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LM46000 sticking in PWM until Vin raised (even with v. light load)

Paul Crouch
Prodigy 20 points
Other Parts Discussed in Thread: LM46000

Hello,

I hope someone can help with this.  I'm using the LM46000 to produce a +3.8V output from an input range of +5V to +30V.  If Vin starts fairly low (say 7-8V) the circuit sticks in PWM mode and its current consumption (even with load disconnected) never goes below 5-7mA.  If, leaving everything else untouched, Vin is raised the circuit suddenly enters PFM mode (at a Vin value of around 12-15V in our case) and current consumption becomes acceptable (around 70 microamps).  If Vin is now lowered back to a low value the circuit stays in PFM mode and current consumption is low ... exactly the behaviour mentioned near the top of page 16 of the datasheet.

We need to somehow avoid this behaviour.  The circuit's output voltage has no trouble reaching the programmed +3.8V output so I don't understand the datasheet comment "... the LM46000 may not enter PFM mode if the output voltage cannot be charged up to provide the trigger to activate the PFM detector."  What isn't being "charged up"?

FYI, the circuit values came from the WebBench online application; L=120uH, R(RT)=33k (so around 1MHz PWM frequency), CFF=10pF ....

Thanks a lot in advance - Paul.

  • 0
    TI__Genius 11085 points

    Paul,

    The behavior is known in this family of parts. But it should be able to get out at a much lower Vin.

    The inductor value is too high in your design. I need to double check with Webench to see how come it is so large.

    Based on the table on page 26 of the datasheet. You should be able to use 15uH inductor, and Cff depends on your Cout type.

    If you need to further lower Vin for PFM, use lower switching frequency helps.

    The description on the datasheet means Vout needs to be slightly higher than regulation (~1%) to trigger the PFM operation. At lower Vin, the peak to peak current is too small, and the parasitic capacitor on SW node charging and discharging consumed the small positive charges. So Vout stays in regulation, no trigger to PFM. When Vin is higher, more positive charges every cycle. Then Vout being charged up a little bit to trigger PFM, then PFM keeps Vout about 1% higher than regulation.

    Hope it helps.

    Yang

  • 0 in reply to YangZhang
    Prodigy 20 points
    Hello Yang,

    Many thanks for your reply. I double-checked the values from WebBench and they still agree - I think WebBench is taking into account the fact that our system has a wide range of Vin (5 to 30V) and it selecting L based on the worst-case end of this range. I'm planning to try using the PGOOD pin to apply a small step change to the voltage divider network to see if I can make the device ramp up to a voltage slightly higher than we need initially and then step back to the "real" +3.8V voltage we need. I'm hoping this will trigger the PFM detector and kick the device into PFM mode (I know there are risks with this - I'll make careful observations).

    Thanks again and best regards - Paul.