Figure 29 shows the simplified version of the feedback network. The standby Iq of the system is dependent on two paths: the ATL431LI-Q1 biasing path and the resistor feedback path. With the given design requirements, the total current through the feedback network cannot exceed 2 mA.
The design goal is to take full advantage of the Imin to set the IKA of the ATL431LI-Q1. The benefit of the ATL431LI-Q1 is its low Imin of 80 µA which allows the IKA to be lower at a full load condition compared to typical TL431LI-Q1 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 the CTR of the optocoupler, voltage, and current at no-load. This can be seen in Equation 2. It is possible to lower IOPTNL to a value of 1.5 mA for a power loss of 30 mW by using an optocoupler with a high CTR.