L6743B

Device description and operation

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5.3

Internal BOOT diode

L6743B embeds a boot diode to supply the high-side driver saving the use of an external

component. Simply connecting an external capacitor between BOOT and PHASE complete

the high-side supply connections.

To prevent bootstrap capacitor to extra-charge as a consequence of large negative spikes,

BOOT pin.

Bootstrap capacitor needs to be designed in order to show a negligible discharge due to the

high-side MOSFET turn-on. In fact it must give a stable voltage supply to the high-side driver

during the MOSFET turn-on also minimizing the power dissipated by the embedded Boot

Diode. Figure 5 gives some guidelines on how to select the capacitance value for the

bootstrap according to the desired discharge and depending on the selected MOSFET.

Figure 5.

Bootstrap capacitance design

5.4

Power dissipation

L6743B embeds high current drivers for both high-side and low-side MOSFETs: it is then

important to consider the power that the device is going to dissipate in driving them in order

to avoid overcoming the maximum junction operative temperature.

Two main terms contribute in the device power dissipation: bias power and drivers' power.

●

supply pins and it is simply quantifiable as follow:

●

Drivers' power is the power needed by the driver to continuously switch ON and OFF

the external MOSFETs; it is a function of the switching frequency and total gate charge

dissipated to switch the MOSFETs dissipated by three main factors: external gate

resistance (when present), intrinsic MOSFET resistance and intrinsic driver resistance.

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:

P

DC

V

CC

I

CC

V

PVCC

I

PVCC

⋅

+

⋅

=

P

SW

F

SW

Q

GHS

PVCC

⋅

Q

GLS

VCC

⋅

+

()

⋅

=