19-0226; Rev 2; 4/10
Transformer Driver for
Isolated RS-485 Interface
_______________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.
____________________________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
MAX253
______________Ordering Information
PART
MAX253CPA
MAX253CSA
MAX253CUA
MAX253C/D
MAX253EPA
MAX253ESA
MAX253ESA/V
MAX253MJA
TEMP RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
PIN-PACKAGE
8 Plastic DIP
8 SO
8 μMAX
Dice*
8 Plastic DIP
8 SO
8 SO
8 CERDIP**
________________________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
*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.
__________Typical Operating Circuit
ON / OFF
4
SD
6
V
CC
D1
1
C3
C2
V
IN
5V
C1
OUTPUT
5V @ 200mA
MAX253
3
FREQUENCY
SWITCH
FS
GND1
2
D2
GND2
7
8
µMAX is a registered trademark of Maxim Integrated Products, Inc.
________________________________________________________________
Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Transformer Driver for
Isolated RS-485 Interface
MAX253
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V
CC
) ...............................................-0.3V to +7V
Control Input Voltages (SD, FS) .................-0.3V to (V
CC
+ 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 (T
A
= +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
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.
ELECTRICAL CHARACTERISTICS
(V
CC
= 5V ±10%, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETER
Switch On-Resistance
Switch Frequency
Operating Supply Current (Note 1)
Shutdown Supply Current (Note 2)
Shutdown Input Threshold
Shutdown Input Leakage Current
FS Input Threshold
FS Input Leakage Current
Start-Up Voltage
High
Low
V
FS
= 0V
FS = V
CC
2.5
10
2.2
2.4
0.8
50
D1, D2; 100mA
FS = V
CC
or open
V
FS
= 0V
No load, V
SD
= 0V, FS low
SD = V
CC
High
Low
10
2.4
0.8
250
150
CONDITIONS
MIN
TYP
1.5
350
200
0.45
0.4
MAX
4.0
500
300
5.0
UNITS
Ω
kHz
mA
µA
V
µA
pA
V
µA
pA
V
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.
2
_______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
__________________________________________Typical Operating Characteristics
(Circuit of Figure 6, V
IN
= 5V ±10%, T
A
= +25°C, unless otherwise noted.)
MAX253
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = LOW)
plot01
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = HIGH)
MEASURED AT TP1
OUTPUT RESISTANCE (Ω)
plot02
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
INCLUDES SWITCH LEAKAGE CURRENTS
SHUTDOWN CURRENT (μA)
0.8
plot03
10.5
MEASURED AT TP1
10.0
OUTPUT RESISTANCE (Ω)
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
-60 -40 -20 0
V
IN
= 5.0V
V
IN
= 4.5V
15
1.0
12
V
IN
= 4.5V
V
IN
= 5.0V
9
0.6
0.4
0.2
6
20 40 60 80 100 120 140
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
0
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = LOW)
plot04
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = HIGH)
plot05
SUPPLY CURRENT vs. TEMPERATURE
(FS = LOW)
V
IN
= 6.0V
plot06
260
480
600
550
240
FREQUENCY (kHz)
V
IN
= 6.0V
FREQUENCY (kHz)
440
V
IN
= 6.0V
SUPPLY CURRENT (μA)
500
V
IN
= 5.5V
450
400
350
300
V
IN
= 4.5V
V
IN
= 4.5V
250
-60 -40 -20 0
20 40 60 80 100 120 140
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
V
IN
= 5.0V
220
V
IN
= 5.5V
400
V
IN
= 5.5V
360
V
IN
= 5.0V
200
180
V
IN
= 5.0V
V
IN
= 4.5V
320
160
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
280
SUPPLY CURRENT vs. TEMPERATURE
(FS = HIGH)
plot07
EFFICIENCY vs. LOAD CURRENT
(FS = LOW)
90
plot08
850
800
SUPPLY CURRENT (μA)
750
V
IN
= 5.5V
V
IN
= 6.0V
100
V
IN
= 5.5V
V
IN
= 4.5V
80
EFFICIENCY (%)
70
60
50
40
30
700
650
600
550
500
450
400
-60 -40 -20 0
V
IN
= 5.0V
V
IN
= 4.5V
20
10
0
20 40 60 80 100 120 140
0
20 40 60 80 100 120 140 160 180 200
LOAD CURRENT (mA)
TEMPERATURE (°C)
_______________________________________________________________________________________
3
Transformer Driver for
Isolated RS-485 Interface
MAX253
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 6, V
IN
= 5V ±10%, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(FS = HIGH)
plot09
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = LOW)
plot10
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = HIGH)
9
8
OUTPUT VOLTAGE (V)
CIRCUIT OF FIGURE 7
V
IN
= 3.3V
TURNS RATIO = 1:2.1
CIRCUIT OF FIGURE 6
V
IN
= 5.0V
TURNS RATIO = 1:1.3
plot11
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0
V
IN
= 4.5V
V
IN
= 5.5V
10
9
8
OUTPUT VOLTAGE (V)
7
6
5
4
3
2
1
0
CIRCUIT OF FIGURE 6
V
IN
= 5.0V
TURNS RATIO = 1:1
CIRCUIT OF FIGURE 7
V
IN
= 3.3V
TURNS RATIO = 1:2.1
CIRCUIT OF FIGURE 6
V
IN
= 5.0V
TURNS RATIO = 1:1.3
10
7
6
5
4
3
2
CIRCUIT OF FIGURE 6
V
IN
= 5.0V
TURNS RATIO = 1:1
MEASURED AT TP1
0 20 40 60 80 100 120 140 160 180 200 220
LOAD CURRENT (mA)
1
0
MEASURED AT TP1
0 20 40 60 80 100 120 140 160 180 200 220
LOAD CURRENT (mA)
20 40 60 80 100 120 140 160 180 200
LOAD CURRENT (mA)
SWITCHING WAVEFORMS
(TWO CYCLES)
SWITCHING WAVEFORMS
(BREAK BEFORE MAKE)
D1
D1
D2
D2
CIRCUIT OF FIGURE 1
CIRCUIT OF FIGURE 1
TIME FROM SHUTDOWN TO POWER-UP
SD
TP1 (OUTPUT VOLTAGE)
CIRCUIT OF FIGURE 6
4
_______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
__________________Pin Configuration
TOP VIEW
_____________________Pin Description
PIN
1
2
NAME
D1
GND1
FS
SD
N.C.
V
CC
GND2
D2
FUNCTION
Open drain of N-channel transformer drive 1.
Ground. Connect both GND1 and GND2
to ground.
Frequency switch. If FS = V
CC
or open,
switch frequency = 350kHz; if V
FS
= 0V,
switch frequency = 200kHz.
Shutdown. Ground for normal operation,
connect 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.
MAX253
+
D1
GND1
1
2
8
7
D2
GND2
V
CC
N.C.
3
4
5
6
FS 3
SD 4
MAX253
6
5
DIP/SO/μMAX
7
8
_______________________________________________________________________________________
5
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