L6491D datasheet(18/24 Pages) STMICROELECTRONICS

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Bootstrap driver

L6491

18/24

DocID024832 Rev 1

Bootstrap driver

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

accomplished by a high voltage fast recovery diode (Figure 9). In the L6491 an integrated

structure replaces the external diode.

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

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

the leakage and quiescent losses.

HVG steady-state consumption is lower than 120

A, so if HVG TON is 5 ms, CBOOT has to

supply CEXT with 0.6 C. This charge on a 1 F capacitor means a voltage drop of 0.6 V.

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 SGND (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 RDS(on) (typical value:

175

). At low frequency this drop can be neglected. Anyway, the rise of frequency has to

take 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.

CEXT

Qgate

Vgate

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

CBOOT>>>CEXT

Vdrop

Ich e

arg RDS on



Vdrop



Qgate

Tch e

arg

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



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