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WEBENCH® Tools/TPS55340: Does Webench account for DC bias derating of Cout when it calculates Rcomp/Ccomp?

Matthew Findlow
Intellectual 880 points
Part Number: TPS55340

Tool/software: WEBENCH® Design Tools

Hi,

Webench parameters:- Vin(min)-10V  Vin(max)-14V  Vout 24.5V Iout 0.37A (370mA)  TPS55340 at 562.300KHz 47uH 96%

Q1:  Webench always puts an electrolytic capacitor for the input capacitor, but the data sheet recommends ceramic.  When I try to change the component the alternative parts list only shows more electrolytic capacitors.  I can't change it for a ceramic.  Why?  Is it safe to ignore this and just put a ceramic in my actual design.

Q2:  Does Webench account for DC bias derating on its output caps?  In the above design Webbench uses a 10uf in 0805 package.  This cap will probably only give 2uf at output voltage of 24.5V.  The data sheet warns to account for DC bias when selecting Cout, but It doesn't appear that Webench does.  Is Webench accounting for DC bias when it calculates Rcomp and Ccomp, or is it calculated using 10uf?

Q3:  The data sheet recommends to include a 100uf bulk input capacior if the supply is more than a few inches away from the IC.  My application is battery powered, from a large-ish 7AH 12V battery through 1 meter of 24AWG cable.  The boost converter powers 7 LEDs in series, with very infrequent load changes.  Can I dispense with the recommended 100uf bulk input capacitance? I was thinking of increasing the recommended 4.7uf ceramic input to a 10uf 1206 35V and leaving the 100uf ali cap off the board. Anyone have experience  with this kind of battery application?

Cheers.

  • 0
    TI__Intellectual 1370 points
    Hi Matthew,
    Thanks for reaching out to us.

    For Q1, notice that the VDC Min rating on alternate parts page is 28V. Reducing VDC Min to 14V or even 20V, you may be able to ceramic cap options. However, it is recommended to use a cap with higher VDC ratings to reduce the effects of dc bias. Refer to section 8.2.1.2.6 in the datasheet.
    www.ti.com/.../tps55340.pdf
    You may choose ceramic caps with similar ratings as long as your VDC is well rated to take the DC bias effects.

    For Q2 and Q3 our team here is looking into them. We will get back to you soon.

    Regards,
    Pavani Jella
    WEBENCH Applications
  • 0 in reply to Pavani Jella
    TI__Mastermind 35115 points

    Hi Matthew,

    The 100 uF recommendation is a generic statement which applies to all power supplies and most of the time less will be fine. Long wires add inductance and if there is a load step, there can be some oscillation on the input due to the inductance. The recommended value is conservative because we really can't predict what every application will look like in regards to the input supply and the cable connecting it to the IC. You will likely be ok with less but you'll have to test out.

    If you'd like to have a better idea before testing out, you could use a transient simulation to simulate the load step. In this simulation, include the estimated inductance and resistance of the 1 meter of cable between the input supply and the TPS55340 bypass capacitors.

    Best Regards,
    Anthony