ISO7141CC: Does this exist as actual 500ohm resistor internally in the ISO7141?

Hi,

I have one more question.

Can "500ohm equivalent resistor" protect the damage from over current, if over current is occurred? 

Please advise us.

Thanks and best regards,
M.HATTORI.

Hi Hattori-san,

Yes, the equivalent circuit will be functioning like a 500Ω resistor and will limit the current as well. The device input circuit consumes only a maximum of 10µA under normal operating conditions as specified in the datasheet. Thanks.

Regards,
Koteshwar Rao

Hi, Rao-san

We can find MAX 10uA on High-level input current or Low-level input current. 　On normal condition, during transient low->high on input pin of ISO7731, is peak current "MAX 10uA"??   Please advise us.

Thanks and best regards,
M.HATTORI.

Hi Hattori-san,

The max 10µA input current is a steady state current that is consumed by device. When there is LOW to HIGH transition at the input, the input capacitance of the pin needs to be charged to let the voltage at the pin reach HIGH and the charging current is only limited by the external source and is not defined by ISO device.

The value of input capacitance (CI) is quoted in datasheet section 8.3.1 as 2.8pF typical and most TTL/CMOS outputs have a typical source resistance of 50Ω. Considering there is no external series resistance used and the input voltage applied as 5V, the peak charging current is expected to be about 100mA (=5V/50Ω) which exponentially drops to ~0mA in about 0.7ns (tcharge = 5*R*C = 5*50*2.8pF). An example capacitor charging current waveform is shown below.

Regards,
Koteshwar Rao

Hi,

Thank you for your reply.

>>The value of input capacitance (CI) is quoted in datasheet section 8.3.1 as 2.8pF typical and most TTL/CMOS outputs have a typical source resistance of 50Ω. 

According to datasheet , you can find a 500Ω resistor as functional equivalent.  So I am confused the above your comment.  I think resistance is 500Ω + 50Ω when doing above calculation.  Please let me know your comments.

Thanks and best regards,
M.HATTORI

Hi Hattori-san,

The 500Ω resistance is a series resistance inside the device and it doesn't really control the input current limit as the input of the MOSFET has much higher impedance which limits the steady state current into the device to 10µA.

The input capacitance (CI) of 2.8pF is the parasitic capacitance measured from external device IN pin to GND pin. Hence, whenever there is an input signal applied that transitions from LOW to HIGH, this capacitor gets charged and the peak charging current value is defined by the external device. I have shared the typical charging current waveform in my previous reply and this current lasts only for about 0.7ns and hence no impact on the system or on the source. I hope this is clear now, thanks.

Regards,
Koteshwar Rao