L6743, L6743Q

Device description and operation

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5.3

Internal BOOT diode

L6743, L6743Q 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

L6743, L6743Q 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:

0.0

0.5

1.0

1.5

2.0

2.5

0

10

203040

5060708090

100

High-Side MOSFET Gate Charge [nC]

Cboot = 47nF

Cboot = 100nF

Cboot = 220nF

Cboot = 330nF

Cboot = 470nF

0

500

1000

1500

2000

2500

0.0

0.2

0.4

0.6

0.8

1.0

Boot Cap Delta Voltage [V]

Qg = 10nC

Qg = 25nC

Qg = 50nC

Qg = 100nC

P

DC

V

CC

I

CC

V

PVCC

I

PVCC

⋅

+

⋅

=

P

SW

F

SW

Q

GHS

PVCC

⋅

Q

GLS

VCC

⋅

+

()

⋅

=