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3V to 20V (150 mA) Boost Converter using LM2735

Francois Birot
Prodigy 40 points

Hello,

I'm using operational amplifiers on my circuit that require -10/+10V. Since I want my circuit to be powered by a lithium battery, I was looking for a boost converter that accepts a Vin of 3V and gives me a Vout of 20V (of at least 120 mA). According to the webbench, the LM2735Y ic seems to be a good match.

I've thus tested the following design (from Webbench) on a prototype board:

http://www.biroweb.com/elec/SchematicLM2735Y.png

While I do get a 20V output, the maximum output current seems to be extremely low. As soon as I try to power an op amp from the +20V output, the LM2735Y heats excessively and Vout drops to the same voltage as Vin. I've tested different LM2735Y but got the same behavior each time.

I'm using a bat43 schottky diode, a ceramic 150pF capacitor, and two aluminium electrolytic capacitors (thus polarized) respectively for the 10uF and 22uF capacitors required.

Any idea of what I'm doing wrong ?

  • 0
    TI__Guru 72545 points

    The layout of the PCB is a critical part of the converter.  If you are using the MSOP or WSON package, be sure to fully solder the DAP to the ground plan.  Also, please follow the layout guudline in the data sheet.  I would use ceramic capacitors for input and output.  The BAT43 looks a little weak in this application; is that the diode that Webench suggetsted?  Send us your layout and we will look at it.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Thank you for your answer. I'm using the SOT-23 package that I have hand soldered to wires plugged in a prototype board. I would actually like to test the component before ordering PCBs.

    I've tried to follow the guidelines as much as possible on a prototype board, but using hand soldered wires would indeed explains a not very well regulated output. Yet, I don't think it would explain that it doesn't work at all.

    I will try to find ceramic capacitors for both the input and output and get a different diode, closer to what is suggested on webbench (I can't take the same ones since the suggested versions are SMDs).

    I'll update this thread as soon as I have received and tested the new components.

    Thanks for your help

  • 0 in reply to Francois Birot
    TI__Guru 72545 points

    You can order an EVM from our website that has a good PCB for you to look at.  Or I can send

    you one free of charge from our laboratory stock.

    FD

     

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Thank you, that would indeed be great if you could send me an EVM from your stock. However will the EVM permits me to support a 20V / 150 mA output ? Could you send me an email, so that I can answer you and give you our address in private ?

  • 0 in reply to Francois Birot
    TI__Guru 72545 points

    My email is frank(dot)de(dot)stasi[at]ti(dot)com you can send your info to that email.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Thanks again for the EVM. It works great and I am able to get a 20V output adding another resistor in parallel.

    I'll thus use the same components and design to integrate it on a PCB.

    Thank you for your help!