MAXIM MAX253CSA

19-0226; Rev 0; 1/94
Transformer Driver for
Isolated RS-485 Interface
________________________Applications
____________________________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
PART
TEMP. RANGE
PIN-PACKAGE
MAX253CPA
0°C to +70°C
8 Plastic DIP
Isolated RS-485/RS-232 Power-Supply
Transformer Driver
MAX253CSA
0°C to +70°C
8 SO
MAX253CUA
0°C to +70°C
8 µMAX
High Noise-Immunity Communications Interface
MAX253C/D
0°C to +70°C
Dice*
Isolated and/or High-Voltage Power Supplies
MAX253EPA
-40°C to +85°C
8 Plastic DIP
Bridge Ground Differentials
MAX253ESA
-40°C to +85°C
8 SO
Medical Equipment
MAX253EUA
-40°C to +85°C
8 µMAX
Process Control
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
VIN
ON / OFF
4
6
SD
VCC
D1
5V
TOP VIEW
C1
OUTPUT
5V @ 200mA
1
C3
C2
MAX253
3
FREQUENCY
SWITCH
FS
__________________Pin Configuration
D1
1
8
D2
GND1
2
7
GND2
6
VCC
5
N.C.
FS 3
D2
GND1
GND2
2
7
8
MAX253
SD 4
DIP/SO/µMAX
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
1
MAX253
_______________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-beforemake 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.
MAX253
Transformer Driver for
Isolated RS-485 Interface
ABSOLUTE MAXIMUM RATINGS
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, 10sec) .............................+300°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
(VCC = 5V ±10%, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
CONDITIONS
PARAMETER
Switch On Resistance
Switch Frequency
MIN
D1, D2; 100mA
TYP
MAX
UNITS
1.5
4.0
Ω
FS = VCC or open
250
350
500
FS = 0V
150
200
300
5.0
Operating Supply Current (Note 1)
No load, SD = 0V, FS low
0.45
Shutdown Supply Current (Note 2)
SD = VCC
0.4
Shutdown Input Threshold
High
FS Input Threshold
FS Input Leakage Current
Start-Up Voltage
V
Low
0.8
10
High
Low
0.8
FS = 0V
50
2.5
V
µA
10
pA
2.2
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
µA
pA
2.4
FS = VCC
mA
µA
2.4
Shutdown Input Leakage Current
kHz
_______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
MEASURED AT TP1
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
1.0
plot02
15
plot01
10.5
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = HIGH)
MEASURED AT TP1
INCLUDES SWITCH LEAKAGE CURRENTS
VIN = 4.5V
9.0
8.5
8.0
VIN = 5.0V
7.5
7.0
SHUTDOWN CURRENT (µA)
OUTPUT RESISTANCE (Ω)
OUTPUT RESISTANCE (Ω)
10.0
9.5
plot03
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = LOW)
12
VIN = 4.5V
VIN = 5.0V
9
0.8
0.6
0.4
0.2
6.5
-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)
TEMPERATURE (°C)
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = LOW)
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = HIGH)
SUPPLY CURRENT vs. TEMPERATURE
(FS = LOW)
FREQUENCY (kHz)
VIN = 5.5V
200
VIN = 5.0V
VIN = 5.5V
360
plot06
plot05
400
VIN = 6.0V
500
VIN = 5.5V
450
VIN = 5.0V
400
350
VIN = 5.0V
320
300
VIN = 4.5V
VIN = 4.5V
VIN = 4.5V
280
160
250
-60 -40 -20 0 20 40 60 80 100 120 140
-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)
TEMPERATURE (°C)
EFFICIENCY vs. LOAD CURRENT
(FS = LOW)
SUPPLY CURRENT vs. TEMPERATURE
(FS = HIGH)
90
VIN = 6.0V
700
VIN = 5.5V
650
600
VIN = 5.0V
550
VIN = 5.5V
80
750
500
plot08
100
plot07
850
800
EFFICIENCY (%)
180
600
550
VIN = 6.0V
440
220
-60 -40 -20 0 20 40 60 80 100 120 140
SUPPLY CURRENT (µA)
VIN = 6.0V
SUPPLY CURRENT (µA)
FREQUENCY (kHz)
480
plot04
260
240
0
6
6.0
70
VIN = 4.5V
60
50
40
30
VIN = 4.5V
450
20
10
0
400
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
0
20 40 60 80 100120 140 160 180200
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
MAX253
__________________________________________Typical Operating Characteristics
(Circuit of Figure 6, VIN = 5V ±10%, TA = +25°C, unless otherwise noted.)
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 6, VIN = 5V ±10%, TA = +25°C, unless otherwise noted.)
9
VIN = 5.5V
70
VIN = 4.5V
60
50
40
30
8
OUTPUT VOLTAGE (V)
80
6
4
2
10
1
0
0
20 40 60 80 100120 140 160 180200
8
5
3
10
9
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1.3
7
20
0
CIRCUIT OF FIGURE 7
VIN = 3.3V
TURNS RATIO = 1:2.1
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1
OUTPUT VOLTAGE (V)
90
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = HIGH)
plot10
10
plot09
100
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = LOW)
plot11
EFFICIENCY vs. LOAD CURRENT
(FS = HIGH)
EFFICIENCY (%)
MAX253
Transformer Driver for
Isolated RS-485 Interface
CIRCUIT OF FIGURE 7
VIN = 3.3V
TURNS RATIO = 1:2.1
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1.3
7
6
5
4
3
2
CIRCUIT OF FIGURE 6
VIN = 5.0V
TURNS RATIO = 1:1
1
MEASURED AT TP1
MEASURED AT TP1
0
0 20 40 60 80 100 120 140 160 180 200 220
LOAD CURRENT (mA)
0 20 40 60 80 100 120 140 160 180 200 220
LOAD CURRENT (mA)
LOAD CURRENT (mA)
SWITCHING WAVEFORMS
(BREAK BEFORE MAKE)
SWITCHING WAVEFORMS
(TWO CYCLES)
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
NAME
FUNCTION
1
D1
Open drain of N-channel transfomer drive 1.
2
GND1
3
FS
4
SD
VIN
5V
N.C.
Not internally connected.
6
VCC
+5V supply voltage.
7
GND2
8
D2
C1
0.1µF
R1
50Ω
6
VCC
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.
5
MAX253
_____________________Pin Description
4
SD
D1
1
ON / OFF
R2
50Ω
MAX253
3
FS
FREQUENCY
SWITCH
D2
GND1
2
8
GND2
7
Ground. Connect both GND1 and GND2
to ground.
Open drain of N-channel transformer drive 2.
VIN
Figure 1. Test Circuit
5V
C1
VCC
F/F
MAX253
Q
C3
N
C2
T
FS
OSC
FREQUENCY
SWITCH
5V @ 200mA
ISO OUTPUT
D1
400kHz/
700kHz
D2
Q
N
SD
GND2
ISO
GND
GND1
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 transformer, optocouplers, and the appropriate Maxim interface product (as described in the Isolated RS-485/RS232 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 disables all internal circuitry, including the oscillator and
both power switches.
Pulling FS low reduces the oscillator frequency and lowers 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-frequency state if not connected.
