PM6681ATR datasheet(33/47 Pages) STMICROELECTRONICS

33 / 47 page

PM6681A

Design guidelines

33/47

Maximum conduction losses are approximately:

Equation 20

where RDS(on) is the drain-source on resistance of the high side MOSFET. Switching losses

are approximately:

Equation 21

where ton and toff are the switching times of the turn off and turn off phases of the MOSFET.

As general rule, high side MOSFETs with low gate charge are recommended, in order to

minimize driver losses.

Below there is a list of possible choices for the high side MOSFET.

The power dissipation of the low side MOSFET is given by:

Equation 22

Maximum conduction losses occur at the maximum input voltage:

Equation 23

Choose a synchronous rectifier with low RDS(on). When high side MOSFET turns on, the fast

variation of the phase node voltage can bring up even the low side gate through its gate

drain capacitance CRSS, causing cross-conduction problems. Choose a low side MOSFET

that minimizes the ratio CRSS/CGS (CGS = CISS - CRSS).

Below there is a list of some possible low side MOSFETs.

Table 14.

High side MOSFET manufacturer

Manufacturer

Type

Gate charge (nC)

Rated reverse voltage (V)

STS12NH3LL

STS17NH3LL

LOAD

min

OUT

DSon

conduction

(max)

)

(max)

(

)

(max)

(

off

LOAD

switching

∆

−

conduction

DLowSide

LOAD

max

OUT

DSon

conduction

(max)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

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