L6392 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6392' PDF : 20 Pages View PDF
L6392
8
Bootstrap driver
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 7 a). In the L6392 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 b.
An internal charge pump (Figure 7 b) 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:
CEXT
=
Q-----g---a---t-e-
Vgate
The ratio between the capacitors CEXT and CBOOT is proportional to the cyclical voltage loss.
It has to be:
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 200 μA, so if HVG TON is 5 ms, CBOOT has
to supply 1 μC to CEXT. This charge on a 1μF 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 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:
Vdrop
=
Ich argeRdson → Vdrop
=
---Q-----g--a---t--e---
Tch arge
Rd
son
where Qgate is the gate charge of the external power MOS, Rdson is the on resistance of the
bootstrap DMOS, and Tcharge is the charging time of the bootstrap capacitor.
Doc ID 14494 Rev 5
15/20
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