Original question:
Hello Team,
What would output from ISO1211, if RTHR is shorted and lead to a large input current and eventual device damage? (High/ Low/ Hi-Z)
We are thinking of using ISO1211 to input DC110V.
Regards,
Taiki Koga
Hi Taiki,
Thanks for reaching out.
What would output from ISO1211, if RTHR is shorted and lead to a large input current and eventual device damage? (High/ Low/ Hi-Z)
For this device, RTHR is responsible for setting the voltage thresholds (VIH and VIL) and limiting the surge current. As shown by the table below, if RTHR is replaced with a short circuit (RTHR = 0 Ω), the voltage thresholds of the device will reduce to the minimum thresholds defined by the datasheet for this specific case (6.5 for VIL and 8.2 for VIH). The signal on the OUT pin of the device will be a logic low if the voltage input on the field side is below VIL or a logic high if the field side input is above VIH.
When RTHR is replaced with a short circuit, the input current should still be limited by the device as long as the following conditions are met:
- The voltage applied to the device is within the -60 V to 60 V range specified by the datasheet
- An RSENSE resistor (which is responsible for limiting the current into the device) that is within the recommended range from 562 to 200 Ω and is connected between the SENSE and IN pins of the ISO1211
As long as the above conditions are met, the ISO1211 should still limit the input current to the level set by the RSENSE resistor whenever RTHR = 0 Ω. For more specific information about the set input current limits for different RSENSE resistors values whenever RTHR = 0 Ω, please refer to the below table from the datasheet.
We are thinking of using ISO1211 to input DC110V.
This implementation is only possible if you use a an RTHR resistor with a large resistance value and connect an additional shunt resistor (RSHUNT) in the configuration shown by the diagram below.
As specified by the ISO1211 datasheet, the maximum voltage than can be applied to the IN and SENSE pins of the device is +/- 60 V. This means that the ISO1211 will only be able to support a 110 DC input if RTHR and RSHUNT resistor are used to create a voltage drop large enough to keep the voltage at the IN and SENSE pins less than 60 V. For a recommendation on what resistor values are required to achieve this implementation with the ISO1211, you can refer to the below app note, which provides example resistor values for this specific type of application:
https://www.ti.com/lit/an/slla382/slla382.pdf
If you have any further questions, please let me know.
Regards,
Kenneth
