MAX253_09 Datasheet, PDF (14/17 Pages) Maxim Integrated Products – Transformer Driver for Isolated RS-485 Interface

Transformer Driver for
Isolated RS-485 Interface
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
CHARACTERISTIC
+5V to ±10V
+5V to +5V
+3.3V to +5V
+5V to +24V
Figure
9a
2, 3, 5, 6
4, 7
8
Turns Ratio
1CT*:1
1CT:1.3CT
1CT:2.1CT
1CT:5CT
Typical
Windings
Primary
Secondary
44CT
44
44CT
56CT
28CT
56CT
44CT
220CT
Primary ET
Product
FS Low
FS High
18.3V-µs
11V-µs
18.3V-µs
11V-µs
12V-µs
7.2V-µs
18.3V-µs
11V-µs
*CT = Center Tapped
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
+5V to ±5V; ±12V
10
1CT:1.5CT:3CT
44CT
66CT, 132CT
18.3V-µs
11V-µs
TRANSFORMERS
BH Electronics
Phone: [phone]
FAX: [phone]
Coilcraft
Phone: [phone]
FAX: [phone]
Coiltronics
Phone: [phone]
FAX: [phone]
TRANSFORMER CORES
Philips Components
Phone: [phone]
FAX: [phone]
Magnetics Inc.
Phone: [phone]
FAX: [phone]
Fair-Rite Products
Phone: [phone]
FAX: [phone]
OPTOCOUPLERS
Quality Technology
Phone: [phone]
FAX: [phone]
Sharp Electronics
Phone: [phone]
FAX: [phone]
Siemens Components
Phone: [phone]
FAX: [phone]
14 ______________________________________________________________________________________