_______________________________________________________________________________________
5
MAX253
Transformer Driver for
Isolated RS-485 Interface
ISOLATION
BARRIER
VIN
5V
C1
0.1µF
6
VCC
D1
ON / OFF
4
SD
2
8
C3
0.1µF
MAX253
D2
GND1
ICT:1.3CT** 1N5817
1
GND2
FS
IN
OUT
C2
22µF
ISO 5V
2
C4
22µF
MAX667
8
1N5817
3
SET
GND
6
SHDN
4
5
7
3.3kΩ
PC410 / 417
6
*74HC04
390Ω
DI
1
5
3.3kΩ
3
8
4
PC357T
*74HC04
390Ω
DE
4
1
4
A
3
*74HC04
3.3kΩ
390Ω
B
1
RO
RE
GND
2
*74HC04 OR EQUIVALENT
** SEE TABLE 2
4
3
Figure 3. Typical RS-485 Application Circuit, 5V Configuration
6
485
I/O
6
1
6
MAX481
MAX483
MAX485
MAX487
DE
2
3
PC410 / 417
5
RO
VCC
DI
_______________________________________________________________________________________
5
7
Transformer Driver for
Isolated RS-485 Interface
MAX253
ISOLATION
BARRIER
VIN
3.3V
C1
0.1µF
5
ON / OFF
D1
N.C.
4
SD
6
ICT:2.1CT** 1N5817
1
8
C3
0.1µF
MAX253
VCC
D2
FS
GND1
2
IN
OUT
C2
22µF
C4
22µF
MAX667
8
3
1N5817
SET
GND2
7
ISO 5V
2
GND
6
1N5817
SHDN
4
5
1N5817
C5
0.1µF
PC410 / 417
3.3kΩ
6
*74HC04
390Ω
DI
1
5
3.3kΩ
3
8
4
PC357T
*74HC04
390Ω
DE
4
1
4
A
3
*74HC04
3.3kΩ
MAX481
MAX483
MAX485
MAX487
DE
2
3
PC410 / 417
1
390Ω
B
1
RO
RE
4
7
GND
2
*74HC04 OR EQUIVALENT
** SEE TABLE 2
6
485
I/O
6
5
RO
VCC
DI
5
3
Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration
_______________________________________________________________________________________
7
MAX253
Transformer Driver for
Isolated RS-485 Interface
VIN
5V
C1
0.1µF 5
ON / OFF
4
ISOLATION
BARRIER
ICT:1.3CT**1N5817
6
VCC
N.C.
SD
D1
1
MAX253
D2
FS
GND1 GND2
2
7
8
2
IN
OUT
C2
22µF
MAX667
C3
0.1µF
ISO 5V
C4
22µF
8
3
SET GND SHDN
6
4
5
1N5817
5 x 3.3kΩ
10 x PC417
*74HC04
390Ω
T1IN
74HC04
6
1
8
5
4
2
390Ω
7
T2IN
74HC04
390Ω
15
VCC
T1IN
GND
3
T1OUT
T2IN
T2OUT
T3IN
T3OUT
T4IN
T4OUT
T5IN
T5OUT
4
2
T3IN
74HC04
390Ω
16
T4IN
74HC04
390Ω
22
T5IN
R1OUT
74HC04
6
5
4
1
390Ω
9
390Ω
6
390Ω
23
R3OUT
74HC04
390Ω
17
R4OUT
74HC04
R5OUT
*74HC04 OR EQUIVALENT
** SEE TABLE 2
R1OUT
R1IN
R2OUT
R2IN
R3OUT
R3IN
R4OUT
R4IN
10
2
R2OUT
74HC04
19
MAX205
5 X 3.3kΩ
74HC04
1
390Ω
14
R5OUT
R5IN
SD
EN
20
21
5
24
18
13
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps)
VCC
1N5711
4N25 6 1N5711
6 4N25
3.3kΩ
3.3kΩ
390Ω
1
1
TIN
ISO ROUT
5
5
TIN
390Ω
74HCO4
*74HC04
2
2 ISO
ISO
4
4 GND
GND
Figure 5. Typical RS-232 Application Circuit
8
_______________________________________________________________________________________
VCC
ISO
ROUT
Transformer Driver for
Isolated RS-485 Interface
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 modifications 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 optocouplers. 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 isolated 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, capable of driving a center-tapped transformer primary from
a 5V or a 3.3V DC power supply (see Figures 6 and 7).
The secondary can be wound to provide any isolated
voltage 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 primary. 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
* 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
_______________________________________________________________________________________
9
MAX253
__________Applications Information
Figures 3–5 are typical isolated RS-485/RS-232 data-interface 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 applications 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 highimpedance nodes, they should be located as close
as possible to the Maxim interface IC. This minimizes stray capacitance and maximizes data rate.
Refer to the µMAX package information for pin spacing
and physical dimensions.
MAX253
Transformer Driver for
Isolated RS-485 Interface
VIN
5V
C1
0.1µF
6
VCC
4
ON / OFF
D1
SD
1
ICT:1.3CT*1N5817
TP1
C3
0.1µF
5V @ 200mA
ISO OUTPUT
C2
22µF
MAX253
3
FS
FREQUENCY
SWITCH
D2
8
1N5817
GND1
GND2
2
7
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
L2
25µH
FILTER
OUTPUT
OUTPUT
C7
2.2µF
*SEE TABLE 2
Figure 6. 5V to Isolated 5V Application Circuit
VIN
3.3V
C1
0.1µF
4
ON / OFF
D1
SD
1
5V @ 100mA
TP1 ISO OUTPUT
ICT:2.1CT* 1N5817
C3
0.1µF
C2
22µF
MAX253
3
FREQUENCY
SWITCH
FS
D2
8
1N5817
GND1
2
GND2
7
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
VCC
L2
25µH
6
1N5817
1N5817
OUTPUT
*SEE TABLE 2
C4
0.1µF
FILTER
OUTPUT
C7
2.2µF
Figure 7. 3.3V to Isolated 5V Application Circuit
10
______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
ISOLATION
BARRIER
6
5V
MAX253
VIN
VCC
D1
24V UNREGULATED
1CT:5CT*
1
1N5817
10µF
MAX253
4
SD
D2
GND1
78L05
1N5817
GND2
2
5V
8
7
1
3
0.1V to 0.5V
MAX480
2
RL
0kΩ to 1kΩ
IL300
7
6
4
2
3
6
4
5
49.9kΩ
ISO
5V
3
49.9kΩ
6
MAX480
2
7
2N3904
4
2N3904
*SEE TABLE 2
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 transferred 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 latter detector is used to generate a feedback signal identical 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 illustrates 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
MAX253
Transformer Driver for
Isolated RS-485 Interface
VIN
INPUT
6
VCC
D1
1
1CT:1CT*
1N5817
+VOUT
≈ 2VIN
OUTPUT
RL+
MAX253
D2
GND2
GND1
2
8
RL+ ≅ RLRL-
7
*SEE TABLE 2
-VOUT ≈ -2V
IN
OUTPUT
1N5817
Figure 9a. Half-Wave Rectifier—Bipolar
VIN
INPUT
6
VCC
D1
1
1CT:1CT*
4 x 1N5817
VOUT ≈ +VIN
OUTPUT
MAX253
D2
GND1
8
GND2
2
7
VOUT ≈ -VIN
OUTPUT
*SEE TABLE 2
Figure 9b. Full-Wave Rectifier—Bipolar
VIN
INPUT
6
VCC
D1
1
D2
2
4 x 1N5817
VOUT ≈ 2 x VIN
OUTPUT
MAX253
GND1
1CT:1CT*
8
GND2
7
*SEE TABLE 2
Figure 9c. Full-Wave Rectifier—Unipolar
12
______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
MAX253
VIN
5V
ISOLATION
BARRIER
1CT : 1.5CT : 3CT*
1
78L05
4 x 1N5817
10µF
ISO
5V
6
8
79 L12
ISO
-12V
D1
MAX253
VCC
D2
SD
GND1
10µF
2
4
ON/OFF
GND2
7
+5V
74HC04
START
8
6N136
7
INPUT CLOCK
1
2
200Ω
QH
3k
10µF
MAX176
0.1µF
ANALOG
INPUT
1
2
3
0.1µF
10µF
VDD
VSS
AIN
CONVST
VREF
CLOCK
4
GND
DATA
14
6
3
5
4
8
8
7
6
5
0.1µF 10µF
SIGNAL
GROUND
6N136
7
11
12
1
QC
RCK
+5V
10
QA
SCLR
13
4
6N136
7
6
5
4
3
2
1
15
16
3
5
1
QE
QB
200Ω
3k
470Ω
74HC595 QF
SCK
QD
2
6
QG
SER
D11(MSB)
D10
D9
D8
+5V
0.1µF
8
74HC04
8
2
7
3
6
4
5
8
QH′
8.2k
14
11
QH
QG
SER
74HC595 QF
QE
SCK
QD
12
QC
RCK
QB
+5V
10
QA
SCLR
7
6
5
4
3
2
1
15
16
13
8
D7
D6
