# Power dissipation for ISO1I811T and ISO1I813T ISOFACE™ digital input - KBA237493

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Community Translated: ISO1I811T および ISO1I813T ISOFACE™ デジタル入力の消費電力 - KBA237493

This article provides the process of calculation of power losses of ISO1I811T and ISO1I813T ISOFACE™ digital input ICs. This loss calculation is applicable for Type 1, 2, and 3 sensors according to IEC 61131-2 standard.

Figure 1 shows the typical application, where Rext is the external resistor that limits the external current through the LED, and Rv is the series resistor that limits the internal current through the IxH pins. The supply voltage to the IC (Vbb) and sensors (Vs) can be different. In the following application circuit, the same voltage is applied to the sensor supply and IC supply.

Figure 1   Typical application circuit

Follow these steps to calculate the power dissipation:

1.  Choose the external resistor and calculate the external LED current

Use the external resistor value of 12 kΩ for Type 1 and 3 sensors, and 8.5 kΩ for Type 2 sensors to have optimum power dissipation, proper sensor detection, and minimum current regulation.

External LED current (Iext) = (Vs - Vled) / Rext

Where,

Vs = Sensor supply voltage (V)

Vbb = Supply to the IC (V)

Vled = LED voltage(V)

Rext = External resistor (Ω)

Internal current (Iint) = Sensor supply current - Iext

2.  Choose the series resistor Rv and calculate the voltage across IxH and IxL pins.

Add an external resistor value of 2 kΩ for Type 1 and 3 sensors, and 1.5 kΩ for Type 2 sensors to have an optimum power dissipation in the digital input IC.

Voltage across IxH and IxL pins (VIxH) = Vs - (Rv * Iint) - Vled

3. Calculate the power dissipation.

Power dissipation across eight channels (PD1) = 8 * (VIxH * Iint)

Power dissipation because of IC supply (PD2) = Vbb * Supply current (typically 11 mA)

Total power dissipation of digital input IC (PD) = PD1 +PD2

4. Calculate the junction temperature and check for the feasibility of usage.

Tj = Ta + PD * Rja

Where,

Tj = Junction temperature

Ta = Ambient temperature

Rja = Thermal resistance (junction to ambient). Typically, 88.6°C/W

Note: ISO1I811T/ISO1I813T on 50 mm x 50 mm x 1.5 mm epoxy PCB FR4 with 2 cm² (one layer, 35 µm thick) copper area. PCB is vertical without blow air).

Calculate Tj with respect to the calculated PD in Step 3 and the maximum ambient temperature. The calculated Tj must be lower than 125°C (recommended maximum Tj at which all the functionalities of the IC are guaranteed).

Table 1 shows a power dissipation calculation example for Type 1/3 sensors with a supply current of 3 mA, sensor supply voltage of 24 V, and ambient temperature of 83°C.

Table  1  Power dissipation

 Parameter Symbol Value Unit Sensor source current Is 3 mA Sensor supply voltage Vbb 24 V LED voltage Vled 2.5 V External resistor Rext 12 kΩ Series resistor Rv 2 kΩ External LED current Iext 1.79 mA Internal current Iint 1.21 mA Voltage across IxH and IxL pins VIxH 21.58 V Power dissipation on eight channels PD1 0.209 W Power dissipation because of IC supply PD2 0.264 W Total power dissipation PD 0.473 W Maximum ambient temperature Ta 83 °C Junction temperature Tj 124.9 °C

In Table 1, you can use the design example ISO1I811T/ISO1I813T until the maximum ambient temperature of 83°C considering IC on 50 mm x 50 mm x 1.5 mm epoxy PCB FR4 with 2 cm² (one layer, 35 µm thick) copper area and PCB is vertical without blow air. To use ISO1I811T/ISO1I813T in applications with the same specification as Table 1 and with an ambient temperature of more than 83°C, increase the copper area by splitting to one more layer of copper.

References

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