AN4354 datasheet(9/14 Pages) STMICROELECTRONICS

9 / 14 page

DocID025239 Rev 2

9/14

AN4354

Gate drivers setup

2.1

Gate current (IGATE), controlled current time (TCC) and turn-

off boost time (TBOOST)

The gate current and the controlled current time define the charge which is forced into the

MOSFET gate through the following formula:

Qgate = Igate × tcc

This value should be greater than the total gate charge (Qg) of the MOSFET when the gate-

source voltage is equal to VCC. The relation between the total gate charge and the gate

voltage is usually reported on the MOSFET datasheet as a graph.

Keeping constant the total gate charge, the switching time of the MOSFET is adjusted

through the tcc parameter (see Figure 8). Higher gate current values allow shorter switching

time reducing the power dissipation of the output stage. In this case the EMI can be

significant. Reducing the gate current the power dissipation is increased due the longer

switching time, but the EMI are lower.

Figure 8. Gate current vs. controlled current time

The boost time (tboost) can be used to shorten the time needed to reach the plateau region.

This way the commutation is faster, but the output slew rate is still controlled by the

programmed gate current. The maximum duration of the boost time is determined by the

Qgs and the Qgd (see Figure 9): the charge sunk by the gate driver during the boost time

(tboost × Iboost) must be lower than the charge required reaching the plateau region

(Qg - Qgd - Qgs). Their value depends on the maximum load current and supply voltage of

the output stage.

AN4354 Datasheet(PDF) 9 Page - STMicroelectronics