DocID030402 Rev 2

11/18

L6399

Bootstrap driver

18

7

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 L6399 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 7. An internal charge pump (Figure 7) 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 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

A, so if HVG TON is 5 ms, CBOOT

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

The internal bootstrap driver gives a great advantage: the external fast recovery diode can

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

This internal diode 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 equivalent resistance of the

internal diode RDSon (typical value: 120 ). 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

where Qgate is the gate charge of the external power MOS.

CEXT

Qgate

Vgate

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

=

Vdrop

Ich e

arg

R



BOOT

Vdrop



Qgate

Tch e

arg

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

R



DSon

==