Datasheet

ManualsBrandsSTMICROELECTRONICS ManualsPMICsPMIC - gate drivers Non-inverting High side, Low side, Sychronous SO 8

Input logic L6388E

8/19 Doc ID 13991 Rev 3

5 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).

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 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.

6.1 C

BOOT

selection and charging

To choose the proper C

BOOT

value, the external MOSFET can be seen as an equivalent

capacitor. This capacitor C

EXT

is related to the MOSFET total gate charge:

Equation 1

The ratio between the capacitors C

EXT

and C

BOOT

is proportional to the cyclical voltage loss.

It must be:

BOOT

>>>C

EXT

E.g.: if Q

gate

is 30 nC and V

gate

is 10 V, C

EXT

is 3 nF. With C

BOOT

= 100 nF the drop is 300

mV.

If HVG must be supplied for a long period, the C

BOOT

selection must also take the leakage

losses into account.

E.g.: HVG steady-state consumption is typical 250 μA, so, if HVG T

is 5 ms, C

BOOT

must

supply 1.25 μC to C

EXT

. This charge on a 1 μF 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 V

OUT

is close to GND (or lower) and, at the same time, the

LVG is on. The charging time (T

charge

) of the C

BOOT

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 R

DS(on

)

(typical value:

125 Ω). This drop can be neglected at low switching frequency, but it should be taken into

account when operating at high switching frequency.

EXT

gate

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