PM6685 datasheet(37/52 Pages) STMICROELECTRONICS

37 / 52 page

PM6685

Monitoring and protections

37/52

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 19

Maximum conduction losses occur at the maximum input voltage:

Equation 20

Choose a synchronous rectifier with low RDSon. 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.

Dual n-channel MOSFETs can be used in applications with a maximum output current of

about 3 A. Below there is a list of some MOSFET manufacturers.

Table 14.

High side MOSFET manufacturer

Manufacturer

Type

Gate charge (nC)

Rated reverse voltage (V)

STS12NH3LL

STS17NH3LL

Table 15.

Low side MOSFET manufacturer

Manufacturer

Type

RDSon (mΩ)

Rated reverse voltage

(V)

STS17NF3LL

5.5

0.047

STS25NH3LL

3.5

0.011

conduction

DLowSide

LOAD

max

OUT

DSon

conduction

(max)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

RSS

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