SLLSEY7F June 2017 – April 2020 ISO1211 , ISO1212
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
The R_{SENSE} resistor limits the current drawn from the field input. A value of 562 Ω for R_{SENSE} is recommended for Type 1 and Type 3 operation, and results in a current limit of 2.25 mA (typical). A value of 200 Ω for R_{SENSE} is recommended for Type 2 operation, and results in a current limit of 6 mA (typical). In each case, a (slightly) lower value of R_{SENSE} can be selected based on the need for a higher current limit or component availability. For more information, see the Electrical Characteristics—DC Specification table and Typical Characteristics section. A 1% tolerance is recommended on R_{SENSE} but 5% resistors can also be used if higher variation in the current limit value is acceptable. The relationship between the R_{SENSE} resistor and the typical current limit (I_{L}) is given by Equation 1.
The R_{THR} resistor sets the voltage thresholds (V_{IL} and V_{IH}) as well as limits the surge current. A value of 1 kΩ is recommended for R_{THR} in Type 3 systems (maximum threshold voltage required is 11 V). A value of 2.5 kΩ is recommended for R_{THR} in Type 1 systems (maximum threshold voltage required is 15 V) and a value of 330 Ω is recommended for R_{THR} in Type 2 systems. The Electrical Characteristics—DC Specification table lists and the Typical Characteristics section describes the voltage thresholds with different values of R_{THR}. For other values of R_{THR}, derive the values through linear interpolation. Use Equation 2 and Equation 3 to calculate the values for the typical V_{IH} values and minimum V_{IL} values, respectively.
The maximum voltage on the SENSE pins of the ISO121x device is 60 V. However, because the R_{THR} resistor drops additional voltage, the maximum voltage supported at the module inputs is higher and given by Equation 4.
Use the ISO121x Threshold Calculator for 9V to 300V DC and AC Voltage Detection to estimate the values of the voltage transition thresholds, the maximum-allowed module input voltage, and module input current for the given values of the R_{SENSE} and R_{THR} resistors.
A value of 0 Ω for R_{THR} also meets Type 1, Type 2 and Type 3 voltage-threshold requirements. The value of R_{THR} should be maximized for best EMC performance while meeting the desired input voltage thresholds. Because R_{THR} is used to limit surge current, 0.25 W MELF resistors must be used.
Figure 18 shows the typical input current characteristics and voltage transition thresholds for 562-Ω R_{SENSE} and 1-kΩ R_{THR}.