AN2785 datasheet(10/51 Pages) STMICROELECTRONICS

10 / 51 page

Bootstrap driver

AN2785

10/51

Doc ID 14785 Rev 1

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 6 a). In the L6393 a patented

integrated structure replaces the external diode. It is realized with a high-voltage DMOS

driven synchronously with the low-side driver (LVG), with a diode in series, as shown in the b

part of Figure 6 on page 11. An internal charge pump (Figure 6 b) provides the DMOS

driving voltage.

5.1

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 total gate charge of the MOS.

Equation 1

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

loss. It must be:

Equation 2

CBOOT >> CEXT

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 has to be supplied for a long time, the CBOOT selection must also take into account

the leakage and quiescent losses.

For example, since the HVG steady state consumption is lower than 200 µA, if HVG TON is

5 ms then CBOOT has to supply 1 µC to CEXT. This charge on a 1 µF capacitor means a

voltage drop of 1 V.

The internal bootstrap driver gives a great advantage: the external fast-recovery high-

voltage diode can be avoided (it usually has great leakage current). This structure works if

VOUT is close to GND (or lower) while LVG is on. The charging time (Tcharge) of 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). At a low frequency

operation this drop can be neglected, but if the frequency is increased the drop must be

taken into account.

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

Equation 3

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

the bootstrap DMOS and TCHARGE is the charging time of the bootstrap capacitor.

GATE

EXT

DSON

CHARGE

GATE

DROP

DSON

CHARGE

DROP

⋅

→

⋅

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