Datasheet

MAX253
Transformer Driver for
Isolated RS-485 Interface
Maxim Integrated | 15www.maximintegrated.com
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 connected high, the guaranteed minimum fre-
quency 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
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Philips Components
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Quality Technology
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Coilcraft
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Magnetics Inc.
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Sharp Electronics
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Coiltronics
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Fair-Rite Products
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Siemens Components
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FAX: [phone]
*CT = Center Tapped