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TLC2652 layout instruction

Bhavin
Prodigy 20 points
Other Parts Discussed in Thread: TLC2652

Hi,

I am using TLC2652 on my evaluation board. I need help for LAYOUT instruction for it. Basically I like to know, if there is ground plane below this amplifier in other layer, will it affect TLC2652? 

Thanks and Regards,

Bhavin

  • 0
    Intellectual 860 points

    Generally a ground plane is a good thing, but you have to be a little careful.  Here's perhaps a starting point:


    First, make sure you decouple both power supplies with a ceramic cap (one supply if you are operating single supply).  0.01uF to 0.1uF usually works.  I also tend to put a large tantalum cap in parallel with the ceramic (10uF to 100uF depending on what I am doing).  The ground connection on the decoupling cap should be as short as possible (multiple vias to ground is always nice) and the ceramic cap should be as close to the power pin as possible.  The tantalum can be a little further away.  Wide traces keep inductance down (inductance is what you are trying to reduce with a ground plane)


    Second, I often remove the ground plane from under the inverting input node to reduce capacitance.  For the TLC2652 this might not be as important (lower GBW) but it is a good habit.  The stray capacitance on this node can introduce a zero in the noise gain (aka a pole in the loop gain AvB) and reduce the phase margin of the amplifier.  Whether this is an issue depends on your circuit.  Test with a square wave input and look at the overshoot and ringing. 

    When laying out the circuit, keep the inverting node as small as possible.  It's OK for the output node traces to be longer since the output impedance is low, but also don't route the output node near the inputs as it can capacitively couple and create an unwanted feedback path (and likely an oscillator). 

    Also, remember Bob Pease's advice...always put a small capacitor across the feedback resistor unless you can prove you don't need it.  Often a few pF is sufficient.


    Hope that helps!

    Tim

  • 0 in reply to Tim Sobering
    TI__Prodigy 20 points

    Thanks Tim, It is really useful

  • 0 in reply to Bhavin
    Intellectual 860 points

    No problem...glad to help.  If you want some more detail including some guidelines for selecting the feedback cap, take a look here:

    http://www.k-state.edu/ksuedl/publications.htm

    Technotes 7 and 9 have good info.  Pease actually gave me some feedback on #9 before I posted it (see footnote 8).

    If you don't mind, mark this as solved if it answered your question.

    Tim