Figure 29 shows the simplified version of the feedback network. The standby Iq of the system is dependent on two paths, ATL431LI biasing path and the resistor feedback path. With the given design requirements the total current through the feedback network cannot exceed 2mA.
The design goal is to take full advantage of the Imin to set the IKA of the ATL431LI. The benefit of the ATL431LI is its low Imin of 80 µA which allows the IKA to be lower at a full load condition compared to typical TL431LI devices. This helps lower the IKA at the no-load condition which is higher than the full load condition due to the dynamic changes in the IKA as the system load varies. The IKA at no-load, IOPTNL, is dependent the value of Rs which is the biasing resistor. Rs is very application specific and is dependent on variables such as optocoupler's CTR, voltage, and current at no-load and this can be seen on Equation 2. By using an optocoupler with a high CTR it is possible to lower IOPTNL to a value of 1.5 mA for a power loss of 30 mW.