ISOM811x opto-emulators are commonly used in the feedback control loops of isolated power supplies. These devices are used to solve the problem of feeding back current while isolating the primary and secondary domains to regulate the output voltage.

In power supplies, the output voltage is isolated from main input voltage using a transformer (for example: flyback converter). For analog power supply units, the controller IC is typically on the primary side of the transformer. For closed loop control, measuring the output voltage on the secondary side and feeding the voltage back to the controller on the primary is necessary. The most common method of achieving this design is using an opto-emulator such as ISOM811x, error amplifier (commonly TL431), and a voltage comparator to form a feedback loop across the isolation barrier

Figure 9-1 illustrates a typical isolated power supply. In this implementation, the output voltage is sensed by an error amplifier using the resistor divider (R1 and R2). Depending on the voltage level that the error amplifier senses, the TL431 can drive the current of the ISOM811x higher or lower which is then compared to a voltage reference. The information is passed across the isolation barrier through ISOM811x to the primary side, where the PWM control circuit modulates the power stage to regulate the output voltage. The TL431 and ISOM811x play an important role for stable feedback and control loop.

The ISOM811x devices enable improvements in transient response, reliability, and stability as compared to commonly used optocoupler as the CTR is stable over wide temperature range providing a small, low-cost, highly reliable, and easy-to-design implementation.