L6388ED(2015) View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
L6388ED Datasheet PDF : 18 Pages
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L6388E
6
Input logic
Input logic
Input logic is provided with an interlocking circuitry which avoids the two outputs (LVG, HVG)
being 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).
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 5 a). In the L6388E 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 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 an
undesirable turn-on.
CBOOT selection and charging
To choose the proper CBOOT value, the external MOSFET can be seen as an equivalent
capacitor. This capacitor CEXT is related to the MOSFET total gate charge:
Equation 1
CEXT = Q-V----gg---aa---tt--ee-
The ratio between the capacitors CEXT and CBOOT is proportional to the cyclical voltage loss.
It must 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 is
300 mV.
If HVG must be supplied for a long period, the CBOOT selection must also take the leakage
losses into account.
E.g.: HVG steady-state consumption is typical 250 A, so, if HVG TON is 5 ms, CBOOT must
supply 1.25 C to CEXT. This charge on a 1 F capacitor means a voltage drop of 1.25 V.
The internal bootstrap driver offers important advantages: the external fast recovery diode
can be avoided (it usually has a high leakage current).
This structure can work only if VOUT is close to GND (or lower) and, at the same time, the
LVG is on. The charging time (Tcharge) of the CBOOT is the time in which both conditions are
fulfilled and it must be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS RDS(on) (typical value:
125 ). This drop can be neglected at low switching frequency, but it should be taken into
account when operating at high switching frequency.
DocID13991 Rev 5
9/18
18
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