L6384E 데이터 시트보기 (PDF) - STMicroelectronics

부품명

상세내역

제조사

L6384E

High-voltage half bridge driver

STMicroelectronics 

Bootstrap driver

4

Bootstrap driver

L6384E

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 L6384E 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

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

=

Q-----g---a---t--e-

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 200µA, so if HVG TON is 5ms, CBOOT has

to supply 1µC to CEXT. This charge on a 1µF capacitor means a voltage drop of 1V.

The internal bootstrap driver gives great advantages: 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

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:

Vdrop

=

Ich arg eRdson → Vdrop

=

---Q-----g---a---t-e----

Tch arge

Rd

so

n

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.

8/17

Share Link: