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TPS54341: Power management forum

Part Number: TPS54341

I already have a case open   CS0175983

I'm trying to understand why the SS does not appear to be working correctly.  

Conditions:
VIN = 24V  (ramps from 0-24V in 9ms)
VOUT = 15V
Css = 22nf  (looking for a 10ms ramp from 0 to +15V in 10ms)
EN pin = floating
Cout = 60uF

When I apply +24V - Vout starts to ramp up to 15V but as it reaches +5V (in ~2ms) the output voltage just jumps to +15V without finishing the ramp.  I have experimented with increasing Css to 200nf and while the Vout ramp is longer the result is the same... when Vout reaches 5V it then jumps directly to 15V.  You can see these scope traces by looking at the case file# provided above.   I took several scope traces of the EN pin, SS/TR pin and FB pin.  I have run WEBENCH and TINA and they both behave as I would expect them to (with a monotonic ramp from 0 to 15V in the time I would have expected it to).  I have also included in the case reports a copy of the circuit.  Layout can also be provided if needed.  I have several boards with this new part and they behave exactly the same.   I had been using the TPS54335 which has performed very well but I needed to change because our input voltage requirement increased above the TPS54335 rating.  I've been struggling with this issue for almost a week and need to resolve it or move to a different part.  Please feel free to reach out and call 717-496-5400.  

Thanks for your support

Bill

  • Hi Bill,

    I can't find out CS0175983.

    Can you upload the circuit schematic and waveform of VIN, SS, SW and Vout?

    Do you have bias load during Vout ramp up?

    B R
    Andy

  • Ignore CH3 on this scope grab....    

    CH3 = SS/TR pin

    CH3 = EN pin

    CH3 = FB pin

    Regarding the output load... 
    The original load was ~200ma   .... I increased it to 1A with same results...

  • Hi Bill,

    CH4 is 24V input current?

    It doesn't make sense that the input peak current is up to 4A, but load current is only 200mA, 

    Can you double check your circuit design and PCB assembly? there must be something wrong when Vin ramps up to 14V

    B R

    Andy

  • Hello Andy,

    CH4 is the 24V input current  -  and it does make sense if you consider that when the input voltage (CH1) reaches ~15V the output voltage (Ch2) quickly ramps from 5V to 15V  -  the output filter capacitors need to be charged.... So the 4 amp spike in current is basically a output capacitor charging transient.   This is precisely why I was hoping the soft start function would work so that we could limit this charging transient.
    Respectfully, Bill
  • Hi Bill,

    TPS54341 is a switching regulator, assume the eff is 85% for 15Vin to 5Vout/3A,

    if input current peak is 4A, then output current peak will be : 15x4*0.85/5=10.2A,

    The averaged charging current is 10.2/2=5A and duration is 0.8ms. 

    So output voltage step is 5A*800us/60uF=66V !!

    that's why we think it doesn't make sense, pls double check the circuit components and assembly.

    B R

    Andy