PM6675AS datasheet(40/48 Pages) STMICROELECTRONICS

40 / 48 page

Application information

PM6675AS

40/48

In buck converters the power dissipation of the synchronous MOSFET is mainly due to

conduction losses:

Equation 43

Maximum conduction losses occur at the maximum input voltage:

Equation 44

The synchronous rectifier should have the lowest RDSon as possible. When the high-side

MOSFET turns on, high dV/dt of the phase node can bring up even the low-side gate

through its gate-drain capacitance CRES, causing cross-conduction problem. Once again,

the choice of the low-side MOSFET is a trade-off between on resistance and gate charge; a

good selection should minimizes the ratio CRSS / CGS where CGS = CISS - CRSS.

Below some tested low-side MOSFETs are listed.

Dual N-MOS can be used in applications with low output current.

Figure 16

shows some suitable dual MOSFETs for applications requiring about 3 A.

Table 14.

Evaluated high-side MOSFETs

Manufacturer

Type

RDSon (mΩ)

Gate charge (Nc)

Rated reverse voltage (V)

STS12NH3LL

10.5

STS7NF60L

IRF7811

Table 15.

Evaluated low-side MOSFETs

Manufacturer

Type

RDSon (mΩ)

Rated reverse voltage

(V)

STS12NH3LL

13.5

0.069

STS25NH3LL

0.011

IRF7811

0.054

Table 16.

Suitable dual MOSFETs

Manufacturer

Type

RDSon (mΩ)

Gate charge (nC)

Rated reverse

voltage (V)

STS8DNH3LL

IRF7313

conduction

DLowSide

≅

MAX

LOAD

MAX

OUT

DSon

conduction

⋅

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

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