AVAILABLE
Functional Diagrams
Pin Configurations appear at end of data sheet.
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
_______________General Description
The MAX253 monolithic oscillator/power-driver is
specifically designed to provide isolated power for an
isolated RS-485 or RS-232 data interface. The device
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
♦♦Power-Supply Transformer Driver for Isolated
RS-485/RS-232 Data-Interface Applications
♦♦Single 5V or 3.3V Supply
♦♦Low-Current Shutdown Mode: 0.4µA
♦♦Pin-Selectable Frequency: 350kHz or 200kHz
♦♦8-Pin DIP, SO, and µMAX®Packages
______________Ordering Information
MAX253
Transformer Driver for
Isolated RS-485 Interface
MAX253
D1
D2FS
GND1 GND2
VCC 1
8
46
27
3
VIN
SD
FREQUENCY
SWITCH
C3
C1
C2
5V @ 200mA
OUTPUT
5V
ON / OFF
__________Typical Operating Circuit
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
MAX253ESA/V -40°C to +85°C 8 SO
MAX253MJA -55°C to +125°C 8 CERDIP**
*
Contact factory for dice specifications.
**
Contact factory for availability and processing to MIL-STD-883.
Devices are also available in a lead(Pb)-free/RoHS-compliant
package. Specify lead-free by adding a (+) to the part number
when ordering.
/V Denotes an automotive qualified part.
µMAX is a registered trademark of Maxim Integrated Products, Inc.
__________Typical Operating Circuit
19-0226; Rev 2; 4/10
MAX253
Transformer Driver for
Isolated RS-485 Interface
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = 5V ±10%, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1: Operating supply current is the current used by the MAX253 only, not including load current.
Note 2: Shutdown supply current includes output switch-leakage currents.
PARAMETER MIN TYP MAX UNITS
Shutdown Input Threshold 0.8 µA
2.4 V
Shutdown Supply Current (Note 2) 0.4 µA
Operating Supply Current (Note 1) 0.45 5.0 mA
Shutdown Input Leakage Current 10 pA
2.4 VFS Input Threshold 0.8
50 µA
Switch On-Resistance 1.5 4.0 Ω
250 350 500 kHzSwitch Frequency 150 200 300
FS Input Leakage Current 10 pA
Start-Up Voltage 2.5 2.2 V
CONDITIONS
FS = VCC
Low
High
SD = VCC
No load, VSD = 0V, FS low
High
Low
VFS = 0V
D1, D2; 100mA
FS = VCC or open
VFS = 0V
Supply Voltage (VCC) ...............................................-0.3V to +7V
Control Input Voltages (SD, FS) .................-0.3V to (VCC + 0.3V)
Output Switch Voltage (D1, D2) .............................................12V
Peak Output Switch Current (D1, D2) ......................................1A
Average Output Switch Current (D1, D2) .........................200mA
Continuous Power Dissipation (TA= +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C)..........................471mW
µMAX (derate 4.10mW/°C above +70°C) .....................330mW
CERDIP (derate 8.00mW/°C above +70°C)..................640mW
Operating Temperature Ranges
MAX253C_ _ ........................................................0°C to +70°C
MAX253E_ _ .....................................................-40°C to +85°C
MAX253MJA ...................................................-55°C to +125°C
Junction Temperatures
MAX253C_ _/E_ _..........................................................+150°C
MAX253MJA .................................................................+175°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow)
PDIP, SO, µMAX lead(Pb)-free .....................................+260°C
PDIP, SO, µMAX, CERDIP containing lead(Pb) ............+240°C
MAX253
2
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
10.5
6.0
-60 -20 60 140
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = LOW)
7.0
9.5
plot01
TEMPERATURE (°C)
OUTPUT RESISTANCE (Ω)
20 100
8.5
8.0
6.5
7.5
10.0
9.0
