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WEBENCH® Tools/TPS61099: TPS610997 Input Cap

John Araujo
Prodigy 160 points
Part Number: TPS61099
Other Parts Discussed in Thread: TPS62840

Tool/software: WEBENCH® Design Tools

Hello,

I did a Webench design using the TPD610997. 1.6V to 3.6v input, 5V output at 100mA. The design selected a very expensive input capacitor: a Panasonic  EEHZC1V151P 150uF, 35V cap which costs $2.59. Is there a particular reason why this cap was selected?  If you look at the datasheet, on page 17 it says that a 10uF is normally recommended. I was wondering if there was something in particular about this design that makes this large cap necessary. Thanks you!

  • 0
    TI__Mastermind 21565 points

    Hi John:

    May I know what's the output ripple you expect? If there is no special requirement, then 10uF is ok.

  • 0 in reply to Minqiu Xie
    Prodigy 160 points

    Hello Minqiu,  Thank you for getting back to me. I don't think I have a special requirement on the output ripple. Here's what I'm trying to do. I have an assembly with two AA alkaline batteries in series. This assembly is going to be collecting digital data from a sensor, but the sesnor maybe be at the end of a 400 foot cable! I will use differential RS485 for the digital comms link. The cable will also provide power and ground to the sensor. Since there will be a IR drop in 400' of power and ground cable, I don't think it's a good idea to send the battery voltage down those cables. The battery voltage is too low! Each cable can have about 8 ohms resistance max, so 16 ohms total. So if my sensor draws 50mA,  50mA * 16 ohms = 0.8V. The battery voltage will start off around 3V, but when the batteries are weak, they can get as low as 1.6V. So if I'm losing 0.8V in the cables, that isn't good. So what I was thinking is to boost the battery voltage in the top assembly (where the batteries are) up to 5V using the TPS610997, then send that 5V down the line. At the sensor assembly, I can dropp the 5V back down to 3.3V with a TPS62840.  I know it's inefficient to boost the voltage then buck it back down, but I can't think of another reliable way to get the voltage down to the sensor assembly. If you have any other ideas I'd love to hear them. Thank you very much! 

  • 0 in reply to John Araujo
    TI__Mastermind 21565 points

    Hi John:

    According to your description, TPS61099x is a good choice, and 10uF is OK. 

    What the voltage that end load need? For example, the load voltage minimum is 3V. Then you could make the output voltage of Boost converter higher than the minimum plus the cable loss, as 3V+0.8V. And better add some margin. If the load need supply precisely, then a buck or LDO is needed. 

  • 0 in reply to Minqiu Xie
    Prodigy 160 points

    Thank you for getting back to me, Minqiu. I think our sensors will operate from 3.3V and shouldn't draw more than 50mA, so if I send 5V down the cable the voltage at the far end should never be less than 4.2V. I can then step that down to 3.3V and still have some margin. Thanks for your help!