DocID14493 Rev 11

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L6390

Bootstrap driver

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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 11.a). In the L6390 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 11.b. An internal charge pump (Figure 11.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:

Equation 2

CBOOT >>> CEXT

E.g.: 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 time, the CBOOT selection must also take the leakage and

quiescent losses into account.

E.g.: HVG steady-state consumption is lower than 240

A, so if HVG TON is 5 ms, CBOOT

must supply 1.2

C to CEXT. This charge on a 1 F capacitor means a voltage drop of 1.2 V.

The internal bootstrap driver offers important advantages: the external fast recovery diode

can be avoided (it usually has a high leakage current).

This structure can work only if VOUT is close to GND (or lower) and, at the same time, 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:

120

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

account when operating at high switching frequency.

CEXT

Qgate

Vgate

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