of the output capacitor is based on the ESR value Rout and the voltage rating

rather than on the capacitor value Cout.

The output capacitor has to satisfy the output voltage ripple requirements. Lower inductor

value can reduce the size of the choke but increases the inductor current ripple

∆I

Since the voltage ripple VRIPPLEout is given by:

Equation 17

A low ESR capacitor is required to reduce the output voltage ripple. Switching sections can

work correctly even with 20 mV output ripple.

However, to reduce jitter noise between the two switching sections it's preferable to work

with an output voltage ripple greater than 30 mV. If lower output ripple is required, a further

compensation network is needed (see Closing the integrator loop paragraph).

Finally the output capacitor choice deeply impacts on the load transient response (see Load

transient response paragraph). Below there is a list of some capacitor manufacturers.

Table 11.

Inductor manufacturer

Manufacturer

Series

Inductor value

(uH)

RMS current

(A)

Saturation current

(A)

Coilcraft

SER1360

1 to 8

6 to 9.5

7 to 31

Coilcraft

MLC

0.7 to 4.5

13.6 to 17.3

11.5 to 26

TDK

RLF12560

1 to 10

7.5 to 14.4

7.5 to 18.5

max

OUT

max

(max)

−

∆

(max)

LOAD

peak

∆

out

RIPPLEout

∆

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