Transformer Driver for

Isolated RS-485 Interface

14

______________________________________________________________________________________

sensor, an extra several hundred milliwatts could easily

be supplied by the circuit, as shown. A +12V supply

could be generated by adding two more diodes to the

ends of the secondary, and a -5V supply could be gen-

erated by connecting additional diodes to the 1/4 and

3/4 tap points on the secondary. For +5V only applica-

tions, the MAX187 is recommended.

______________Component Selection

Transformer Selection

The transformer primary used with the MAX253 must be

a center-tapped winding with sufficient ET product to

prevent saturation at the worst-case lowest selected

frequency. The MAX253’s guaranteed minimum fre-

quency with the FS pin held low is 150kHz, equating to

a maximum period of 6.67µs. The required ET product

for half the primary is simply the product of the maxi-

mum supply voltage and half the maximum period.

With FS tied high, the guaranteed minimum frequency

is 250kHz, giving a maximum period of 4µs.

The secondary winding may or may not be center

tapped, depending on the rectifier topology used. The

phasing of the secondary winding is not critical. In

some applications, multiple secondaries might be

required. Half-wave rectification could be used, but is

discouraged because it normally adds a DC imbalance

to the magnetic flux in the core, reducing the ET prod-

uct. If the DC load is imbalanced, full-wave rectification

is recommended, as shown in Figure 9b.

The transformer turns ratio must be set to provide the

minimum required output voltage at the maximum

anticipated load with the minimum expected input volt-

Table 2. Typical Transformer Characteristics

Table 3. Transformer, Transformer Core, and Optocoupler Suppliers

CHARACTERISTIC

+5V to ±10V

+5V to +5V

+3.3V to +5V

+5V to +24V

+5V to ±5V; ±12V

Figure

9a

2, 3, 5, 6

4, 7

8

10

Turns Ratio

1CT*:1

1CT:1.3CT

1CT:2.1CT

1CT:5CT

1CT:1.5CT:3CT

Primary

44CT

44CT

28CT

44CT

44CT

Typical

Windings

Secondary

44

56CT

56CT

220CT

66CT, 132CT

FS Low

18.3V-µs

18.3V-µs

12V-µs

18.3V-µs

18.3V-µs

Primary ET

Product

FS High

11V-µs

11V-µs

7.2V-µs

11V-µs

11V-µs

TRANSFORMERS

TRANSFORMER CORES

OPTOCOUPLERS

BH Electronics

Phone: (507) 532-3211

FAX: (507) 532-3705

Philips Components

Phone: (407) 881-3200

FAX: (407) 881-3300

Quality Technology

Phone: (408) 720-1440

FAX: (408) 720-0848

Coilcraft

Phone: (708) 639-6400

FAX: (708) 639-1469

Magnetics Inc.

Phone: (412) 282-8282

FAX: (412) 282-6955

Sharp Electronics

Phone: (206) 834-2500

FAX: (206) 834-8903

Coiltronics

Phone: (407) 241-7876

FAX: (407) 241-9339

Fair-Rite Products

Phone: (914) 895-2055

FAX: (914) 895-2629

Siemens Components

Phone: (408) 777-4500

FAX: (408) 777-4983

*CT = Center Tapped