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UCC28600: UCC28600 regarding SLOU293C Reference Design Standby Function

Jim Albert
Intellectual 350 points
Part Number: UCC28600
Other Parts Discussed in Thread: UCC25600

I am using the UCC28600 in a design based on the SLOU293C Reference Design (we are not using PFC). I would like to understand the standby function described in the reference design user guide. In section 4.4 "Standby" the reference design document states the following; 

"The UCC28600 supplies the bias power to the PFC and DC/DC converter. When the power consumption
on the ±15V rails falls below approx. 1.5W, the UCC28600 turns off the bias power to the main power
converters and enters low power/standby mode.
During this mode the ±15V is "alive" but the main power rail (50V/25V) is off. When power consumption on
the auxiliary voltages goes up again the main power rail is turned on."

Can someone help me understand how this is accomplished as there is no indication from the UCC28600 datasheet nor the Reference design document on how this functions or is adjusted.

I believe I have a current design that may have an issue with the UCC28600 inadvertently starting in standby since my +/- 15V rails come up fine but the status pin is not going low. As a result the UCC25600 LLC controller is not starting. This occurs on a small minority of power supplies but I need to determine the root cause.

  • 0
    TI__Mastermind 40360 points
    Hi Jim,

    The STATUS output of the UCC28600 responds to the state of the FB input, per section 7.3.2 of the datasheet (page 15).
    Coming out of stand-by state, the FB voltage must rise above the Greenmode OFF threshold, nominally 1.4V. But this threshold parameter can vary from 1.2 to 1.6V over full temperature range and lot-to-lot variations (page 5).

    Perhaps some of the units that appear to power the +/-15V rails but not the LLC have a higher threshold at FB, and Vfb is just barely not reaching it. Please check the FB voltage under your operating conditions where you want the LLC to turn on, and determine if adjustments need to be made to reliably get Vfb > 1.6 in those conditions. For example, maybe the current sense resistor needs to be raised a bit, if the UCC28600 power stage is perhaps overdesigned for its normal power output and the stand-by threshold power level drives Vfb too low.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Intellectual 350 points
    Ulrich,

    Thank you for your comments

    The difficulty is that I cannot repeat the behavior consistently. I find the problem typically occurs on a "cold" start; meaning, if the power supply is off for a long period and then started. Once the power supply has been started once, subsequent starts seem to be normal in that the status pin remains low, the FB pin is about 1.7 volts and therefore the LLC controller powers up. Based on your comments this is very close to the 1.6V threshold.

    I have monitored the FB pin on a"failure" to start (status = high) and I have seen the FB pin go from 5V on start-up to about 600mV immediately. I know this will certainly cause the status to go high as you have pointed out but was not sure "why" the FB was having this behavior during a cold start.

    I have adjusted Rcs to a larger value; Initially it was set to 1.8 Ohms so I adjusted it to 2.0 Ohms but still have the problem. The FB voltage = 1.8V when it powers up correctly but, as before, drops to 600mV on a cold start. Perhaps I need to increase the value more to accommodate the lower power on the +/-15 rails on power-up.
  • 0 in reply to Jim Albert
    Intellectual 350 points
    I recalculated the Rcs (and other values; Rpl, Cvdd etc.) to almost 5 ohms since my power requirement for the +/-15 V supply is only 2.5 Watts. I just did another "cold" start and it seems much happier now. The Vfb exceeds 1.6 volts reliably now. I'll continue testing to verify the problem is completely gone.

    Thanks for the help pointing me in the right direction!