• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 170 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5085: Effects of pulling RT pin to about 2.5V through 3k

Scott Willis
Prodigy 130 points
Part Number: LM5085

Hi, I inherited a DAC-controlled power supply that uses the LM5085.  RT (from RT pin to VIN) is 383k.

The DAC control voltage:

  • Is buffered to VCTRL_BUFD
  • Fed to FB pin through 2.00k (I get this part; lower VCTRL_BUFD will raise the output voltage)
  • For reasons I don't yet understand, if VCTRL_BUFD is > 2.50V, a comparator turns on a small FET that connects the RT pin to VCTRL_BUFD through 3.01k.

I would very much appreciate some idea of how to expect the LM5085 to behave if the RT pin has 383k to VIN and 3.01k to 2.5..5V.

Thank you!

Scott

  • 0
    Prodigy 130 points
    VIN is 60V.
    There is a separate FET circuit to ground the RT/SHDN pin to disable the power supply.
  • 0 in reply to Scott Willis
    TI__Mastermind 25590 points
    Scott,

    Here's what is going on:

    The LM5085 operates in normal constant-on-time operation while VCTRL_BUFD < 2.5V. RT voltage is about 2V (see figure 8 in the datasheet).

    Then VCTRL_BUFD reaches 2.5V. The FET turns on and connects RT to 2.5V through a 3k resistor which will pull the RT voltage up to 2.5V (not all the way but pretty much 2.5V). This reduces the on-time (see figure 6 in correlation to fig 8) and increases the frequency of operation proportional to VCTRL_BUFD.

    This means as the DAC drives the output voltage lower, switching frequency increases. Increased switching frequency reduces the output ripple and if your ripple spec is stated as a percentage of VOUT, then lower VOUT would require smaller ripple to get the same percentage spec.

    Cool circuit! Let me know if you have any questions.
    -Sam
  • 0 in reply to Scott Willis
    Prodigy 130 points
    Found out this is intended to raise the switching frequency at low output voltages, to reduce output ripple.