Bootstrap driver

A6387

8/14

DocID023386 Rev 5

5

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 5 a). In the A6387 device

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 a diode in series, as

shown in Figure 5 b. An internal charge pump (Figure 5 b) provides the DMOS driving

voltage.

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:

Equation 1

The ratio between the capacitors CEXT and CBOOT is proportional to the cyclical voltage loss.

It must be:

C

BOOT

>>>C

EXT

For example: if Qgate is 30 nC and Vgate is 10 V, CEXT is 3 nF. With CBOOT = 100 nF the drop

would be 300 mV.

If HVG must be supplied for a long period, the CBOOT selection must take into account also

the leakage and quiescent losses.

For example: HVG steady-state consumption is lower than 100

A, therefore, if HVG TON is

5 ms, CBOOT must supply 0.5 C to CEXT. This charge on a 1 F capacitor means a voltage

drop of 0.5 V.

The internal bootstrap driver offers a big advantage: the external fast recovery diode can be

avoided (it usually has very high leakage current).

This structure can work only if VOUT is close to GND (or lower) and, in the meantime, the

LVG is on. The charging time (Tcharge) of the CBOOT is the time in which both conditions are

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

The bootstrap driver introduces a voltage drop due to the DMOS RDSon (typical value:

125

). This drop can be neglected at low switching frequency, but it should be taken into

account when operating at high switching frequency.

Equation 2 is useful to compute the drop on the bootstrap DMOS:

Equation 2

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.

CEXT

Qgate

Vgate

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

=

Vdrop

Ich e

arg Rdson

Vdrop



Qgate

Tch e

arg

-------------------Rdson

==