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TPS61081 DC to DC Boost Converter

Steven Medina
Prodigy 50 points
Other Parts Discussed in Thread: TPS61081, TPS61087

Hello,

I have built a circuit that converts 3.3V to 18V using the TPS61081 IC. My problem is that when I put a load across it, the output voltage drops to 5.5V and low current at 40mA. One thing that I didnt use were the recommended capacitors but would that cause the internal switching?

  • 0
    TI__Guru**** 150690 points

    Yes, capacitors are absolutely required on the TPS61081.  Could you post a schematic of your circuit?

    Also, make sure that you followed the layout recommendations in the datasheet.  You can order the EVM to test the IC on a known good layout.

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Chris,

    I did use capacitors, just not the ones recommended. The datasheet suggested low ESR capacitors. For budget purposes, I did not purchase those, instead I used ceramic. About the EVM. This is a project that I'm working on for school so I cant afford to buy an EVM.

    Also, the same problem occurred using the TPS61087. So for both IC we get the 18V except when we put a load on it, it goes through some internal switching. I am trying to accomplish at least 15V @ 300mA output. The schematic that I used is on the datasheet, top of page 17. http://focus.ti.com/lit/ds/symlink/tps61081.pdf

     

     

  • 0 in reply to Steven Medina
    TI__Guru**** 150690 points

    Ceramic capacitors are low ESR capacitors, so no worries there.  Which schematic are you referring to?  Figure 21?  I assume you didn't copy it exactly, so what changes did you make to it? (it is setup to generate 12V out).

    What do you mean by "it goes through some internal switching"?  Can you attach waveforms of Vin, Vout, L and SW when it does this?

    Can you post your board layout?  Unfortuneately, these high frequency power supplies cannot be implemented on the regular breadboards that we all used in school.  You must have a PCB for them to work properly.

    Make sure and check the current limit on the 3.3V input supply.  To get 15V at 300 mA, you will need to be able to supply at least 1.7 or 2A.

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Yes, figure 21. Correct, I did not copy the same values, I calculated new values with the formulas given in the data sheet. I made changes to get 18V. 

    I'm not sure if its going through some sort of internal switching, as I read the datasheet it stated that it switches from high to low. I can work on getting those waveforms for you. 

    I currently have it built on a through hole board. My next step was to create a PCB layout if it worked. 

    For that 1.7 and 2A current, is that the current constant or peak? I order the PCB layout and see if i get the same results.

  • 0 in reply to Steven Medina
    TI__Guru**** 150690 points

    Ordering the EVM will be a simpler solution than making your own PCB.  They are in stock and ship overnight.  If you do your own layout, make absolutely sure that you follow the layout recommendations given on page 16 of the datasheet.  Your circuit may not work if you do a bad layout.

    The average input current can be determined from your output power and efficiency.  Output power = Vout * Iout = 18 * 300mA = 5.4W.  Assuming 80% efficiency, this is 6.75W on input power.  At 3.3V, this is over 2A of input current.  The peak current will be higher due to the ripple current in the inductor.

    The average current calculated above is greater than the IC's current limit.  This is discussed on page 12 of the datasheet.  This IC will not be able to provide 18V at 300 mA.