MAX253 Datasheet, PDF (11/16 Pages) Maxim Integrated Products – Transformer Driver for Isolated RS-485 Interface

VIN
5V
6
VCC
1
D1
MAX253
4
SD
8
D2
GND1 GND2
2
7
Transformer Driver for
Isolated RS-485 Interface
ISOLATION
BARRIER
1CT:5CT*
1N5817
24V UNREGULATED
10µF
1N5817
78L05
5V
3
7
1
IL300
0.1V to 0.5V
6
2
MAX480
2
4
3
6
4
5
49.9kΩ
*SEE TABLE 2
RL
0kΩ to 1kΩ
ISO
5V
3
6
49.9kΩ _MAX480 ⁷
2
4
2N3904
2N3904
10kΩ
24.9Ω
Figure 8. Typical 4mA to 20mA Application Circuit
Output-Ripple Filtering
A simple lowpass pi-filter (Figures 6 and 7) can be added
to the output to reduce output ripple noise to about
10mVp-p. The cutoff frequency shown is 21kHz. Since the
filter inductor is in series with the circuit output, minimize its
resistance so the voltage drop across it is not excessive.
Isolated 4mA to 20mA Analog Interface
The 4mA to 20mA current loop is a standard analog
signal range that is widely used in the process-control
industry for transducer and actuator control signals.
These signals are commonly referred to a distant
ground that may be at a considerably higher voltage
with respect to the local ground.
An analog signal in the range of 0.1V to 0.5V is applied
to the first MAX480 to generate a signal current in the
range of 20µA to 100µA. This low-level signal is trans-
ferred across the barrier by the Siemens IL300 linear
optocoupler. This device is unique in that it corrects
the dominant nonlinearity present in most optocou-
plers—the LED efficiency variation. The IL300 is really
two optocouplers in the same package sharing the same
LED; one detector is across the isolation barrier, the
other is on the same side as the LED (Figure 8). The lat-
ter detector is used to generate a feedback signal identi-
cal to the signal on the isolated side of the barrier. The
current signal transferred across the barrier is converted
back to a voltage that matches the input in the 100mV to
500mV range. This voltage is then transformed to the
final 4mA to 20mA current signal range by the second
MAX480, Darlington stage, and the 20Ω resistor.
Isolated ADC
Almost any serial-interface device is a candidate for
operation across an isolation barrier; Figure 10 illus-
trates one example. The MAX176 analog-to-digital
converter (ADC) operates from +5V and -12V supplies,
provided by the multiple-tapped secondary and linear
regulators. If some additional isolated power is needed
for signal conditioning, multiplexing, or possibly for a
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