-40 0 8040 120
VIN = 4.5V
VIN = 5.0V
MEASURED AT TP1
15
6
-60 -20 60 140
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = HIGH)
plot02
TEMPERATURE (°C)
OUTPUT RESISTANCE (Ω)
20 100
9
12
-40 0 8040 120
VIN = 4.5V
VIN = 5.0V
MEASURED AT TP1
1.0
0
-60 -20 60 140
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
plot03
TEMPERATURE (°C)
SHUTDOWN CURRENT (μA)
20 100
0.4
0.8
-40 0 8040 120
0.6
0.2
INCLUDES SWITCH LEAKAGE CURRENTS
260
160
-60 -20 60 140
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = LOW)
plot04
TEMPERATURE (°C)
FREQUENCY (kHz)
20 100
200
240
-40 0 8040 120
220
180
VIN = 5.5V
VIN = 4.5V
VIN = 5.0V
VIN = 6.0V
850
400
-60 -20 60 140
SUPPLY CURRENT vs. TEMPERATURE
(FS = HIGH)
500
750
plot07
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
20 100
650
600
450
550
800
700
-40 0 8040 120
VIN = 5.5V
VIN = 6.0V
VIN = 4.5V
VIN = 5.0V
480
280
-60 -20 60 140
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = HIGH)
plot05
TEMPERATURE (°C)
FREQUENCY (kHz)
20 100
360
440
-40 0 8040 120
400
320
VIN = 4.5V
VIN = 5.0V
VIN = 6.0V
VIN = 5.5V
550
250
-60 -20 60 140
SUPPLY CURRENT vs. TEMPERATURE
(FS = LOW)
plot06
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
20 100
350
-40 0 8040 120
300
400
500
450
600
VIN = 4.5V
VIN = 5.0V
VIN = 5.5V
VIN = 6.0V
100
0
0 40 120 200
EFFICIENCY vs. LOAD CURRENT
(FS = LOW)
20
80
plot08
LOAD CURRENT (mA)
EFFICIENCY (%)
80 160
60
40
10
30
90
70
50
20 60 140100 180
VIN = 5.5V
VIN = 4.5V
__________________________________________Typical Operating Characteristics
(Circuit of Figure 6, VIN = 5V ±10%, TA = +25°C, unless otherwise noted.)
MAX253
Maxim Integrated
3
Transformer Driver for
Isolated RS-485 Interface
10
0
0 40 220
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = LOW)
2
8
plot10
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
80
6
4
1
3
9
7
5
20 60 14012 0100 160 180 200
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1.3
CIRCUIT OF FIGURE 7
VIN = 3.3V
TURNS RATIO = 1:2.1
MEASURED AT TP1
10
0
0 40 220
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = HIGH)
2
8
plot11
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
80
6
4
1
3
9
7
5
20 60 14012 0100 160 180 200
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1.3
CIRCUIT OF FIGURE 7
VIN = 3.3V
TURNS RATIO = 1:2.1
MEASURED AT TP1
CIRCUIT OF FIGURE 1
SWITCHING WAVEFORMS
(TWO CYCLES)
D1
D2
CIRCUIT OF FIGURE 1
SWITCHING WAVEFORMS
(BREAK BEFORE MAKE)
D1
D2
CIRCUIT OF FIGURE 6
TIME FROM SHUTDOWN TO POWER-UP
SD
TP1 (OUTPUT VOLTAGE)
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 6, VIN = 5V ±10%, TA = +25°C, unless otherwise noted.)
100
0
0 40 120 200
EFFICIENCY vs. LOAD CURRENT
(FS = HIGH)
20
80
plot09
LOAD CURRENT (mA)
EFFICIENCY (%)
80 160
60
40
10
30
90
70
50
20 60 140100 180
VIN = 5.5V
VIN = 4.5V
MAX253
4
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
1
2
3
4
8
7
6
5
D2
GND2
VCC
N.C.
SD
FS
GND1
D1
DIP/SO/
μ
MAX
TOP VIEW
MAX253
+
__________________Pin Configuration _____________________Pin Description
Not internally connected.N.C.5
5V supply voltage.VCC
6
GND27
Open drain of N-channel transformer drive 2.D28
SD4
FS3
PIN
GND12
Open drain of N-channel transformer drive 1.D11
FUNCTIONNAME
Ground. Connect both GND1 and GND2
to ground.
Frequency switch. If FS = VCC or open,
switch frequency = 350kHz; if VFS = 0V,
switch frequency = 200kHz.
Shutdown. Ground for normal operation,
connect high for shutdown.
Ground. Connect both GND1 and GND2
to ground.
MAX253
Maxim Integrated
5
Transformer Driver for
Isolated RS-485 Interface
_______________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 (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 be set to the high-
frequency state if not connected.
