hi sir
I'm trying to use UCC28950 in PSFB converter.
can i leave pin no.9 (TMIN) unused?
if so, how should i handle this pin?
can it remains floating(connet to nothing) or conneting to gnd?
Hello Kim,
Thank you for your interest in UCC28950.
It is normally not a good idea to leave the input pin floating or NC.
In UCC28950, the voltage at TMIN pin sets a min pulse width which is then compared with the pulse width demanded by the feedback loop at the COMP output.
If the PWM Comp pulse is longer than the TMIN - then the PWM Comp pulse is used to generate the various Gate Drive outputs.
If the PWM Comp pulse is shorter than the TMIN - then the TMIN pulse is used to generate the Gate drive outputs AND the controller enters burst mode of operation.
Leaving the pin NC will result in PWM COMP pulse always longer than the TMIN pulse and will force the device to never trigger burst mode which will affect the light load efficiency also the controller will not support it. In UCC28950 Burst mode cannot be turned OFF.
The Burst Mode can be minimized by using the minimum RTmin value.
The minimum allowable RTmin value is 10kohms from the graph below, and that calculates Tmin to 59.2nsec from the equation 8 of the datasheet.
I hope this answers your question.
Regards,
Sonal
