AN4043 datasheet(33/60 Pages) STMICROELECTRONICS

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AN4043

Electrical characteristics and functions

Doc ID 022726 Rev 2

33/60

Considering the limit cases during the PWM control and further leakages and dispersions in

the board layout, the capacitance value to use in the bootstrap circuit must be selected two

or three times higher than the CBOOT calculated in the graph of Figure 21. The bootstrap

capacitor should be with a low ESR value for a good local decoupling, therefore, in case an

electrolytic capacitor is used, one parallel ceramic capacitor placed directly on the SLLIMM-

nano pins is strictly recommended.

2.3.13

Initial bootstrap capacitor charging

During the startup phase, the bootstrap capacitor must be charged for a suitable time to

complete the initial charging time (tCHARGE), which is, at least, the time VCBOOT needs to

exceed the turn-on undervoltage threshold VBS_thON, as already stated in Equation

9Equation 9. For a normal operation, the voltage across the bootstrap capacitor must never

drop down to the turn-off undervoltage threshold VBS_thOFF throughout the working

conditions. For the period of startup, only the low-side IGBT is switched on and, just after

this phase, the PWM is run, as shown in the following steps of

Figure 22:

●

t1: the bootstrap capacitor starts to charge through the low-side IGBT (LVG)

●

t2: the voltage across the bootstrap capacitor (VCBOOT) reaches its turn-on

undervoltage threshold VBS_thON

●

t3: the bootstrap capacitor is fully charged; this enables the high-side IGBT and the

CBOOT capacitor starts to discharge in order to provide the right IGBT gate charge. The

bootstrap capacitor recharges during the on-state of the low-side IGBT (LVG).

Figure 22.

Initial bootstrap charging time

The initial charging time is given by

Equation 12 Equation 12 and must be, for safety

reasons, at least three times longer than the calculated value.

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