IL300 View Datasheet(PDF) - Siemens AG
Part Name
Description
Manufacturer
IL300 Datasheet PDF : 8 Pages
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The isolated amplifier provides the PWM control signal 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 +3V signal. Given this infor-
mation, 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 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 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 (IFq) and the servo gain
(K1). For this design, IFq=12 mA. Figure 4 shows the servo pho-
tocurrent at IFq is found to be 100 µA. With this data R3 can be
calculated.
R3=
Vb
-I--P---l
=
3V
1----0---0----µ----A--
=
30KΩ
Figure 17. Switch mode power supply
Figure 16. Isolated control amplifier
To Control
Input
ISO
AMP
+1
R1
Voltage
Monitor
R2
For best input offset compensation at U1, R2 will equal R3. The
value of R1 can easily be calculated from the following.
R1=
R2
VMONITOR
-----------V----a------------
–
1
20KΩ=
30KΩ
5V
3----V--
–
1
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, Siemens offers K3 graded
into ±5 % bins. R5 can determined using the following equa-
tion,
R5=
VOUT
V----M-----O----N---I--T---O----R--
•
R3 (R1 + R2)
-----------R-----2---K-----3------------
Or if a unity gain amplifer is being designed (VMONI-
TOR=VOUT, R1=0), the euation simplifies to:
R3
R5= K-----3--
110/
220
MAIN
AC/DC
RECTIFIER
SWITCH
XFORMER
DC OUTPUT
AC/DC
RECTIFIER
SWITCH
MODE
REGULATOR
TDA4918
CONTROL
ISOLATED
FEEDBACK
Figure 18. DC coupled power supply feedback amplifier
Vmonitor
R1
20 KΩ
R2
30 KΩ
3+
7 VCC
Va
U1
6
R4
100 Ω
LM201
Vb 2 -
1
8
VCC
4
100 pF
R3
30 KΩ
IL300
1
8
2
K1
3
K2 7
6
4
5
VCC
Vout
R5
30 KΩ
To
control
input
IL300
5–6
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