L6386D datasheet(5/10 Pages) STMICROELECTRONICS

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BOOTSTRAP DRIVER

A bootstrap circuitry is needed to supply the high

voltage section. This function is normally accom-

plished by a high voltage fast recovery diode (fig.

4a). In the L6386 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 fig. 4b

An internal charge pump (fig. 4b) provides the

DMOS driving voltage .

The diode connected in series to the DMOS has

been added to avoid undesirable turn on of it.

CBOOT selection and charging:

To choose the proper CBOOT value the external

MOS can be seen as an equivalent capacitor.

This capacitor CEXT is related to the MOS total

gate charge :

CEXT =

Qgate

Vgate

The ratio between the capacitors CEXT and CBOOT

is proportional to the cyclical voltage loss .

It has to be:

CBOOT>>>CEXT

e.g.: if Qgate is 30nC and Vgate is 10V, CEXT is

3nF. With

CBOOT = 100nF the drop would be

300mV.

If HVG has to be supplied for a long time, the

CBOOT selection has to take into account also the

leakage losses.

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

200

µA, so if HVG TON is 5ms, CBOOT has to

supply 1

µCto CEXT. This charge on a 1µFca-

pacitor means a voltage drop of 1V.

The internal bootstrap driver gives great advan-

tages: the external fast recovery diode can be

avoided (it usually has great leakage current).

This structure can work only if VOUT is close to

GND (or lower) and in the meanwhile the LVG is

on. The charging time (Tcharge ) of the CBOOT is

the time in which both conditions are fulfilled and

it 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 frequency this drop can be ne-

glected. Anyway

increasing the frequency

must be taken in to account.

The following equation is useful to compute the

drop on the bootstrap DMOS:

Vdrop

= IchargeRdson

→ Vdrop =

Qgate

Tcharge

Rdson

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 5

µs. In fact:

Vdrop =

30nC

µs

⋅ 125Ω ~ 0.8V

Vdrop has to be taken into account when the volt-

age 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.

TO LOAD

D99IN1056

H.V.

HVG

LVG

HVG

LVG

CBOOT

TO LOAD

H.V.

CBOOT

DBOOT

VBOOT

VOUT

VBOOT

VOUT

Figure 4. Bootstrap Driver.

L6386

5/10

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