AN4354 datasheet(11/14 Pages) STMICROELECTRONICS

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DocID025239 Rev 2

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AN4354

Gate drivers setup

the overcurrent detection is based on the voltage drop on the on resistance of the MOSFET:

increasing the resistance value the protection is triggered at lower load currents.

This condition can also cause problems during the turn-off of the MOSFET as shown in

Figure 11. At the beginning of the turn off the gate voltage may have reached the VCC value,

in particular for the low-side MOSFETs where the gate current is not limited at the end of the

tCC time (see Section 1.2: Turn-on sequence on page 5 statement). Starting from VCC the

gate voltage is decreased down to the Miller plateau region, but the gate charge is not

enough to complete the plateau and the Miller clamp is closed before the Vds commutation

is ended. The MOSFET is immediately turned off and as a consequence the output of the

power stage is subjected to a strong slew rate which could cause critical issues.

In the second case the gate charge is not enough to complete the Miller plateau region (see

Figure 12). In this case the output commutation is not completed and the overcurrent

protection, which measures the voltage drop on the MOSFET, is triggered even if very low

or even no load current is present.

Figure 10. Turn-on with low gate charge

AN4354 Datasheet(PDF) 11 Page - STMicroelectronics