PM6680ATR datasheet(34/48 Pages) STMICROELECTRONICS

34 / 48 page

Design guidelines

PM6680A

34/48

Equation 22

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.

A rectifier across the low side MOSFET is recommended. The rectifier works as a voltage

clamp across the synchronous rectifier and reduces the negative inductor swing during the

dead time between turning the high-side MOSFET off and the synchronous rectifier on. It

can increase the efficiency of the switching section, since it reduces the low side switch

losses. A shottky diode is suitable for its low forward voltage drop (0.3 V). The diode reverse

voltage must be greater than the maximum input voltage VINmax. A minimum recovery

reverse charge is preferable. Below there is a list of some shottky diode manufacturers.

Table 10. Low side MOSFET manufacturer

Manufacturer

Type

RDSon (mΩ)

Rated reverse voltage

(V)

STS7NF60L

[VC11] 19

0.0625

Table 14.

Dual MOSFET manufacturer

Manufacturer

Type

RDSon (mΩ)

Gate charge

(nC)

Rated reverse voltage

(V)

STS4DNF60L

Table 15.

Schottky diode manufacturer

Manufacturer

Series

Forward voltage

(V)

Rated reverse

voltage (V)

Reverse current

(uA)

STPS1L40M

0.5

LOAD

max

OUT

DSon

conduction

(max)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

RSS

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