Datasheet

MAX253
Transformer Driver for
Isolated RS-485 Interface
Maxim Integrated | 6www.maximintegrated.com
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.
MAX253
D1
D2FS
GND1 GND2
V
CC
1
8
4
6
27
3
SD
FREQUENCY
SWITCH
ON / OFF
R2
50Ω
R1
50Ω
V
IN
5V
C1
0.1μF
Figure 1. Test Circuit
MAX253
D1
D2
FS
GND2 GND1
V
CC
FREQUENCY
SWITCH
C3
C1
C2
5V @ 200mA
ISO OUTPUT
5V
N
N
Q
Q
OSC
F / F
V
IN
SD
ON / OFF
400kHz/
700kHz
T
ISO
GND
Figure 2. Block Diagram