AN993 datasheet(14/27 Pages) STMICROELECTRONICS

14 / 27 page

Device block description

AN993

14/27

Doc ID 5656 Rev 10

If the HVG needs to be supplied for a long period of time, the Cboot selection also has to take

into account the leakage losses.

The internal bootstrap driver has great advantages: the external fast-recovery diode can be

avoided (it usually has great leakage current). This structure can only work if Vout is close to

GND (or lower) and if 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 RDS(on) (typical value at

25°C is 150

Ω). At low frequencies, this drop can be neglected, but must be taken into

account when the frequency is increased.

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

Equation 14

where Qgate is the gate charge of the external power MOSFET, RDS(on) is the on resistance

of the bootstrap DMOS, and Tcharge is the charging time of the bootstrap capacitor.

For example: using a power MOSFET with a total gate charge of 30 nC, the drop on the

bootstrap DMOS is about 1 V if the Tcharge is 5 µs. In fact:

Equation 15

Vdrop has to be taken into account when the voltage drop on Cboot is calculated: if this drop

is too high or if the charging time is insufficient, an external diode can be used.

Figure 11.

External bootstrap diode

connection

Figure 12.

L6574 integrated bootstrap

diode connection

drop

arg

DS on

()

drop

gate

arg

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

DS on

()

→

AM01319v1

TO LOAD

H.V.

HVG

LVG

CBOOT

DBOOT

VBOOT

VOUT

AM01320v1

EX_D99IN1056

HVG

LVG

TO LOAD

H.V.

CBOOT

VBOOT

VOUT

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