Datasheet IL300 (Vishay) - 6

IL300
www.vishay.com Vishay Semiconductors
APPLICATION CONSIDERATIONS
In applications such as monitoring the output voltage from a 5 45 line powered switch mode power supply, measuring dB bioelectric signals, interfacing to industrial transducers, or Phase 0 0 making floating current measurements, a galvanically isolated, DC coupled interface is often essential. The IL300 - 5 - 45 can be used to construct an amplifier that will meet these needs. - 10 - 90 The IL300 eliminates the problems of gain nonlinearity and I = 10 mA Fq drift induced by time and temperature, by monitoring LED Mod = ± 4.0 mA - 135 output flux. - 15 T = 25 °C A Ø - Phase Response (°) Amplitude Response (dB) R = 50 Ω L A pin photodiode on the input side is optically coupled to the - 20 - 180 LED and produces a current directly proportional to flux 103 104 105 106 107 falling on it. This photocurrent, when coupled to an amplifier, iil300_13 F - Frequency (Hz) provides the servo signal that controls the LED drive current. Fig. 8 - Amplitude and Phase Response vs. Frequency The LED flux is also coupled to an output PIN photodiode. The output photodiode current can be directly or amplified to satisfy the needs of succeeding circuits.
ISOLATED FEEDBACK AMPLIFIER
- 60 The IL300 was designed to be the central element of DC - 70 coupled isolation amplifiers. Designing the IL300 into an amplifier that provides a feedback control signal for a line - 80 powered switch mode power is quite simple, as the following example will illustrate. - 90 See figure 12 for the basic structure of the switch mode - 100 supply using the Infineon TDA4918 push-pull switched - 110 power supply control cChip. Line isolation are provided by the high frequency transformer. The voltage monitor - 120 CMRR - Rejection Ratio (dB) isolation will be provided by the IL300. - 130 The isolated amplifier provides the PWM control signal 106 101 102 103 104 105 which is derived from the output supply voltage. Figure 13 iil300_14 F - Frequency (Hz) more closely shows the basic function of the amplifier. Fig. 9 - Common-Mode Rejection 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 V 14 signal. Given this information, the amplifier circuit topology shown in figure 14 is selected. 12 The power supply voltage is scaled by R1 and R2 so that 10 there is + 3 V at the non-inverting input (Va) of U1. This 8 voltage is offset by the voltage developed by photocurrent flowing through R3. This photocurrent is developed by the 6 optical flux created by current flowing through the LED. 4 Thus as the scaled monitor voltage (Va) varies it will cause a Capacitance (pF) change in the LED current necessary to satisfy the 2 differential voltage needed across R3 at the inverting input. The first step in the design procedure is to select the value 0 0 2 4 6 8 10 of R3 given the LED quiescent current (IFq) and the servo iil300_15 Voltage (V ) gain (K1). For this design, IFq = 12 mA. Figure 4 shows the det servo photocurrent at IFq is found to be 100 mA. With this Fig. 10 - Photodiode Junction Capacitance vs. data R3 can be calculated. Reverse Voltage V 3 V R3 b = --- = --------- = 30 k I 100 μA PI Rev. 1.8, 02-Jun-14
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