AM335x Output load capacitance for Touchscreen Controller

As Biser pointed out, we do not specify the maximum capacitive load for AIN terminals operating as outputs.  I will ask the ADC designer if he can provide any suggestions related to this question.  This may take him a few days if it is necessary to run simulations to derive an answer. 

I would partially disagree with Biser on his comment about external ESD protection not being required for the AIN terminals.  Our internal ESD protection is only meant to protect the device from ESD events resulting from handling during assembly.  Customers should provide ESD protection for system related ESD events.  I somewhat agree with Biser's comment related to the touchscreen resistance limiting the current, but this depends where the ESD event is applied to the touchscreen.  For example, resistance from the corner of the touchscreen to the AIN terminals may be very small if the ESD event is applied to the corners of the screen.

Regards,
Paul

The designer said the touchscreen controller was designed with the capability of driving high capacitance loads.  The output drivers are a few ohms, so the RC time constant would only be a couple of ns.

So there should not be a problem with the 2200pf loads presented by the ESD protection devices.

Regards,
Paul

Paul,

you are joking, right?

If the ports are outputs, capacitive load is no problem.

But in input mode (ADC working), we have a RC low pass with R = resistance of touch foils and C = capacitive load.

So there will be some delay here, which is more than just a few ns.

With R = 1000 Ohms, C = 2200pf, t = 2.2us.

You may use a low cap. TVS diode array like SRV05-4 from semtech.

regards

Wolfgang

The original question asked the maximum load capacitance that could be driven by the AM335x outputs.  This is a question a customer would expect TI to answer, so my answer was with respect to AM335x ability to drive a capacitive load.

I realize the system level answer depends on other variables like the touch screen impedance and expected response time.  However, this is something the product designer needs to understand.  If the 2200pf capacitive load causes the response time to be too slow, they will need select a ESD protection device with lower capacitance.  But this decision is based on system level considerations not AM335 output considerations.

Regards,
Paul