max253cuat Maxim Integrated Products, Inc., max253cuat Datasheet - Page 5

• Manufacturer Part Number: max253cuat
• Description: Max253 Transformer Driver For Isolated Rs-485 Interface
• Manufacturer: Maxim Integrated Products, Inc.

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-
_____________________Pin Description
Figure 2. Block Diagram
_______________Detailed Description
PIN
1
2
3
4
5
6
7
8
NAME
GND1
GND2
N.C.
V
SD
D1
FS
D2
CC
FREQUENCY
SWITCH
Open drain of N-channel transfomer drive 1.
Ground. Connect both GND1 and GND2
to ground.
Frequency switch. If FS = V
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.
C1
_______________________________________________________________________________________
FS
400kHz/
700kHz
MAX253
OSC
FUNCTION
T
F / F
CC
Q
Q
SD
ON / OFF
or open,
V
IN
V
CC
5V
GND2
Isolated RS-485 Interface
GND1
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.
Figure 1. Test Circuit
Transformer Driver for
V
5V
N
N
IN
D1
D2
FREQUENCY
ON / OFF
SWITCH
C1
0.1µF
4
3
SD
FS
GND1
MAX253
2
V
CC
6
GND2
C3
7
D1
D2
1
8
50
ISO
GND
R1
5V @ 200mA
C2
ISO OUTPUT
50
R2
5