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PDF MAX253 Data sheet ( Hoja de datos )
| Número de pieza | MAX253 | |
| Descripción | Transformer Driver for Isolated RS-485 Interface | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-0226; Rev 0; 1/94
Transformer Driver for
Isolated RS-485 Interface
_______________General Description
The MAX253 is a monolithic oscillator/power-driver,
specifically designed to provide isolated power for an
isolated RS-485 or RS-232 data interface. It
drives a center-tapped transformer primary from a 5V
or 3.3V DC power supply. The secondary can be
wound to provide any isolated voltage needed at power
levels up to 1W.
The MAX253 consists of a CMOS oscillator driving a
pair of N-channel power switches. The oscillator runs
at double the output frequency, driving a toggle flip-flop
to ensure 50% duty cycle to each of the switches.
Internal delays are arranged to ensure break-before-
make action between the two switches.
The SD pin puts the entire device into a low-power
shutdown state, disabling both the power switches and
oscillator.
________________________Applications
Isolated RS-485/RS-232 Power-Supply
Transformer Driver
High Noise-Immunity Communications Interface
Isolated and/or High-Voltage Power Supplies
Bridge Ground Differentials
Medical Equipment
Process Control
____________________________Features
o Power-Supply Transformer Driver for Isolated
RS-485/RS-232 Data-Interface Applications
o Single +5V or +3.3V Supply
o Low-Current Shutdown Mode: 0.4µA
o Pin-Selectable Frequency: 350kHz or 200kHz
o 8-Pin DIP, SO, and µMAX Packages
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX253CPA
0°C to +70°C
8 Plastic DIP
MAX253CSA
0°C to +70°C
8 SO
MAX253CUA
0°C to +70°C
8 µMAX
MAX253C/D
0°C to +70°C
Dice*
MAX253EPA
-40°C to +85°C
8 Plastic DIP
MAX253ESA
-40°C to +85°C
8 SO
MAX253EUA
-40°C to +85°C
8 µMAX
MAX253MJA
-55°C to +125°C 8 CERDIP**
* Contact factory for dice specifications.
**Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit
ON / OFF
VIN 5V
46
SD VCC 1
D1
MAX253
3
FREQUENCY
SWITCH
FS
GND1
2
8
D2
GND2
7
C1
OUTPUT
5V @ 200mA
C3
C2
__________________Pin Configuration
TOP VIEW
D1 1
GND1 2
FS 3
SD 4
MAX253
8 D2
7 GND2
6 VCC
5 N.C.
DIP/SO/µMAX
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
1 page  
Transformer Driver for
Isolated RS-485 Interface
_____________________Pin Description
PIN NAME
1 D1
2 GND1
3 FS
4 SD
5 N.C.
6 VCC
7 GND2
8 D2
FUNCTION
Open drain of N-channel transfomer drive 1.
Ground. Connect both GND1 and GND2
to ground.
Frequency switch. If FS = VCC or open,
switch frequency = 350kHz; if FS = 0V,
switch frequency = 200kHz.
Shutdown. Ground for normal operation,
tie high for shutdown.
Not internally connected.
+5V supply voltage.
Ground. Connect both GND1 and GND2
to ground.
Open drain of N-channel transformer drive 2.
VIN
5V C1
0.1µF
4 SD
ON / OFF
6
VCC
MAX253
R1
50Ω
1
D1
R2
50Ω
3
FREQUENCY
SWITCH
FS
GND1
2
8
D2
GND2
7
Figure 1. Test Circuit
C1
MAX253
FREQUENCY
SWITCH
FS
OSC
400kHz/
700kHz
VIN 5V
F/F
Q
T
VCC
Q
D1
N
D2
N
SD GND2 GND1
5V @ 200mA
ISO OUTPUT
C3 C2
ISO
GND
ON / OFF
Figure 2. Block Diagram
_______________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 (see 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 float to the high-fre-
quency state if not connected.
_______________________________________________________________________________________ 5
5 Page 
VIN
5V
6
VCC 1
D1
MAX253
4
SD
8
D2
GND1 GND2
27
Transformer Driver for
Isolated RS-485 Interface
ISOLATION
BARRIER
1CT:5CT*
1N5817
24V UNREGULATED
10µF
1N5817
78L05
5V
0.1V to 0.5V
37
MAX480
2
4
6
1
2
3
4
49.9kΩ
*SEE TABLE 2
IL300
6
5
RL
0kΩ to 1kΩ
3
6
49.9kΩ
2
MAX480
4
ISO
5V
7
2N3904
2N3904
10kΩ
24.9Ω
Figure 8. Typical 4mA to 20mA Application Circuit
Output-Ripple Filtering
A simple lowpass pi-filter (Figures 6 and 7) can be added
to the output to reduce output ripple noise to about
10mVp-p. The cutoff frequency shown is 21kHz. Since the
filter inductor is in series with the circuit output, minimize its
resistance so the voltage drop across it is not excessive.
Isolated 4mA to 20mA Analog Interface
The 4mA to 20mA current loop is a standard analog
signal range that is widely used in the process-control
industry for transducer and actuator control signals.
These signals are commonly referred to a distant
ground that may be at a considerably higher voltage
with respect to the local ground.
An analog signal in the range of 0.1V to 0.5V is applied
to the first MAX480 to generate a signal current in the
range of 20µA to 100µA. This low-level signal is trans-
ferred across the barrier by the Siemens IL300 linear
optocoupler. This device is unique in that it corrects
the dominant nonlinearity present in most optocou-
plers—the LED efficiency variation. The IL300 is really
two optocouplers in the same package sharing the same
LED; one detector is across the isolation barrier, the
other is on the same side as the LED (Figure 8). The lat-
ter detector is used to generate a feedback signal identi-
cal to the signal on the isolated side of the barrier. The
current signal transferred across the barrier is converted
back to a voltage that matches the input in the 100mV to
500mV range. This voltage is then transformed to the
final 4mA to 20mA current signal range by the second
MAX480, Darlington stage, and the 20Ω resistor.
Isolated ADC
Almost any serial-interface device is a candidate for
operation across an isolation barrier; Figure 10 illus-
trates one example. The MAX176 analog-to-digital
converter (ADC) operates from +5V and -12V supplies,
provided by the multiple-tapped secondary and linear
regulators. If some additional isolated power is needed
for signal conditioning, multiplexing, or possibly for a
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet MAX253.PDF ] | |
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