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TLV752: FB Pins as Remote Sense

IDK
Prodigy 230 points
Part Number: TLV752

Tool/software:

Hello,

Just double checking - I should theoretically be able to use the FB pin of the TLV752 sort of as a remote sense line if the sensed/connected point to the output voltage is at the load right? Do you foresee any issues with this, even if the distance to the load were a couple of inches away?

Thanks,

Matthew

  • 0
    TI__Guru 50025 points

    Hi Matthew,

    In general this can be done, but the issues begin arising as the load point gets further from the device. The concern with doing this is that the traces that connect the load point back to the feedback divider add a lot of parasitic inductance, which can affect stability. I used an online calculator and estimated about 170nH for a 4-inch trace (2 inches both ways) with some other assumptions, and I'm running a simulation now (with a different device) to see how much this can affect the loop stability. I'm also curious whether using a feedforward capacitor near the device can help with the stability issue so that's included in the simulation. It should be done by tomorrow, so I'll update you then.

    Regards,

    Nick

  • +1
    TI__Guru 50025 points

    Hi Mathew,

    Sorry for the delayed response. 

    After running several simulations, it seems that the extra inductance, which mostly shows up as ESL for the top feedback resistor (assuming the OUT net is sized like a power net with low inductance), does not affect stability. The delay in the signal back to the error amplifier input is on the order of picoseconds for a 4-inch round trip, which is far faster than the LDO sees it so there is no issue there. Using a feedforward cap near the device (instead of near the load) helps to short out any stray inductance and speeds up the transient as usual. I would think that the load will have its own decoupling caps near the supply pin, but in case it doesn't it would be worthwhile to leave a pad there in case you need some extra cap to help get the supply output impedance down.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 230 points

    Hey Nick,

    Got it, thanks for looking into this! I'll proceed to use the FB connection sort of as a remote sense line with a feedforward cap near the regulator.

    Thanks,

    Matthew

  • 0 in reply to IDK
    TI__Guru 50025 points

    Hi Matthew,

    Sure thing! It was an excuse to examine an interesting topic. Slight smile

    I didn't mention it in my last response, but the output cap and feedback network should still be located close to the device. 

    Regards,

    Nick