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UCC3808-2: UCC3808 Slope compensation

David Cano30
Prodigy 210 points
Part Number: UCC3808-2

Hello everyone,

I am using the UCC3808 controller in a Push-Pull Topology. As the duty cycle at steady state would be greater than 50% it is almost imperative the use of slope compensation. The circuitry that I am using to achieve that goal is shown next:

According to the datasheet of the 3800 (not the 3808) the timing resistor Rt could be broken up into two series resistors to bias up the NPN follower.

I would like to know if you consider that mandatory to the 3808 case. The voltage at the base should be around 1V, is not it?

(Page 23, http://www.ti.com/lit/ds/symlink/ucc3800.pdf )

Thank you for reading.

Best regards.

David.

  • 0
    TI__Expert 3095 points
    Hi David Cano30,
    They are correct. If you want to have a nice uniform ramp signal at the emitter of the transistor then it is necessary to split the RT resistor. If you do not then at the bottom of the ramp there will not be sufficient voltage to forward bias properly the VBE drop of the transistor.
    Fortunately for adequate slope compensation you do not need a nice uniform ramp signal from the start of the ON pulse. It is only necessary to have a uniform ramp of the correct slope from the 50% duty cycle point onwards. Hence I am pretty confident that you can get away without splitting the resistor.
    I hope this helps.
    Joe Leisten