Other Parts Discussed in Thread: LM5146, , PMP11257, LM5146-Q1
Hi,
I am concerned about SW node ringing when the high side MOSFET turns on. My Vin is 87V (abs max) but I want to use 100VDS MOSFETs if it all possible. I therefore need to carefully ensure there is very limited SW node ringing.
One approach to this as identified in the datasheet is to use a gate resistor up to 2.2R to slow down the high side MOSFET drive. In comparison however the LM5146 datasheet which has an almost identical topology suggests placing the same 2.2R in series with the boost capacitor. This second option seems better to me as is slows down turn on but not turn off, this seems a more power efficient approach. Can you advise?
Also if you can please explain how the gate driver can experience negative voltages as per the LM5116 datasheet version SNVS499H:
"In some applications it may be desirable to slow down the high-side MOSFET turnon time in order to control switching spikes. This may be accomplished by adding a resistor is series with the HO output to the high-side gate. Values greater than 10 Ω should be avoided so as not to interfere with the adaptive gate drive. Use of an HB resistor for this function should be carefully evaluated so as not cause potentially harmful negative voltage to the high-side driver, and is generally limited to 2.2-Ω maximum"
Interestingly a TI reference design PMP11257 uses a 10R resistor which would seem to not adhere to the above datasheet recommendation.
Many Thanks,
Shane