PM6670AS datasheet(45/54 Pages) STMICROELECTRONICS

45 / 54 page

PM6670AS

Application information

45/54

where RDSon is the drain-source on-resistance of the control MOSFET.

Switching losses are approximately given by:

Equation 41

where tON and tOFF are the turn-on and turn-off times of the MOSFET and depend on the

gate-driver current capability and the gate charge Qgate. A greater efficiency is achieved with

low RDSon. Unfortunately low RDSon MOSFETs have a great gate charge.

As general rule, the RDSon x Qgate product should be minimized to find out the suitable

MOSFET.

Logic-level MOSFETs are recommended, as long as low-side and high-side gate drivers are

greater than the maximum input voltage .VINmax.

Table 15 lists tested high-side MOSFETs.

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

conduction losses:

Equation 42

Maximum conduction losses occur at the maximum input voltage:

Equation 43

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 CRRS, causing a 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 minimize the ratio CRSS / CGS where

Equation 44

Table 9. Evaluated high-side MOSFETs

Manufacturer

Type

RDSon

Ω)

Gate charge

(nC)

Rated reverse

voltage (V)

STS12NH3LL

10.5

IRF7811

)

(max)

(

)

(max)

(

off

LOAD

switching

⋅

∆

⋅

∆

−

⋅

conduction

DLowSide

≅

MAX

LOAD

MAX

OUT

DSon

conduction

⋅

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

RSS

ISS

−

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