Device description and operation

L6741

10/16

This last term is the important one to be determined to calculate the device power

dissipation.

The total power dissipated to switch the mosfets results:

When designing an application based on L6741 it is recommended to take into

consideration the effect of external gate resistors on the power dissipated by the driver.

External gate resistors helps the device to dissipate the switching power since the same

resulting in a general cooling of the device.

Referring to Figure 6, classical mosfet driver can be represented by a push-pull output stage

with two different mosfets: P-MOSFET to drive the external gate high and N-MOSFET to

reach the driving voltage (PVCC for HS and VCC for LS). This capacitance is charged and

The total power Psw is dissipated among the resistive components distributed along the

driving path. According to the external Gate resistance and the power-MOSFET intrinsic

gate resistance, the driver dissipates only a portion of Psw as follow:

The total power dissipated from the driver can then be determined as follow:

Figure 6.

Equivalent circuit for MOSFET drive.

P

SW

F

SW

Q

GHS

PVCC

⋅

Q

GLS

VCC

⋅

+

()

⋅

=

P

SW

HS

–

1

2

--- C

GHS

PVCC

2

Fsw

R

hiHS

R

hiHS

R

GateHS

R

iHS

++

----------------------------------------------------------------

R

loHS

R

loHS

R

GateHS

R

iHS

++

----------------------------------------------------------------

+

⎝⎠

⎛⎞

⋅⋅

⋅

⋅

=

P

SW

LS

–

1

2

--- C

GLS

VCC

2

Fsw

R

hiLS

R

hiLS

R

GateLS

R

iLS

++

--------------------------------------------------------------

R

loLS

R

loLS

R

GateLS

R

iLS

++

--------------------------------------------------------------

+

⎝⎠

⎛⎞

⋅⋅

⋅

⋅

=

PP

DC

P

SW

HS

–

P

SW

LS

–

++

=

VCC

LS DRIVER

LS MOSFET

GND

LGATE

BOOT

HS DRIVER

HS MOSFET

PHASE

HGATE

PVCC