L6395 datasheet(11/16 Pages) STMICROELECTRONICS

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L6395

Bootstrap driver

A bootstrap circuitry is needed to supply the high voltage section. This function is normally

accomplished using a high voltage fast recovery diode (Figure 6). In the L6395 device

a patented integrated structure replaces the external diode. It is implemented using a high

voltage DMOS, driven synchronously with the low-side driver (LVG), with a diode in series,

as shown in Figure 7. An internal charge pump provides the DMOS driving voltage.

CBOOT selection and charging

To select 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

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

300 mV.

If HVG needs to be supplied for an extended period, the CBOOT selection has to take into

account also the leakage and quiescent losses.

E.g.: HVG steady state consumption is lower than 220

A, so if HVG TON is 5 ms, CBOOT

must supply 1.1

C to CEXT. This charge on a 1 F capacitor means a voltage drop of 1.1 V.

The internal bootstrap driver offers some 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 has to be long enough to charge the capacitor.

The bootstrap driver introduces a voltage drop due to the DMOS RDS(on) (typical value:

120

). At low switching frequency, this drop can be neglected but, operating at high

switching frequency, it should 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 MOSFET, RDS(on) is the on-resistance

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

CEXT

Qgate

Vgate

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

CBOOT CEXT

Vdrop

Ich e

arg

RDS on



Vdrop

Qgate

Tch e

arg

------------------- RDS on









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