MAX253
D1
D2FS
GND1 GND2
VCC
1
8
4
6
27
3
SD
FREQUENCY
SWITCH
ON / OFF
R2
50Ω
R1
50Ω
VIN
5V C1
0.1µF
Figure 1. Test Circuit
MAX253
D1
D2
FS
GND2 GND1
VCC
FREQUENCY
SWITCH
C3
C1
C2
5V @ 200mA
ISO OUTPUT
5V
N
N
Q
Q
OSC
F / F
VIN
SD
ON / OFF
400kHz/
700kHz
T
ISO
GND
Figure 2. Block Diagram
MAX253
6
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
C3
0.1μF
C1
0.1μF
C2
22μF
ISO 5V
ISOLATION
BARRIER
5V
VIN
MAX253
D1
D2
FS
GND1 GND2
VCC
1
8
6
27
31N5817
SD
4
ON / OFF
390Ω
*74HC04
DE
RO
390Ω
*74HC04
DI
3.3kΩ
1
3
5
4
6
1
2
4
3
5
4
1
3
6
*74HC04
*74HC04 OR EQUIVALENT
** SEE TABLE 2
MAX667
564
SHDNSET GND
MAX481
MAX483
MAX485
MAX487
52
RE GND
C4
22μF
IN OUT
82
1N5817
390Ω
4
3.3kΩ
3.3kΩ
3
1
8
6
7
DI
DE
RO
A
B
PC410 / 417
PC357T
PC410 / 417
1CT:1.3CT**
VCC
485
I/O
Figure 3. Typical RS-485 Application Circuit, 5V Configuration
MAX253
Maxim Integrated
7
Transformer Driver for
Isolated RS-485 Interface
C3
0.1µF
C1
0.1µF
C2
22µF
ISO 5V
ISOLATION
BARRIER
3.3V
VIN
MAX253
D1
D2
GND1 GND2
VCC
1
8
6
27
N.C.
5
1N5817
1N5817
C5
0.1µF
SD
4
ON / OFF
390Ω
*74HC04
DE
RO
390Ω
*74HC04
DI
3.3kΩ
1
3
5
4
6
1
2
4
3
5
4
1
3
6
*74HC04
*74HC04 OR EQUIVALENT
** SEE TABLE 2
MAX667
564
SHDNSET GND
MAX481
MAX483
MAX485
MAX487
52
RE GND
C4
22µF
IN OUT
82
1N5817
390Ω
4
3
1
8
6
7
DI
DE
RO
A
B
PC410 / 417
PC357T
PC410 / 417
1N5817
FS 3
1CT:2.1CT**
VCC
3.3kΩ
3.3kΩ
485
I/O
Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration
MAX253
8
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
C3
0.1µF
C2
22µF
ISO 5V
ISOLATION
BARRIER
5V
VIN
1
8
5
C1
0.1µF
1N5817
MAX253
D1
D2
GND1 GND2
VCC
6
27
N.C.
SD
4
ON / OFF
10 x PC417
*74HC04 OR EQUIVALENT
** SEE TABLE 2
MAX205
ENSD
C4
22µF
82
1N5817
MAX667
564
SHDNSET GND
IN OUT
390Ω
83
T1IN T1OUT
390Ω
*74HC04
T1IN
1
24
5
6
1
24
5
6
390Ω
74HC04
T2IN
390Ω
74HC04
T3IN
390Ω
74HC04
T4IN
390Ω
74HC04
T5IN
74HC04
R1OUT
74HC04
74HC04
74HC04
74HC04
R2OUT
R3OUT
R4OUT
R5OUT
5 X 3.3kΩ
390Ω
390Ω
390Ω
390Ω
74
T2IN T2OUT
15 2
T3IN T3OUT
16 1
T4IN T4OUT
22 19
T5IN T5OUT
910
R1OUT R1IN
65
R2OUT R2IN
23 24
R3OUT R3IN
17 18
R4OUT R4IN
14 13
R5OUT R5IN
VCC
2021
GND
5 x 3.3kΩ
1N5711 6
5
4
1
2
390Ω
3.3kΩ
VCC
ISO
ROUT
ROUT
1N5711
1
2
6
5
4
390Ω3.3kΩ
VCC
ISO
TIN
TIN
*74HC04
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps)
FS 3
74HCO4
4N25
ISO
GND
4N25
ISO
GND
1CT:1.3CT**
Figure 5. Typical RS-232 Application Circuit
MAX253
Maxim Integrated
9
Transformer Driver for
Isolated RS-485 Interface
* PC-Series Optocouplers, Sharp Electronics
USA Phone: (206) 834-2500
FAX: (206) 834-8903
Sharp Electronics, Europe GmbH
Germany Phone: (040) 2376-0
FAX: (040) 230764
__________Applications Information
Figures 3–5 are typical isolated RS-485/RS-232 data-inter-
face circuits. These circuits withstand 1800VRMS (1sec)
and are intended for industrial communications and control
applications where very high voltage transients, differential
ground potentials, or high noise may be encountered.
