L6388E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6388E' PDF : 18 Pages View PDF
Input logic
5
Input logic
L6388E
Input logic is provided with an interlocking circuitry which avoids the two outputs (LVG, HVG)
to be active at the same time when both the logic input pins (LIN, HIN) are at a high logic
level. In addition, to prevent cross conduction of the external MOSFETs, after each output is
turned-off the other output cannot be turned-on before a certain amount of time (DT)
(see Figure 3).
6
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 5 a). In the L6388E 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 in series a diode, as shown in
Figure 5 b. An internal charge pump (Figure 5 b) provides the DMOS driving voltage. The
diode connected in series to the DMOS has been added to avoid undesirable turn on of it.
6.1
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 30nC and Vgate is 10V, CEXT is 3nF. With CBOOT = 100nF the drop would be
300mV.
If HVG has to be supplied for a long time, the CBOOT selection has to take into account also
the leakage losses.
e.g.: HVG steady state consumption is lower than 200µA, so if HVG TON is 5ms, CBOOT has
to supply 1µC to CEXT. This charge on a 1µF capacitor means a voltage drop of 1V.
The internal bootstrap driver gives great advantages: 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: 125
Ω). At low frequency this drop can be neglected. Anyway increasing the frequency it must be
taken in to account.
8/18
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