TCA8418: Pull up resistor min max value

Hi David,

We answered this question a while back in the thread here. Customer can expect a variation of pull-up resistance +/- 20% of the typical value (105k ohm). 

The leakage that the customer found calculating out the pullup value to be around 5kohm seems a bit off. May we have a little more information on how the customer arrived at this conclusion? What amount of leakage current was measured? 

Regards,

Tyler

Hi Tyler,

Thanks for the response!

The customer pulled up pin 18 to 3.3V through a 4.7kohm resistor, as the power of TCA8418 off the voltage observed at pin 18 was 1.6V.

For other pins, pin 1-15 were connected to SW open, pin 16, 17 were pulled up to the power(turned off) through 12kohm resistor.

Pin 20 was pulled up to the power(turned off) through 12kohm resistor, pin 22-24 are connected to the ground. The resistance between ground and power was 175kohm.

According to the voltage drop at pin 18 the resistor value does not seem to be correct, is there any other components (e.g. parasitic diode) in the pin 18?

Regards

Hi David,

David Huang said:

The customer pulled up pin 18 to 3.3V through a 4.7kohm resistor, as the power of TCA8418 off the voltage observed at pin 18 was 1.6V.

When the power was turned off, what was the voltage measured at VCC on the chip? Are there any other connections on pin 18 other than the 4.7kohm pull-up resistor to 3.3V? 

Powering off the TCA8418 wouldn't generate a 1.6V output voltage on the GPIO pin 18. There must be an additional connection to this pin to have caused this bias. 

David Huang said:

According to the voltage drop at pin 18 the resistor value does not seem to be correct, is there any other components (e.g. parasitic diode) in the pin 18?

Input voltage range listed in the absolute maximum ratings of the datasheets shows a Vi(min) = -0.5V which indicates to mean there is some ESD diode present. The leakage from this diode should be accounted for in the input leakage spec listed in the datasheet.

May I see a fuller view of the schematic specifically showing the connections made on pin 18? 

Regards,

Tyler

Hi Tyler,

Thanks for the response!

Tyler Townsend said:

The leakage from this diode should be accounted for in the input leakage spec listed in the datasheet.

I didn't find the input leakage spec in the datasheet, do you mean by Iinput in page 6 marked as 1uA?

I will try to get a schematic, but I think there's no other connection to pin 18 except for the pull up resistor. For pin 18, I think the customer just connected the pin to another 3.3V power through a 4.7kohm resistor, and as the power of the TCA turned off, the voltage on this pin dropped to 1.6V, meaning there's an internal pull down resistor in the pin with a value close to 5kohm. The Vcc voltage in this case should be 0V.

Regards,

Hi David,

David Huang said:

I didn't find the input leakage spec in the datasheet, do you mean by Iinput in page 6 marked as 1uA?

This is correct. A GPIO pin configured as high-impedance input with voltage input = VCC or GND, has a current draw of 1uA. This tells of the maximum leakage that could result from a GPIO pin. 

David Huang said:

I will try to get a schematic, but I think there's no other connection to pin 18 except for the pull up resistor. For pin 18, I think the customer just connected the pin to another 3.3V power through a 4.7kohm resistor, and as the power of the TCA turned off, the voltage on this pin dropped to 1.6V, meaning there's an internal pull down resistor in the pin with a value close to 5kohm. The Vcc voltage in this case should be 0V.

I agree with this statement that the voltage should be close to 0V since the internal pull-up is very weak and not close to 5kohm. 

Does the customer see this issue only on P5, or does this appear across multiple input pins? 

Has the customer tried another TCA8418 device to see if P5 fails in the same way? If customer can track the failure between "A" and "B" devices, there is potential that the device failed and has some type of permanent damage to the pin. 

Regards,

Tyler

Hi Tyler,

Thanks for the response.

I double checked with the customer, it seems like he's done ABA swap but the voltage drop was still the same.

The voltage on pin 18 dropped from 3.3V to 1.98V or 2V with the setup I mentioned(through a 4.7ohm pull up resistor, and nothing else connected). The device had no power supply during the whole test.

Could you double check on the internal structure of the device? (I think it's better if you could run a quick test on this? The result should be replicated easily with a 4.7kohm resistor and a 3.3V power source)

Regards,

Hi David,

David Huang said:

I double checked with the customer, it seems like he's done ABA swap but the voltage drop was still the same.

Thank you for the update. 

David Huang said:

The voltage on pin 18 dropped from 3.3V to 1.98V or 2V with the setup I mentioned(through a 4.7ohm pull up resistor, and nothing else connected). The device had no power supply during the whole test.

Could you double check on the internal structure of the device? (I think it's better if you could run a quick test on this? The result should be replicated easily with a 4.7kohm resistor and a 3.3V power source)

I can conduct this test in lab. Please give me till Wednesday to get results to you. There are a couple of other customer tests in the que at the moment. 

Regards,

Tyler

Hi David,

Working with design to get you an answer. I replicated what the customer was seeing and found that my pull-up resistance was around 1kohm instead of the typical 105kohm. 

Will try to get you an answer by Wednesday next week.

Regards,

Tyler

Hi Tyler,

Thanks for the update. Kindly let me know the result once you have it!

Regards,

Hi David,

Working on an explanation with Design. Can you please message me on my email t-townsend@ti.com? Since this is the public forum? 

Regards,

Tyler