5–6

IL300

Figure 16. Isolated control amplifier

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, Siemens offers K3 graded

into

% bins. R5 can determined using the following equa-

tion,

Or if a unity gain amplifer is being designed (V

MONI-

TOR=VOUT, R1=0), the euation simplifies to:

+

-

Voltage

Monitor

R1

R2

To Control

Input

ISO

AMP

+1

R1

R2

V

MONITOR

V

a

---------------------------

1

–







=

20K

Ω 30KΩ

5V

3V

-------1

–





=

5

±

R5

V

OUT

V

MONITOR

---------------------------

R3 R1

R2

+

()

R2K3

-------------------------------------

•

=

R5

R3

K3

-------

=

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

calculated.

R3

V

b

I

Pl

------

3V

100

µA

------------------

=

=

30K

Ω

=

Figure 17. Switch mode power supply

Figure 18. DC coupled power supply feedback amplifier

SWITCH

XFORMER

SWITCH

MODE

REGULATOR

TDA4918

ISOLATED

FEEDBACK

CONTROL

110/

220

MAIN

DC OUTPUT

AC/DC

RECTIFIER

AC/DC

RECTIFIER

8

7

6

5

100 pF

4

3

1

2

8

6

7

K1

V

CC

V

CC

1

2

3

4

K2

V

CC

Vmonitor

R1

20 K

Ω

R2

30 K

Ω

R3

30 K

Ω

R4

100

Ω

Vout

To

control

input

R5

30 K

Ω

IL300

Vb

Va

+

-

U1

LM201