PM6685TR View Datasheet(PDF) - STMicroelectronics

Part Name

Description

MFG CO.

PM6685TR

Dual step-down main supply controller with auxilary voltages for notebook system power

STMicroelectronics 

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PM6685

Monitoring and protections

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.

Table 14. High side MOSFET manufacturer

Manufacturer

Type

Gate charge (nC)

ST

STS12NH3LL

10

ST

STS17NH3LL

18

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

Equation 19

Rated reverse voltage (V)

30

30

PDLowSide = Pconduction

Maximum conduction losses occur at the maximum input voltage:

Equation 20

Pconduction

= RDSon × ⎜⎜⎝⎛1−

VOUT

VINmax

⎟⎟⎠⎞ × ILOAD (max)2

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.

Table 15. Low side MOSFET manufacturer

Manufacturer

Type

RDSon (mΩ)

ST

STS17NF3LL

5.5

ST

STS25NH3LL

3.5

C RSS

CGS

0.047

0.011

Rated reverse voltage

(V)

30

30

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.

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