IL300
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Vishay Semiconductors
Rev. 1.8, 02-Jun-14
6
Document Number: 83622
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Fig. 8 - Amplitude and Phase Response vs. Frequency
Fig. 9 - Common-Mode Rejection
Fig. 10 - Photodiode Junction Capacitance vs.
Reverse Voltage
APPLICATION CONSIDERATIONS
In applications such as monitoring the output voltage from a
line powered switch mode power supply, measuring
bioelectric signals, interfacing to industrial transducers, or
making floating current measurements, a galvanically
isolated, DC coupled interface is often essential. The IL300
can be used to construct an amplifier that will meet these
needs.
The IL300 eliminates the problems of gain nonlinearity and
drift induced by time and temperature, by monitoring LED
output flux.
A pin photodiode on the input side is optically coupled to the
LED and produces a current directly proportional to flux
falling on it. This photocurrent, when coupled to an amplifier,
provides the servo signal that controls the LED drive current.
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
The IL300 was designed to be the central element of DC
coupled isolation amplifiers. Designing the IL300 into an
amplifier that provides a feedback control signal for a line
powered switch mode power is quite simple, as the
following example will illustrate.
See figure 12 for the basic structure of the switch mode
supply using the Infineon TDA4918 push-pull switched
power supply control cChip. Line isolation are provided by
the high frequency transformer. The voltage monitor
isolation will be provided by the IL300.
The isolated amplifier provides the PWM control signal
which is derived from the output supply voltage. Figure 13
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 V
signal. Given this information, the amplifier circuit topology
shown in figure 14 is selected.
The power supply voltage is scaled by R1 and R2 so that
there is + 3 V at the non-inverting input (V
a
) 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 (V
a
) varies it will cause a
change in the LED current necessary to satisfy the
differential 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 (I
Fq
) and the servo
gain (K1). For this design, I
Fq
= 12 mA. Figure 4 shows the
servo photocurrent at I
Fq
is found to be 100 mA. With this
data R3 can be calculated.
iil300_13
dB
Phase
Ø - Phase Response (°)
10
3
10
4
10
5
10
6
10
7
5
0
- 5
- 10
- 15
- 20
45
0
- 45
- 90
- 135
- 180
F - Frequency (Hz)
Amplitude Response (dB)
I
Fq
= 10 mA
Mod = ± 4.0 mA
T
A
= 25 °C
R
L
= 50 Ω
iil300_14
- 130
- 120
- 110
- 100
- 90
- 80
- 70
- 60
F - Frequency (Hz)
CMRR - Rejection Ratio (dB)
10
6
10
1
10
2
10
3
10
4
10
5
iil300_15
0
2
4
6
8
10
12
14
Voltage (V
det
)
Capacitance (pF)
048
2610
R3
V
b
I
PI
------
3 V
100 μA
------------------
30 k== =
