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UCC2804PW BiCMOS Current-Mode PWM Controller Timing Resistor

calvin payne
Prodigy 10 points

I'm using the controller in the subject line for a fly-back converter with a lower than suggested value for the timing resistor 6.8kΩ versus 10kΩ min which I hadn't noticed that in the datasheet during the design.  I wondered what the limitation is due to, and what issues could develop as a result of using a smaller value than 10kΩ.  The oscillation frequency is programmed to be about 550kHz using a 390pF cap (switching frequency is half that).

Thanks!

  • 0
    TI__Genius 9130 points
    Calvin,
    It's not clear from the datasheet where the 10 k limitation comes from. But as the Rt value is decreased, this will increase the load on the VREF pin, and will also cause the internal discharge transistor inside the CT pin to get more heavily loaded - it will have to handle a larger peak current, since it has to discharge the Ct cap and also sink current from VREF through Rt. Since the freq is increasing with smaller Rt, the rms current into the CT pin will also increase a lot, besides the peak.
    However, since the IC is rated to operate at up to 1 Mhz, you can still easily achieve your target 550 kHz using an Rt of 10 k, by dropping the Ct cap vlaue down to approx 270 pF. This design will then comply with the TI datasheet.
    Thanks,Bernard
  • 0 in reply to Bernard Keogh
    TI__Genius 15910 points

    The minimum value for the external timing resistor is often limited by the internal Rdson of the devices used in the oscillator circiut.  The external resistor is recommended to be much much greater than the internal device impedance so that there will be very little switching frequency variation or drift as the IC warms up because the external resitor value will always dominate.  The internal PMOS structures used for the oscillator circuit often have fairly large Rdson values that vary widely with temperature and current.  It's always best to stick with the IC manufacturer's recommendations.