The ISO121x devices are rated for 60 V on the SENSE and IN pins with respect to FGND. However, larger voltages on the module input can be supported by dropping extra voltage across an external resistor, RTHR. Because the current drawn by the SENSE and IN pins is well controlled by the built-in current limit, the voltage drop across RTHR is well controlled as well. However, increasing the RTHR resistance also correspondingly raises the voltage transition threshold. An additional resistor, RSHUNT (see Figure 21), provides the flexibility to change the voltage transition thresholds independently of the maximum input voltage. The current through the RSHUNT resistor is less near the voltage transition threshold, but increases with the input voltage, increasing the voltage drop across the RTHR resistor, and preventing the voltage on the ISO121x pins from exceeding 60 V. With the correct value selected for the RTHR and RSHUNT resistors, the voltage transition thresholds and the maximum input voltage supported can be adjusted independently.
A 1-nF or greater CIN capacitor is recommended between the SENSE and FGND pins to slow down the transitions on the SENSE pin, and to prevent overshoot beyond 60 V during transitions.
For more information, refer to the How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC Detection TI TechNote. Use the ISO121x Threshold Calculator for 9V to 300V DC and AC Voltage Detection to estimate the values of voltage transition thresholds, the maximum-allowed module input voltage, and module input current for given values of the RSENSE, RTHR, and RSHUNT resistors.
Another way to increase the maximum module input voltage without changing the voltage transition thresholds is to use a 60-V Zener diode to limit the voltage on the ISO121x pins to less than 60 V as shown in Figure 22. In this case, when the module input is greater than 60 V, the Zener diode must be designed to sink the additional current, and the RTHR resistor must be designed to drop a higher voltage.
For example, with a 2.5-kΩ RTHR and 560-Ω RSENSE, the voltage transition threshold is 15 V, and the ISO121x input current is 2.25 mA. If the module voltage reaches 100 V, the voltage drop across the RTHR resistor is 40 V, and the current through the Zener diode is approximately 14 mA.