L6385E

Bootstrap driver

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4

Bootstrap driver

A bootstrap circuitry is needed to supply the high voltage section. This function is normally

accomplished by a high voltage fast recovery diode (Figure 4 a). In the L6385E a patented

integrated structure replaces the external diode. It is realized by a high voltage DMOS,

driven synchronously with the low side driver (LVG), with in series a diode, as shown in

Figure 4 b. An internal charge pump (Figure 4 b) provides the DMOS driving voltage. The

diode connected in series to the DMOS has been added to avoid undesirable turn on of it.

4.1

It has to be:

300mV.

the leakage losses.

e.g.: HVG steady state consumption is lower than 200

µA, so if HVG T

to supply 1

µC to C

The internal bootstrap driver gives great advantages: the external fast recovery diode can

be avoided (it usually has great leakage current).

fulfilled and it has to be long enough to charge the capacitor.

Ω). At low frequency 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:

C

EXT

Q

gate

V

gate

---------------

=

V

drop

I

ch

e

V

drop

→

Q

gate

T

ch

e

arg

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

dson

==