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TLV3502: About TLV3502's PCB layout

Yatao Ling1
Expert 1840 points
Part Number: TLV3502

Hi TI expert,

I want to use high-speed comparator TLV3502. As you know, special care must be taken as to high-speed op amp's PCB design. For example, the internal plane's copper surrounding op amp's input and output terminal must be removed to diminish parasitic capacitance, thus preserve op amp's high-speed performance. 

However, no reference or advice has been given about TLV3502's PCB design. I want to consult you to for help. Are there any special measures, as in the op amp's case, to keep TLV3502's perfect performace?

Regards 

Yatao

  • 0
    TI__Mastermind 49966 points
    Hello Yato,

    Just follow good high-speed layout design practices - as you would for any high frequency gate. Assume this is a >1GHz device...the output edge rates are up in that area (~1ns) , anyways.

    Use controlled impedance traces and solid ground planes. The output impedance if the TLV3502 is close to 50 ohms, so size your traces for 50 ohm impedance if running for more than a few cm.

    Keep the input traces away from the outputs.

    Place the supply bypass capacitors directly at the supply pins and return to a solid ground. I recommend using a small (< 0603) 1000pF cap directly at the supply pin, and a 0.1uF cap as close as possible, and a 4.5-10uF "bulk" cap near by. The idea there is to stagger the self resonance of the capacitors. Supply bypassing is critical because of the large supply current transients that occur when the output transitions. Use thick supply traces to minimize inductance and prevent "ringing".

    Use only as much output swing as needed. The greater the output swing, the greater the possible ringing. If you can run from 3.3V instead of 5V, then do so..you can also go faster since the lower output voltage means less time slewing for the same edge rate.
  • 0 in reply to Paul Grohe
    Expert 1840 points

    Hi Paul,
    Thx for your reply.
    In my design, TLV3502's output pin is about 3cm away from the digital controller. I've never done "controlled impedence" before, can you offer me some instractions or tutorials.
    Besides, just to confirm, I don't need tgo remove the inner power or ground's copper around TLV3502's input and output pin ?

    Best regards
    Yatao

  • 0 in reply to Yatao Ling1
    TI__Mastermind 49966 points
    Hello Yatao,

    There are several resources - a quick Google Search of "controlled impedance trace" will bring up many hits. Also called "microstrip" when on the surface and "stripline" when buried within the layers.

    Here are a few TI resources:

    www.ti.com/.../slyp173.pdf
    www.ti.com/.../scaa082a.pdf

    Basically, it is finding the proper trace-width over the ground plane, to create the desired trace impedance. That width depends on many, many factors, too many to discuss here..Training courses are designed around the topic. Many PCB layout suites have tools/wizards to calculate the correct trace width.

    Controlled impedance will require a ground plane under the trace. The whole idea of the stripline is to balance the inductance of the trace and the capacitance to the ground plane to create a transmission line (like a coax cable).

    Removing the ground plane under the chip will decrease the capacitance to the traces in the immediate area, but it then makes the traces more inductive. It also puts a hole in the ground plane - which may not be desired. It also breaks the shielding between the input and output.

    So removing the copper under the device depends on the overall node impedance you are trying to create. If the node impedance is over 1k, such as the feedback circuit of a high-speed op-amp, then it may be beneficial to remove the copper. There is no set rule - it's an engineering call depending on your circuit needs.

    If the signal is only going less than a cm, then controlled impedance may not be necessary. But over a few cm, it is *highly* recommended.
  • 0 in reply to Paul Grohe
    Expert 1840 points
    Thx! Paul .

    Best regards
    Yatao