When using an optocoupler in a feedback control loop for an isolated power supply, many variables can affect how to properly use the optocoupler, including the output voltage of the power supply and the type of controller the feedback signal is being sent to. For this example, let’s assume the output voltage of this power supply, V_OUT, is 5 V, and the PWM controller being used has an integrated error amplifier with a COMP pin that acts as the output of this amplifier.

Sizing R_PULLUP

The transistor output of ISOM811x will operate in active, saturation, reverse, and cut-off regions, just like a regular transistor. To ensure the output does not get damaged when it is saturated, the minimum value of R_PULLUP can be calculated for a given pull-up voltage, V_PULLUP, in Equation 1 below:

For the example of a feedback loop application, we can calculate the minimum required value for R_PULLUP for a given V_PULLUP of 10 V, the max output voltage of the error amplifier (V_COMP(MAX) of 2.5 V, and the max output current of the error amplifier is internally clamped at 1.6 mA. The equation to calculate R_PULLUP is shown in Equation 2 below:

Sizing R_IN

The input side of ISOM811x is current-driven. To limit the amount of current flowing into the AN pin, it is recommended that a series resistor, R_IN, is used in series with the input as shown in Figure 10-1.

Depending on how the ISOM811x device is being used, the value of R_IN can vary quite a bit. However, at a high level, to make sure the input does not get damaged, the minimum value of R_IN can be calculated for a given input voltage, V_IN, in Equation 3 below:

However, in the use case of a feedback loop, R_IN directly affects the mid-band gain of the loop. Let’s assume the TL431 has been configured to give a reference voltage, V_REF, of 2.5 V and R_PULLUP is 5 kΩ. Equation 4 is used to calculate the maximum value of R_IN ensuring that V_COMP voltage on the primary side can be pulled to the saturation voltage of the ISOM811x, V_CE(SAT).