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UCC28742: UCC28742

Naizeng Wang
Expert 5380 points
Part Number: UCC28742

Hi team,

I have several questions below about UCC28742:

(1) Vs pin can compensate the current-sense threshold across the AC-input range. What's the meaning?

(2) The gate drive turn-off current is internally limited to about 400 mA when DRV is above about 4 V. At lower DRV voltages the current will reduce, eventually being limited by the low-side on resistance, RDS(on). Why driving current is affected by Rdson?

(3) If additional blanking time is needed, a small bypass capacitor, up to 30 pF, can be placed on between CS pin and GND pin. How to explain bypass capacitor can influence blanking time?

Thanks.

  • 0
    TI__Guru* 85616 points

    Hello,

    1.  The IVLS current when the switch is on is multiplied by IVSLS/KLC and is source out of the CS pin.  This current goes through RLC resistor and the current sense resistor (RCS).   VCST = Ivls*(Rlc+Rcs)+ Ipeak*Rcs.

    • Reviewing the block diagram and the applications schematic will help understand where this equation comes from.
    • The RLC resistor is used to ensure the gate drive turns off at the same peak current no matter what the input voltage to the flyback converter is.

    2. You are correct the Rdson of the pull down FET will limit the current.

    3. There is current sense leading edge blanking in this device of 270 ns.  If you have noisy system you might want to add capacitance to the CS pin to ground in combination with  the RLC resistor will form a low pass filter.  The filter pole = 1/(2*3.14*CCS*RCS) 

    Regards,