L6388 datasheet(5/11 Pages) STMICROELECTRONICS

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L6388

INPUT LOGIC

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 con-

duction 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 4).

Figure 6. Typical Rise and Fall Times vs. Load

Capacitance

Figure 7. Quiescent Current vs. Supply

Voltage

For both high and low side buffers @25˚C Tamb

C (nF)

100

150

200

250

time

(nsec)

D99IN1054

2468

16 VS(V)

102

103

104

(

µA)

D99IN1055

3.1 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 (fig. 8a). In the L6388 a patented integrated structure replaces the external di-

ode. It is realized by a high voltage DMOS, driven synchronously with the low side driver (LVG), with in series

a diode, as shown in fig. 8b

An internal charge pump (fig. 8b) provides the DMOS driving voltage .

The diode connected in series to the DMOS has been added to avoid undesirable turn on of it.

3.2 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 :

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 200

µA, 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

EXT

gate

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