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IL300-X009 Datasheet(PDF) 9 Page - Vishay Siliconix |
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IL300-X009 Datasheet(HTML) 9 Page - Vishay Siliconix |
9 / 15 page VISHAY IL300 Document Number 83622 Rev. 1.4, 26-Oct-04 Vishay Semiconductors www.vishay.com 9 The isolated amplifier provides the PWM control sig- nal which is derived from the output supply voltage. Figure 16 more closely shows the basic function of the amplifier. The control amplifier consists of a voltage divider and a non-inverting unity gain stage. The TDA4918 data sheet indicates that an input to the control amplifier is a high quality operational amplifier that typically requires a +3.0 V signal. Given this information, the amplifier circuit topology shown in Figure 18 is selected. The power supply voltage is scaled by R1 and R2 so that there is + 3.0 V at the non-inverting input (Va) of U1. This voltage is offset by the voltage developed by photocurrent flowing through R3. This photocurrent is developed by the optical flux created by current flowing through the LED. Thus as the scaled monitor voltage (Va) varies it will cause a change in the LED current necessary to satisfy the dif- ferential voltage needed across R3 at the inverting input. The first step in the design procedure is to select the value of R3 given the LED quiescent current (IFq) and the servo gain (K1). For this design, IFq = 12 mA. Fig- ure 4 shows the servo photocurrent at IFq is found to be 100 µA. With this data R3 can be calculated. For best input offset compensation at U1, R2 will equal R3. The value of R1 can easily be calculated from the following. The value of R5 depends upon the IL300 Transfer Gain (K3). K3 is targeted to be a unit gain device, however to minimize the part to part Transfer Gain variation, Infineon offers K3 graded into ± 5 % bins. R5 can determined using the following equation, Or if a unity gain amplifier is being designed (VMON- ITOR = VOUT, R1 = 0), the equation simplifies to: Figure 16. Isolated Control Amplifier R3 = Vb IPI = 3V 100 µA =30K Ω 17164 iil300_16 + - Voltage Monitor R1 R2 To Control Input ISO AMP +1 R1=R2 ( VMONITOR Va - 1 ) 17165 R5 = VOUT VMONITOR • R3(R1 + R2) R2K3 17166 R5 = R3 K3 17190 |
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