PM6680 datasheet(42/49 Pages) STMICROELECTRONICS

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Device description

PM6680

42/49

7.13.8

Design example

The following design example considers an input voltage from 7 V to 16 V. The two switching

outputs are OUT1=1.5 V and OUT2=1.05 V and must deliver a maximum current of 5 A. The

selected switching frequencies are about 290 kHz for OUT1 section and about 425 kHz for

OUT2 section (see Table 7 on page 30).

Inductor selection

OUT1: ILOAD = 5 A, 35 % ripple current.

Equation 36

We choose standard value L = 2.5 uH.

∆I

L(max) = 1.8 A @VIN =12 V

ILRMS = 5.03 A

IPEAK = 5 A + 0.9 A = 5.9 A

OUT2: ILOAD = 5 A, 30 % ripple current.

Equation 37

∆I

L(max) = 1.6 A @VIN = 12 V.

ILRMS = 5.02 A

IPEAK = 5 A + 0.8 A = 5.8 A

Output capacitor selection

We would like to have an output ripple smaller than 25 mV.

OUT1: POSCAP 6TPB330M

OUT2: POSCAP 6TPB330M

Power MOSFETS

OUT1: High side: STS12NH3LL

Low side: STS12NH3LL

OUT2:High side: STS12NH3LL

Low side: STS12NH3LL

Current limit

OUT1:

Equation 38

KHz

290

)

(

≈

⋅

−

⋅

KHz

425

)

(

≈

⋅

−

⋅

(min)

(max)

(min)

LOAD

Lvalley

∆

−

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