Table 2 lists transformer characteristics for the applica-
tions of Figures 3–10. Some suggested manufacturers
of transformers, transformer cores, and optocouplers
are listed in Table 3, along with their respective phone
and fax numbers.
Important layout considerations include:
♦♦For maximum isolation, the “isolation barrier” should not
be breached. Connections and components from one
side should not be located near those of the other side.
♦♦Since the optocoupler outputs are relatively high-
impedance nodes, they should be located as close
as possible to the Maxim interface IC. This mini-
mizes stray capacitance and maximizes data rate.
Refer to the µMAX package information for pin spacing
and physical dimensions.
Isolated RS-485 Data Interface
The MAX253 power-supply transformer driver is
designed specifically for isolated RS-485 data-interface
applications. The application circuits of Figures 3 and 4
combine the MAX253 with a low-dropout linear regulator,
a transformer, several high-speed optocouplers, and a
Maxim RS-485 interface device. With a few modifica-
tions to these circuits, full-duplex communications can
be implemented by substituting the MAX481/MAX485
with the MAX490/MAX491 (for data rates up to 2.5Mbps)
or substituting the MAX483/MAX487 with the
MAX488/MAX489 (for data rates up to 250kbps).
The data transfer rates of the application circuits in
Figures 3 and 4 are critically limited by the optocou-
plers. Table 1 lists suggested optocouplers and the
appropriate Maxim interface device for data-transfer
rates up to 2.5Mbps.
Refer to the MAX1480 data sheet for a complete isolat-
ed RS-485 solution in one package.
Isolated RS-232 Data Interface
The MAX253 is ideal for isolated RS-232 data-interface
applications requiring more than four transceivers. The
1W power output capability of the MAX253 enables it to
drive more than 10 transceivers simultaneously. Figure 5
shows the typical application circuit for a complete
120kbps isolated RS-232 data interface. The figure
also shows how the Sharp PC417 optocouplers can be
replaced by the lower-cost 4N25 devices to achieve
data-transfer rates up to 9.6kbps.
For 3.3V operation, substitute the primary portion of
Figure 5 with the circuit of Figure 7.
For applications requiring two transceivers or fewer,
refer to the MAX250/MAX251 or MAX252 data sheet.
Isolated Power Supplies
The MAX253 is a versatile isolated power driver, capa-
ble of driving a center-tapped transformer primary from
a 5V or a 3.3V DC power supply (Figures 6 and 7). The
secondary can be wound to provide any isolated volt-
age needed at power levels up to 1W with a 5V supply,
or 600mW with a 3.3V supply. Figure 6 shows a typical
5V to isolated 5V application circuit that delivers up to
200mA of isolated 5V power.
In Figure 7, the MAX253 is configured to operate from a
3.3V supply, deriving a “boost” VCC for the MAX253 by
connecting diodes to both ends of the transformer pri-
mary. This produces nearly double the input supply,
and may be useful for other applications, as shown in
Figure 4. The average current in each MAX253 switch
must still be limited to less than 200mA, so the total
power available is approximately 600mW.
