L6391

Bootstrap driver

Doc ID 17892 Rev 1

17/21

The following equation is useful to compute the drop on the bootstrap DMOS:

Equation 3

For example: using a power MOS with a total gate charge of 30nC the drop on the bootstrap

Equation 4

is too high, or the circuit topology doesn’t allow a sufficient charging time, an external diode

can be used.

Figure 8.

Bootstrap driver with high voltage fast recovery diode

Figure 9.

Bootstrap driver with internal charge pump

V

drop

I

ch

e

V

drop

→

Q

gate

T

ch

e

arg

------------------R

dson

==

V

drop

30nC

5

μs

--------------- 120

Ω 0.7V

∼

⋅

=

TO LOAD

H.V.

HVG

LVG

BOOT

V

CC

OUT

b

HVG

LVG

TO LOAD

H.V.

V

CC

OUT

BOOT