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AM3352 Touch Screen Inputs (Capacitance / ADC Input Specification)

Chris Yorkey
Genius 16340 points

Other Parts Discussed in Thread: AM3352

Sitara Team,

 We just received a call from our customer.   They are testing their AM3352 board with 16.5KV (Air Gap) and seeing failures on the AM3352 Touch Screen Inputs.   We reviewed the design below and the ESD devices they are using.   

 ESD Device – Schematic shows 5V bidirectional device from AVX.  They have switched to 3.3V bidirectional device with no better results.

 Part Number is VC060303A100DP

 http://datasheets.avx.com/tguard.pdf

 They have also replaced the zero ohm jumpers in the schematic (R1010-R1013) with 600 ohm ferrite inductors.

 Part Number BLM18E601SH1D

 http://www.murata.com/~/media/webrenewal/support/library/catalog/products/emc/emifil/c51e.ashx?la=en-us

They would like to increase the capacitance shown on the touch screen interface but needs to figure out the limitation of the ADC input.   A quick look at the AM3352 Datasheet and TRM shows this is not a trivial question.   The touch screen inputs are highly programmable and it is unclear how the device operates and what the impedances are in the touch screen mode.   We'd, therefore, like to reach out on the Forums for more guidance.

 We’ll be looking at this closer myself and would like to reach back out this afternoon to the customer.   Let me know if anyone has any input/guidance?

Thanks!

C. Yorkey (on behalf of customer)

 

  • 0
    TI__Genius 16340 points
    Team,

    I should add that we consider the ESD problem to be more of a customer system level design issue. The real question we have for the factory is how much parallel capacitance and series resistance can be added between the touch screen and the AM3352 interface?

    Please advise and thanks again for your input.

    -C. Yorkey
  • 0 in reply to Chris Yorkey
    TI__Guru**** 393215 points
    Hi Chris,

    I will forward this to the factory team.
  • 0 in reply to Biser Gatchev-XID
    TI__Genius 16340 points

    Thanks Biser,

    I did also just find this related post but we'd still be looking for more conclusive information:

    http://e2e.ti.com/support/arm/sitara_arm/f/791/p/328318/1144273?keyMatch=AM335 Touch Screen Controller&tisearch=Search-EN-Everything

    Thanks again,

    Chris

  • 0 in reply to Chris Yorkey
    TI__Guru 64015 points

    We do not specify maximum capacitance on the AIN terminals. However, we define a maximum Drive Current of 25mA for the AIN outputs. If capacitors are connected to the AIN terminals, the peak current required to charge/discharge these capacitors must be limited to 25mA when the touch screen controller applies voltage to the touch screen.

    As you mentioned, there are many system dependencies which the product designer needs to consider when designing the associated ESD protection circuits. For example, the post you referenced mentions a concern with adding capacitance. If too much capacitance is added the RC time constant created by of the touch panel resistance and capacitance on the AIN terminals may create a delayed touch response that may not provide the expected user experience. If the delayed response is not an issue and the peak current is too high, they may be able to add low value series resistors between the AIN terminals and the shunt capacitors. This will limit the charge/discharge current without reducing the shunt capacitance.

    One of the biggest problems with ESD is allowing the large spike of ESD current to flow through a large loop area with high inductance. This allows the energy to couple into other circuits. The energy from the ESD event needs to be shunted to ground through a very low inductance path as close as possible to the source.

    Regards,
    Paul