L6385 datasheet(5/9 Pages) STMICROELECTRONICS

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leakage losses.

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

200

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

supply 1

µC to CEXT. This charge on a 1µF ca-

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.

-45

-25

100

125

100

150

200

250

Tj (°C)

Typ.

@ Vcc = 15V

Figure 5. Turn On Time vs. Temperature

-45

-25

100

125

100

150

200

250

Tj (°C)

Typ.

@ Vcc = 15V

Figure 6. Turn Off Time vs. Temperature

L6385

5/9

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