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has to be long enough to charge the capacitor.

The bootstrap driver introduces a voltage drop due to the DMOS RDSON (typical value: 125 Ohm). At low fre-

quency this drop can be neglected. Anyway increasing the frequency it must be taken in to account.

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

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

circuit topology doesn't allow a sufficient charging time, an external diode can be used.

Figure 8. Bootstrap Driver.

V

drop

I

ch

e

V

drop

→

Q

gate

T

ch

e

arg

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

dson

==

V

drop

30nC

5

µs

--------------- 125

Ω 0.8V

∼

⋅

=

TO LOAD

H.V.

HVG

a

b

LVG

HVG

LVG

TO LOAD

H.V.