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Original question:

LM5175: Problem. unwanted turn off on HS in

LM5175: Problem, Standby current LM5175

Peter Stephenson
Prodigy 190 points
Part Number: LM5175
Other Parts Discussed in Thread: , LM34936, TPS25740B

I have been testing the LM34936 as a possible replacement for the LM5175 and noticed the no load current was rather high. I also have a LM5175EVM which I decided to apply some tests related to the problem with the LM34936.

With a supply voltage of 12V and 5V is requested with a few mA load and then the load is removed, the LM5175 continues to switch and draws 45mA. 

With the supply voltage unchanged, 9V is requested and the small load then removed, the LM5175 stops switching and draws negligible no load current.

If the LM5175 is disabled by shorting the EN to 0V, the LM5175 will commence switching with no load applied and draws, as before, about 45mA.

I did not expect an evaluation board to exhibit this behaviour.

Is there a way of ensuring either the LM5175 ( or an LM34936 ) will revert to the non switching state when load is removed regardless of supply voltage or voltage output requested ? I am currently disabling the I.C. for no load condition but this adds extra components to an already small board.

  • 0
    TI__Mastermind 21720 points
    Hi Peter,

    Thanks for reaching out with your questions.

    How are you measuring the input current during light load operation? Are you measuring the current on the VIN pin or just from the input terminals of the board? If you are measuring from the input terminals the leakage current from the capacitors and the UVLO resistor divider need to be taken into account.

    Also how is the board configured? Are you operating in diode emulation mode or inf forced PWM mode? I'm guessing that you are operating in diode emulation mode.

    When the EN pin is pulled low (<0.7V) the VIN pin should be pulling less than 10uA of current. This also means that the output voltage will not be regulated and there will be no switching.

    During a no load condition where the output voltage is still regulated the VIN pin is going to pull no more than 4mA of current. However since the MOSFETs are active the gate driver will need current to drive the MOSFETs on and off.

    Please let me know if you have any questions.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 190 points

    Hi Garrett,

    Thanks for responding so quickly. I only discovered this by chance when putting a TPS25740BEVM through some tests. The current was measured at the supply ( VIN ) and this was not changed. What did change, was the LM5175 deciding to either keep switching or not when the Type-C was removed.

    We are switching to the LM34936 anyway but was using the TPS25740BEVM as a test bed for using the ENSRC ( pin 11 ) to act as EN for the LM5175 via an FET. and inputting aux. power to VDD on the TPS25740B. I think the LM34936 will need to be disabled for no load.

    What had caused a problem, which may well be related, was slight overshoot upon start-up ( 24V bucked down to the requested 5V ) where the TPS25740B decided to switch off and turn off the EN signal. This reduced the signal time for EN to a repeating pulse of 34mS. Increasing the soft start has cured this and for now, there is no longer a problem.

    Any further comments will be greatly appreciated. You may wish to tell me what the TAG is about though.

    Regards, Peter