L6388 datasheet(6/11 Pages) STMICROELECTRONICS

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L6388

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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:

where Qgate is the gate charge of the external power MOS, Rdson is the on resistance of the bootstrap DMOS,

and Tcharge is the charging time of the bootstrap capacitor.

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

1V, if the Tcharge is 5µs. In fact:

Vdrop has to be taken into account when the voltage drop on CBOOT is calculated: if this drop is too high, or the

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

Figure 8. Bootstrap Driver.

drop

arg Rdson

drop

→

gate

arg

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

dson

drop

30nC

µs

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

Ω 0.8V

∼

⋅

TO LOAD

H.V.

HVG

LVG

HVG

LVG

CBOOT

TO LOAD

H.V.

CBOOT

DBOOT

VBOOT

VOUT

VBOOT

VOUT

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