The common practice of using opto couplers in the feedback path of switching power supplies has become easier and more cost effective with the introduction of the TPS5904 Opto-Isolated Feedback Amplifier.
Typical Application
The TPS5904 consists of the industry standard TL431 with 0.4% reference voltage tolerance and an optocoupler. It is primarily intended for use as the error/reference/isolation amplifier element in isolated AC/DC power supplies and DC/DC converters. The optocoupler is a GaAs LED and a silicon phototransistor. The LED emits infrared light at 940 nm and has a typical forward voltage of 1.2 V.
The TPS5904 operates as a comparator with an optically coupled output. Texas Instruments provides these devices in two different ranges of current transfer ratios (CTR). The CTR ranges from 100% to 400% for the standard version and an "A" version is provided with a CTR range of 150% to 300% for applications that require higher performance. With the TPS5904 and the TPS5904A, power supply designers can reduce their component count and save space in tightly packaged designs. The tight tolerance of the voltage reference eliminates the need for adjustments in many applications.
The suggested resale price is quoted per device in quantities of 1,000
TPS5904 : $0.87
TPS5904A : $0.99
Vol 12, Summer, 1995