D5
D4
D3
D2
D1
D0(LSB)
+5V
0.1µF
*SEE TABLE 2
Figure 10. Typical Isolated ADC Application
______________________________________________________________________________________
13
MAX253
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 generated by connecting additional diodes to the 1/4 and
3/4 tap points on the secondary. For +5V only applications, 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 frequency 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 maximum 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 product. 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
+5V to ±5V; ±12V
Figure
9a
2, 3, 5, 6
4, 7
8
10
Turns Ratio
1CT*:1
1CT:1.3CT
1CT:2.1CT
1CT:5CT
1CT:1.5CT:3CT
Typical
Windings
Primary
44CT
44CT
28CT
44CT
44CT
Secondary
44
56CT
56CT
220CT
66CT, 132CT
Primary ET
Product
FS Low
18.3V-µs
18.3V-µs
12V-µs
18.3V-µs
18.3V-µs
FS High
11V-µs
11V-µs
7.2V-µs
11V-µs
11V-µs
*CT = Center Tapped
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
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: (407) 241-7876
FAX: (407) 241-9339
Fair-Rite Products
Phone: (914) 895-2055
FAX: (914) 895-2629
Siemens Components
Phone: (408) 777-4500
FAX: (408) 777-4983
14
______________________________________________________________________________________
Transformer Driver for
Isolated RS-485 Interface
Diode Selection
The MAX253’s high switching frequency demands
high-speed rectifiers. Schottky diodes are recommended. Ensure that the Schottky diode average current 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 recommended for surface-mount applications. Use the
higher frequency setting to reduce ripple.
Output Filter Capacitor
In applications sensitive to output-ripple noise, the output 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 provide 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
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
* Nihon Inter Electronics Corp.
USA Phone: (805) 867-2555
FAX: (805) 867-2556
Japan Phone: 81-3-3494-7411
FAX: 81-3-3494-7414
______________________________________________________________________________________
15
MAX253
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 occurring (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 minimize radiated noise, use common closed-magneticpath 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 about 22 turns on
each side of the center tap, or 44 turns total. This will
result in a nominal primary inductance of about 832µH.
The secondary can be scaled to produce the required
DC output.
MAX253
Transformer Driver for
Isolated RS-485 Interface
___________________Chip Topography
D1
D2
0.085"
(2.159mm)
GND1
GND2
FS
V CC
SD
0.058"
(1.4732mm)
TRANSISTOR COUNT: 31;
SUBSTRATE CONNECTED TO VCC.
________________________________________________________Package Information
DIM
E
A
A1
B
C
D
E
e
H
L
α
H
INCHES
MAX
MIN
0.036
0.032
0.008
0.004
0.014
0.010
0.007
0.005
0.120
0.116
0.120
0.116
0.0256
0.198
0.188
0.026
0.016
6°
0°
MILLIMETERS
MIN
MAX
0.81
0.91
0.10
0.20
0.25
0.46
0.13
0.18
2.95
3.04
2.95
3.04
0.65
4.90
––
0.55
––
0°
6°
21-0036
D
C
α
A
.127mm
.004 in
e
B
A1
8-PIN µMAX
PACKAGE
L
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.
16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1994 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.