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates
DATA RATE FULL DUPLEX
RS-485 IC
HALF DUPLEX
RS-485 IC
OPTOCOUPLER
FOR DI / RO
OPTOCOUPLER
FOR DE
250kbps MAX488/MAX489 MAX483/MAX487 PC417* PC357T*
2.5Mbps MAX490/MAX491 MAX481/MAX485 PC410* PC357T
MAX253
10
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
C3
0.1µF
C1
0.1µF
C2
22µF
5V @ 200mA
ISO OUTPUT
5V
VIN
L2
25µH
MAX253
D1
D2FS
GND1 GND2
VCC
1
8
6
27
3
FREQUENCY
SWITCH
SD
4
C7
2.2µF
FILTER
OUTPUT
OUTPUT
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
TP1
1N5817
1CT:1.3CT*
1N5817
ON / OFF
*SEE TABLE 2
Figure 6. 5V to Isolated 5V Application Circuit
C3
0.1µF
C1
0.1µF
C2
22µF
5V @ 100mA
ISO OUTPUT
3.3V
VIN
L2
25µH
MAX253
D1
D2FS
GND1 GND2 VCC
1
8
627
3
FREQUENCY
SWITCH
SD
4
C7
2.2µF
FILTER
OUTPUT
OUTPUT
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
TP1
1N5817
1CT:2.1CT*
1N5817
ON / OFF
1N5817
1N5817
C4
0.1µF
*SEE TABLE 2
Figure 7. 3.3V to Isolated 5V Application Circuit
MAX253
Maxim Integrated
11
Transformer Driver for
Isolated RS-485 Interface
10µF
5V
MAX253
D1
D2
GND1 GND2
VCC 1
8
6
27
1N5817
SD
4
1N5817
1CT:5CT*
49.9kΩ
7
6
3
24
7
6
3
24
10kΩ
24.9Ω
2N3904
MAX480
MAX480
5V
0.1V to 0.5V
ISO
5V
78L05
IL300
4
3
2
1
5
6
24V UNREGULATED
49.9kΩ
RL
0kΩ to 1kΩ
2N3904
*SEE TABLE 2
ISOLATION
BARRIER
VIN
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 approximately
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 20Ωresistor.
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
MAX253
12
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
-VOUT
OUTPUT ≈ -2VIN
VIN
INPUT
MAX253
GND1 GND2
VCC 1
8
6
27
1N5817
1CT:1CT*
1N5817
D1
D2
+VOUT
OUTPUT ≈ 2VIN
RL+
RL-
RL+ ≅ RL-
*SEE TABLE 2
Figure 9a. Half-Wave Rectifier—Bipolar
VOUT ≈ -VIN
OUTPUT
VIN
INPUT
MAX253
GND1 GND2
VCC 1
8
6
27
1CT:1CT*
D1
D2
VOUT ≈ +VIN
OUTPUT
4 x 1N5817
*SEE TABLE 2
Figure 9b. Full-Wave Rectifier—Bipolar
VIN
INPUT
MAX253
GND1 GND2
VCC 1
8
6
27
1CT:1CT*
D1
D2
VOUT ≈ 2 x VIN
OUTPUT
4 x 1N5817
*SEE TABLE 2
Figure 9c. Full-Wave Rectifier—Unipolar
MAX253
Maxim Integrated
13
Transformer Driver for
Isolated RS-485 Interface
10μF
6N136
MAX253
D1
D2
GND1 GND2
VCC
4
27
8
1
SD
6
6N136
6N136
10μF
79 L12
78L05
1
2
3
4
8
7
6
5
1
2
3
4
8
7
6
5
1
2
3
4
8
7
6
5
1
2
3
4
8
7
6
5
CONVST
VDD VSS
CLOCK
DATA
AIN
VREF
GND
MAX176
3kΩ
3kΩ
470Ω
0.1μF10μF
0.1μF
10μF
0.1μF
10μF
5V
200Ω
200Ω
8.2kΩ
74HC04
7
6
5
4
3
2
1
15
16
QH
QG
QF
QE
QD
QC
QB
QA
SER
SCK
RCK
SCLR
14
11
12
10
74HC595
13 8
5V
0.1μF
D11(MSB)
D10
D9
D8
5V
7
6
5
4
3
2
1
15
16
QH
QG
QF
QE
QD
QC
QB
QA
SER
SCK
RCK
SCLR
14
11
12
10
74HC595
13 8
5V
0.1μF
D7
D6
D5
D4
D3
D2
D1
D0(LSB)
5V
74HC04
ON/OFF
START
INPUT CLOCK
1CT : 1.5CT : 3CT*
4 x 1N5817
VIN
5V
SIGNAL
GROUND
ANALOG
INPUT
ISO
5V
ISO
-12V
8
QH′
ISOLATION
BARRIER
*SEE TABLE 2
Figure 10. Typical Isolated ADC Application
MAX253
14
Maxim Integrated
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 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, 7 810
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: (516) 241-7876
FAX: (516) 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
MAX253
Maxim Integrated
15
Transformer Driver for
Isolated RS-485 Interface
age. In addition, include in the calculations an
allowance for worst-case losses in the rectifiers. Since
the turns ratio determined in this manner will ordinarily
produce a much higher voltage at the secondary under
conditions of high input voltage and/or light loading, be
careful to prevent an overvoltage condition from occur-
ring (see Output Voltage vs. Load Current in the
Typical
Operating Characteristics
).
