ISOM811x opto-emulators have analog output behavior, and are design to replace optocouplers commonly used to transmit isolated signals in the feedback control loops of isolated power supplies.

In these 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 usually on the primary side of the transformer, and for closed loop control, the output voltage on the secondary side is measured and fed back to the controller on the primary for regulation. The most common way of isolating this analog feedback control signal is using an analog-output optocoupler, replaced by ISOM8110 in this example, an error amplifier (commonly TL431), and a voltage comparator to form a feedback loop across the isolation barrier

Figure 4-1 illustrates a typical isolated power supply. In this implementation, the output voltage is sensed by an error amplifier through the resistor divider. Depending on the voltage level that it 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 of -55°C to 125°C providing a small, low-cost, highly reliable, and easy-to-design design as highlighted in the Isolating Feedback Signals in Power Supplies product overview.

Figure 4-1 Typical Isolated Power Supply Application Using ISOM811x