Design Feature: September 1, 1995

Adding a single resistor and optoisolator to the standard dc/dc converter trim in Fig 1a results in an isolated trim configuration (Fig 1b). In Fig 1a, V_OUT equals V_TRIM×V_NOM/2.5V. In this example, V_NOM is 15V, and V_TRIM is 0 to 2.5V. This configuration is acceptable for most applications, except those in which the trim signal must be totally isolated from the converter.

For Fig 1b, many types of analog optoisolators are available, but the VTL2C2 provides the necessary characteristics. R₁ biases IC₁ for a 0-to-5V trim signal that you can change as necessary. Fig 1c shows the isolated circuit's output voltage as a function of trim voltage.