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What size capacitor is needed across the charge pump LCDCAPx pins for the FR4133 LCD_E Module??

Nick Jensen
Prodigy 20 points

The datasheet says 0.1uF but the EXP430FR4133 Launchpad Schematic shows a 1.0uF cap on that pin.  In both cases, a 0.1uF cap is specified for the three LCD resistor I/O pins

  • 0
    Guru 25210 points
    Personally I would use the value recommended in the datasheet - 0.1uF
  • 0
    TI__Guru* 83531 points
    Hi Nick,

    I checked the datasheet and it says 0.1uF across LCDCAP0/1, as well as the required 0.1uF caps on R13/R23/R33. However, I can see as well in the Launchpad schematic that 1uF was used. While I would be inclined to say the datasheet should be correct, I will consult with the team about this discrepancy to confirm why there is this difference. Thanks for pointing it out, and I'll let you know as soon as I have an answer.

    Regards,
    Katie
  • 0 in reply to Katie Pier
    Guru 25210 points
    When talking about charge pumps, there is a minimum capacitance necessary to transfer the electric charge. I would assume that the 0.1uF value specified in the datasheet is a conservatively-derated minimum.

    When layout out a board, a designer will try to minimize the number of unique parts on the Bill Of Materials. Generally for an application like this, there is nothing wrong with bumping up a 0.1uF to a 1.0uF capacitor. I suspect that's what they did on the launchpad for cost reasons.
  • 0 in reply to Brian Boorman
    TI__Guru* 83531 points
    Hi Brian,

    This too would be my suspicion, but I want to double-check with the team. It seems odd to me that they would use the larger value when there are already the 3 other required caps at 0.1uF (the datasheet recommendation would have had them all at the same size). I want to see if there's any other reason they made this decision, and just to double-check that the datasheet is correct while we're at it.

    -Katie
  • 0 in reply to Katie Pier
    TI__Guru* 83531 points

    After talking to the team that designed the Launchpad, it sounds like they experimented with a few different size capacitors with the particular display used on the Launchpad, and found that the larger capacitor helped them to achieve better contrast with this particular LCD glass. If you already have a particular LCD in mind, it might be a good idea to wire it up and do some of your own tests with different values to try to see what gives you the best result (with a minimum of 0.1uF). This kind of testing is a good thing to do for an LCD design anyway, because LCDs can need a lot of fine-tuning depending on the display to get the desired contrast/appearance that you are looking for - it would let you experiment with other things as well (such as whether you can use the internal bias caps or need to use external ones) before locking in on a final design. You can find more information on different things that affect contrast and LCD appearance in the app note www.ti.com/lit/pdf/slaa654
     
    Regards,
    Katie

  • 0 in reply to Katie Pier
    Prodigy 20 points

    I just noticed that the LCD_E section of the Famly Guide does address this question on page 385: "When the internal charge pump is used, a 100-nF or larger capacitor must be connected between the LCDCAP0 and LCDCAP1 pins."

  • 0 in reply to Nick Jensen
    TI__Expert 7015 points

    Hi, Katie and Nick,

    Yes, 0.1uF or larger capacitor should be for LCDCAP0 and LCDCAP1 pins.

    The selection guide for the capacitor value: 0.1uF can get lowest power consumption for LCD_E. But with low charge pump frequency, the 0.1uF may result in LCD contrast not very good. For this case, increasing the capacitor value can help. So it is a tradeoff between LCD contrast and low power consumption for LCD. You can select appropriate capacitro value based on the charge pump frequency setting, LCD contrast and the power consumption requirement.

    Best Regards,

    Lixin

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