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LIA135STR Datasheet(PDF) 8 Page - IXYS Corporation |
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LIA135STR Datasheet(HTML) 8 Page - IXYS Corporation |
8 / 13 page INTEGRATED CIRCUITS DIVISION LIA135 / LIA136 8 www.ixysic.com R01 2. Functional Description The LIA135 and LIA136 are optically isolated error amplifiers. Each comprises the three primary components necessary to implement feedback for an isolated power supply in a single package. These components are: an error amplifier; a voltage reference; and an optocoupler. The LIA135 and LIA136 are the functional equivalent of a 431 type precision shunt regulator and an optocoupler in the same package. Understanding how the LIA135 and LIA136 function and are used is best understood by referencing the simplified application circuit shown in Figure 10: Typical Isolated Power Supply Using an Optical Feedback Error Amplifier. The function of the LIA135 / LIA136 is to sample the power supply output to be regulated, generate an error signal, and transmit the error signal across the isolation barrier to the power supply’s control circuitry. Power for the input side circuits, consisting of an LED; a shunt regulator; and an error amplifier, is provided by the power supply’s rectified secondary output (VOUT) via the series current limiting resistor (RLED) as shown in the application circuit. Regulation of VOUT is made possible by applying a scaled sample of its voltage to pin FB, the error amplifier’s non-inverting input. The error amplifier compares this scaled voltage (VFB) against an internal high accuracy reference voltage and generates an output which in turn sets the LED drive current. As VOUT increases, the error amplifier’s input voltage VFB will also increase. Ramping of VFB beyond the internal reference voltage causes the error amplifier to generate the LED drive current (IF) necessary to cause the optocoupler’s NPN output transistor to conduct. Increasing the LED drive current results in an increase of the output transistor’s collector current (IC) which in turn decreases the voltage seen at the collector (VC). This voltage is also commonly referred to as the Error Voltage (VE). Likewise, a reduction of VOUT results in a lessening of IF causing VC to increase. The power supply’s control circuitry uses the error voltage presented by the optocoupler’s output transistor to interpret the power needs of the secondary side load and to maintain regulation. Figure 10: Typical Isolated Power Supply Using an Optical Feedback Error Amplifier LIA135 / LIA136 + _ + _ LIA136 only + V OUT V IN PWM Control R1 R2 + COMP LED FB GND C E B V CC R C R LED I F |
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