Transformer Driver for

Isolated RS-485 Interface

6

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_______________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

D2

FS

GND1

GND2

1

8

4

6

27

3

SD

FREQUENCY

SWITCH

ON / OFF

R2

50

Ω

R1

50

Ω

5V

C1

0.1µF

Figure 1. Test Circuit

MAX253

D1

D2

FS

GND2

GND1

FREQUENCY

SWITCH

C3

C1

C2

5V @ 200mA

ISO OUTPUT

5V

N

N

Q

Q

OSC

F / F

SD

ON / OFF

400kHz/

700kHz

T

ISO

GND

Figure 2. Block Diagram