MAX253ESA Maxim Integrated Products, MAX253ESA Datasheet - Page 12

• Manufacturer Part Number: MAX253ESA
• Description: IC XFRMR DRIVER RS485 8-SOIC
• Manufacturer: Maxim Integrated Products
• Type: Driverr

Specifications of MAX253ESA

• Number Of Drivers/receivers: 1/0
• Protocol: RS232, RS485
• Voltage - Supply: 4.5 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

A simple lowpass pi-filter (Figures 6 and 7) can be added
to the output to reduce output ripple noise to approximately
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.
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-
Transformer Driver for
Isolated RS-485 Interface
Figure 8. Typical 4mA to 20mA Application Circuit
12
Isolated 4mA to 20mA Analog Interface
______________________________________________________________________________________
V
IN
5V
5V
0.1V to 0.5V
*SEE TABLE 2
4
3
2
SD
MAX480
GND1
2
MAX253
V
CC
7
4
Output-Ripple Filtering
6
GND2
7
49.9kΩ
6
D1
D2
1
8
1
2
3
4
ISOLATION
1CT:5CT*
BARRIER
IL300
1N5817
1N5817
6
5
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.
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
24V UNREGULATED
10µF
49.9kΩ
3
2
MAX480
4
6
7
ISO
5V
24.9Ω
78L05
10kΩ
2N3904
Isolated ADC
2N3904
R
0kΩ to 1kΩ
L