MAX253_10 Datasheet, PDF (6/18 Pages) Maxim Integrated Products – Transformer Driver for Isolated RS-485 Interface

Transformer Driver for
Isolated RS-485 Interface
V_IN
5V
C1
0.1µF
6
V_CC
4 _SD
ON / OFF
MAX253
R1
50Ω
1
D1
R2
50Ω
3
FREQUENCY
SWITCH
FS
GND1
2
8
D2
GND2
7
Figure 1. Test Circuit
C1
MAX253
FREQUENCY
SWITCH
FS
OSC
400kHz/
700kHz
V_IN 5V
F/F
V_CC
D1
Q
N
T
D2
Q
N
SD
GND2 GND1
5V @ 200mA
ISO OUTPUT
C3
C2
ISO
GND
ON / OFF
Figure 2. Block Diagram
_______________Detailed Description
The MAX253 is an isolated power-supply transformer
driver specifically designed to form the heart of a fully
isolated RS-485 data interface. Completely isolated
communications are obtained by combining the
MAX253 with a linear regulator, a center-tapped trans-
former, optocouplers, and the appropriate Maxim inter-
face product (as described in the Isolated RS-485/RS-
232 Data Interface section).
The MAX253 consists of an RC oscillator followed by a
toggle flip-flop, which generates two 50% duty-cycle
square waves, out-of-phase at half the oscillator fre-
quency (Figure 2). These two signals drive the ground-
referenced output switches. Internal delays ensure
break-before-make action between the two switches.
Ground SD for normal operation. When high, SD dis-
ables all internal circuitry, including the oscillator and
both power switches.
Pulling FS low reduces the oscillator frequency and low-
ers the supply current (see Supply Current vs.
Temperature in the Typical Operating Characteristics).
FS includes a weak pull-up, so it will be set to the high-
frequency state if not connected.
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