AN993 datasheet(13/27 Pages) STMICROELECTRONICS

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AN993

Device block description

Doc ID 5656 Rev 10

13/27

3.3

Bootstrap section

Bootstrap circuitry is needed to supply the high-voltage section. This function is normally

accomplished by a high-voltage fast-recovery diode (Figure 11). In the L6574, a patented

integrated structure replaces the external diode. It is realized by means of a high-voltage

DMOS, driven synchronously with the low-side driver (LVG), with a diode connected in

series, as shown in Figure 12. An internal charge pump (Figure 12) provides the DMOS

driving voltage.

The diode connected in series to the DMOS has been added to avoid it being unintentionally

turned on.

3.3.1

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 total gate charge of the MOS.

Equation 12

The ratio between the capacitors Cext and Cboot is proportional to the cyclical voltage loss. It

has to be:

Equation 13

For example, if Qgate is 30 nC and Vgate is 10 V, Cext is 3 nF. With Cboot = 100 nF, the drop

would be 300 mV.

Figure 10.

Startup timing diagram and EN2 function

AM01317v1

V(Cpre)

EN2

TPRE

VSupply

Time

ignition

fMAX

fMIN

preheating

ignition

Foult-ignition

preheating

TIGN (TSH)

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