Transformer Driver for
Isolated RS-485 Interface
_____________________Pin Description
PIN
1
2
3
4
5
6
7
8
NAME
D1
GND1
FS
SD
N.C.
V
CC
GND2
D2
FUNCTION
Open drain of N-channel transfomer drive 1.
Ground. Connect both GND1 and GND2
to ground.
Frequency switch. If FS = V
CC
or open,
switch frequency = 350kHz; if FS = 0V,
switch frequency = 200kHz.
Shutdown. Ground for normal operation,
tie high for shutdown.
Not internally connected.
+5V supply voltage.
Ground. Connect both GND1 and GND2
to ground.
Open drain of N-channel transformer drive 2.
MAX253
V
IN
5V
C1
0.1µF
4
ON / OFF
6
V
CC
SD
D1
R1
50Ω
1
R2
50Ω
D2
8
MAX253
3
FREQUENCY
SWITCH
FS
GND1
2
GND2
7
Figure 1. Test Circuit
V
IN
C1
F/F
V
CC
Q
T
OSC
FREQUENCY
SWITCH
400kHz/
700kHz
Q
5V
D1
N
C3
5V @ 200mA
ISO OUTPUT
C2
MAX253
FS
D2
N
SD
GND2
GND1
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 (see 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 float to the high-fre-
quency state if not connected.
_______________________________________________________________________________________
5
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