L6398 datasheet(11/16 Pages) STMICROELECTRONICS

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L6398

Bootstrap driver

Doc ID 18199 Rev 3

11/16

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). In the L6398 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 diode in series, as shown in

Figure 7. An internal charge pump (Figure 7) provides the DMOS driving voltage.

8.1

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:

Equation 1

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

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

the leakage and quiescent losses.

e.g.: HVG steady state consumption is lower than 190

μA, so if HVG T

ON is 5 ms, CBOOT has

to supply 1

μC to C

EXT. This charge on a 1 μF capacitor means a voltage drop of 1V.

The internal bootstrap driver gives a great advantage: 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:

120

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:

Equation 3

EXT

gate

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

drop

arg Rdson

drop

→

gate

arg

------------------R

dson

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