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UVLO behavior on TPS54331

Todd Semke1
Prodigy 30 points
Other Parts Discussed in Thread: TPS54331

I'm trying to set the voltage divider to turn off the TPS54331 when the input voltage drops below a threshold.  I've tried using the equations for Ren1 & Ren2 from the datasheet on page 9.

Ren1 = (Vstart - Vstop) / 3uA    (Resister between Vin and En pins)

Ren2 = Ven / ((Vstart - Ven)/Ren1 + 1uA)    (Resistor between En pin and gnd)

I'm trying to set Vstop (shutdown voltage) to 6V.  Vout is 5V. ( Ro1= 10k  , Ro2 =  1.87k )

I set Ren1  1% 333K and Ren2 68K. ( and a couple of other combos )

Anyway, I tested this setup and it does not turn completely off ( no switching)  until VIN level is down to 3.1 V. This is true of both rising Vin and falling Vin. I slowly turn up an external power supply to vin , and there starts some slight switching at 3.1V . As I increase Vin to 3.8V these is continuous switching, although Vout is spikey and less than the 5V set point. Finally , when Vin is raised to 5.3V, then the output is steady 5V as expected and this continoues good VOut of 5V as raise Vin up to 12V input that will normally be used.. However as I lower Vin the output contiunes to behave as described above ( in the Vin rising case). There appears to be no UVLO where the switching should turn off completely if Vin falls below a certain threshold.

 

Can you get the switching to completely stop at a controlled Vin level ?

Todd

Values used ( designators based on the DS) : Ren1 =333k , Ren2= 68k , Css=10nF,

c1comp=1nF, c2comp=27pF, R3comp=32k ,

C1vin=10uF+10nF, cboot=100nF , Ro1=10k, Ro2=1.87k, Lo=12uH , Cout=47uf+10uF+100nF

 

 

 

  • 0
    TI__Guru**** 191445 points

    I'll take a look at it...  if I have to build something it may take a couple days.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    Using Ren1 = 333k and Ren2 = 68k you should get Vstart = 7 V and Vstop = 6 V.  Is it possible you have something miswired?  Can you post some waveforms?  How many have you tried?  If it is only one board, you may have a damaged device.

  • 0 in reply to JohnTucker
    Prodigy 30 points

    It happens on several boards after a test burn in period.

    After turn on , the VIN ( yellow) is lowered down to 3.8V , and the output i( blue) continues to switch

    3073.Doc1.docx

     

     

    Is this normal behavior ?

    Should the switching totally stop on the output at <6V Vin ?

    Could internal circuitry connected to  the enble pin being damaged account fot this behavior ?/

    /

     

  • 0 in reply to Todd Semke1
    TI__Guru**** 191445 points

    There are two UVLO functions for TPS54331.  There is an external EN function that is adjustable, and an internal UVLO on Vin.  The internal UVLO is 3.5 V maximum.  It sounds like the internal UVLO is functioning in your circuit.  Can I see your actual schematic?  You may have something connected wrong.

  • 0 in reply to JohnTucker
    Prodigy 30 points

    The following schematic is slightly outdated.  Now the R160 is 330k  , and R161 is   76k mounted .

     

    Also , I see in the data sheet that the max voltage on the EN pin is 5V. If 12V was put on this pin then I assume it would damage the UVLO circuit functionality ?

  • 0 in reply to Todd Semke1
    TI__Guru**** 191445 points

    Yes the abs max is 5 V.  You can actually pull it up higher as long as your R160 is high impedance (>100 k, or so).  If you have previously pulled it directly up to 12 V, then perhaps you have damaged the IC and now only the internal UVLO is functioning.

  • 0 in reply to JohnTucker
    Prodigy 10 points

    Hi,John.

    I met the same problem with Todd,I'm trying to set the voltage divider to turn off the TPS54331 when the input voltage drops below a threshold. It Seems to be no UVLO where the switching should turn off completely when Vin falls below a certain threshold,but reopened after about 15ms actually.Values used in my circuits are Ren1 =100k , Ren2= 39k,and Vstart = 4.35V and Vstop =4.05V. This is the power down sequence,the green line is the waveform of Vin,and the yellow line is Vout.

    Could you give me some suggestion about it?

  • 0 in reply to tingyun wen
    TI__Guru**** 191445 points

    I t looks like your VIN is rising back above the threshold.  I would recommend that you make the start and stop voltages further apart.  I usually like 1 V of difference.