Transformers used with the MAX253 will ordinarily be
wound on high-permeability magnetic material. To min-
imize radiated noise, use common closed-magnetic-
path physical shapes (e.g., pot cores, toroids, E/I/U
cores). A typical core is the Philips 213CT050-3B7,
which is a toroid 0.190” in diameter and 0.05” thick.
For operation with this core at 5.5V maximum supply
voltage, the primary should have approximately 22
turns on each side of the center tap, or 44 turns total.
This will result in a nominal primary inductance of
approximately 832µH. The secondary can be scaled to
produce the required DC output.
Diode Selection
The MAX253’s high switching frequency demands
high-speed rectifiers. Schottky diodes are recom-
mended. Ensure that the Schottky diode average cur-
rent rating exceeds the load-current level. The 1N5817
is a good choice for through-hole applications, and the
NIEC* SB05W05C dual in an SOT-23 package is rec-
ommended for surface-mount applications. Use the
higher frequency setting to reduce ripple.
Output Filter Capacitor
In applications sensitive to output-ripple noise, the out-
put filter capacitor C2 should have a low effective
series resistance (ESR), and its capacitance should
remain fairly constant over temperature. Sprague 595D
surface-mount solid tantalum capacitors and Sanyo
OS-CON through-hole capacitors are recommended
due to their extremely low ESR. Capacitor ESR usually
rises at low temperatures, but OS-CON capacitors pro-
vide very low ESR below 0°C.
In applications where output ripple is not critical, a
0.1µF chip or ceramic capacitor is sufficient. Refer to
Table 4 for suggested capacitor suppliers. Use the
higher frequency setting to reduce ripple.
Input Bypass Capacitor
The input bypass capacitor C1 is not critical. Unlike
switching regulators, the MAX253’s supply current is
fairly constant, and is therefore less dependent on the
input bypass capacitor. A low-cost 0.1µF chip or
ceramic capacitor is normally sufficient for input
bypassing.
* Nihon Inter Electronics Corp.
USA Phone: (805) 867-2555
FAX: (805) 867-2556
Japan Phone: 81-3-3494-7411
FAX: 81-3-3494-7414
Table 4. Suggested Capacitor Suppliers
PRODUCTION METHOD CAPACITORS
Surface Mount
Matsuo
267 series (low ESR)
USA Phone: (714) 969-2491, FAX: (714) 960-6492
Sprague Electric Co.
595D/293D series (very low ESR)
USA Phone: (603) 224-1961, FAX: (603) 224-1430
Murata Erie
Ceramic
USA Phone: (800) 831-9172, FAX: (404) 436-3030
High-Performance
Through Hole
Sanyo
OS-CON series (very low ESR)
USA Phone: (619) 661-6835, FAX: (619) 661-1055
Japan Phone: 81-7-2070-1005, FAX: 81-7-2070-1174
Through Hole
Nichicon
PL series (low ESR)
USA Phone: (708) 843-7500, FAX: (708) 843-2798
Japan Phone: 81-7-5231-8461, FAX: 81-7-5256-4158
MAX253
16
Maxim Integrated
Transformer Driver for
Isolated RS-485 Interface
___________________Chip Information
PROCESS: CMOS
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
8 µMAX U8+1 21-0036
8 PDIP P8+1 21-0043
8 SO S8+4 21-0041
8 CDIP J8-2 21-0045
Package Information
For the latest package outline information and land patterns,
go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package
drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
MAX253
Maxim Integrated
17
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 1/94 Initial release —
1 8/09 Deleted the MAX253EUA part number from the Ordering Information table 1
2 4/10 Added automotive qualified part number to the Ordering Information table 1
Transformer Driver for
Isolated RS-485 Interface
MAX253
18 Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.
Maxim reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical
Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
© 2010 Maxim Integrated The Maxim logo and Maxim Integrated are trademarks of Maxim Integrated Products, Inc.
