MAX253ESA+ Maxim, MAX253ESA+ Datasheet - Page 14
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MAX253ESA+
Manufacturer Part Number
MAX253ESA+
Description
Transformer Dvr for Iso. RS-485/RS-232 I'face; Single Supply 5.0 or 3.3 V, SOIC8
Manufacturer
Maxim
Datasheet
1.MAX253ESA.pdf
(16 pages)
Specifications of MAX253ESA+
Lead Free Status / Rohs Status
RoHS Compliant part
Electrostatic Device
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.
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
Table 2. Typical Transformer Characteristics
*CT = Center Tapped
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
14
______________Component Selection
Figure
Turns Ratio
Typical
Windings
Primary ET
Product
BH Electronics
Phone: (507) 532-3211
FAX: (507) 532-3705
Coilcraft
Phone: (708) 639-6400
FAX: (708) 639-1469
Coiltronics
Phone: (407) 241-7876
FAX: (407) 241-9339
______________________________________________________________________________________
CHARACTERISTIC
TRANSFORMERS
Primary
Secondary
FS Low
FS High
Transformer Selection
9a
1CT*:1
44CT
44
18.3V-µs
11V-µs
+5V to ±10V
Philips Components
Phone: (407) 881-3200
FAX: (407) 881-3300
Magnetics Inc.
Phone: (412) 282-8282
FAX: (412) 282-6955
Fair-Rite Products
Phone: (914) 895-2055
FAX: (914) 895-2629
TRANSFORMER CORES
2, 3, 5, 6
1CT:1.3CT
44CT
56CT
18.3V-µs
11V-µs
+5V to +5V
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-
4, 7
1CT:2.1CT
28CT
56CT
12V-µs
7.2V-µs
+3.3V to +5V
Quality Technology
Phone: (408) 720-1440
FAX: (408) 720-0848
Sharp Electronics
Phone: (206) 834-2500
FAX: (206) 834-8903
Siemens Components
Phone: (408) 777-4500
FAX: (408) 777-4983
8
1CT:5CT
44CT
220CT
18.3V-µs
11V-µs
+5V to +24V
OPTOCOUPLERS
+5V to ±5V; ±12V
10
1CT:1.5CT:3CT
44CT
66CT, 132CT
18.3V-µs
11V-µs