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LM5088 instability at no load or Ioal < 40mA

Ron Choy1
Prodigy 90 points
Other Parts Discussed in Thread: LM5088

All,

  I have a board using x4 =>  LM5088 controllers.  I am seeing Vout instabilty at no laod on one switcher, a Vout = 20V, Imax = 5A design. I am seeing Vout instability (approx. 4-6Vpp ripple) for Iload < 50mA.  Also, looking at the SW node I see oscillations between FET on pulses.  I see this on the other three switchers also but their Vout is stable at the set voltage withy no load.  At > 30mA the Vout is stable >500mA the SW node is very clean and crisp, no ringing, even without the snubber .  I have seen many post about no load instability but no solutions.  My layout is very tight and I am slowing down the FETrising edge gate dirve with a 20 ohm res. and diode/0 ohm for the turn off.  I have increased the compensation and loop gain to shift out the zero crossing, using anPSPICE  average macro encripted LM5088 model also indicates instability at very load load (don't really trust the encripted model anyway).  The design components are per the spec. sheet design methods.

Any  ideas ??

  • 0
    TI__Guru 59001 points

    Hi 

    First, please check whether the issue is caused by low Cboot voltage or not. Easiest way is to connect an external 8V battery between SW and BOOT and see any improvement. 

    Regards,

  • 0 in reply to EricLee
    Prodigy 90 points

    Eric,

      A 9V battery across Cboot does help greatly, I believe your 8V batt is a typo.

    Once I saw your orgional messgae I checked/monitored VCC and VCboot during the no load instability and saw droop mainly on VCC and low Vcboot so I  increased Cvcc from 2.2uF to  10uF and Cboot from .1uF to 2.2uF =>  this mitigated the VCC droop and increased the lower VCboot voltage a bit.  I change the loop comp. a bit and that also helped to a point now where I have regulaiton/stability at no load or at least to good regulaiton down to Vin = 26V with a 5mA dummy load .  My  Vin range should be from 23-39V, with Iload range from 0 to 5A.

    Now, sometimes,  I still get some instability on power up at no load low Vin, etc.

    Lets discuss more during our 11am conference call.  I send schematics to Hector and David.

    Thanks ERIC King => I used Ron Choy forum login as he was logged in on this computer at the time I wrote the original message.