MAX253C/D [MAXIM]
Transformer Driver for Isolated RS-485 Interface; 用于隔离型RS - 485接口的变压器驱动器
| 型号： | MAX253C/D |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Transformer Driver for Isolated RS-485 Interface |
| 文件： | 总16页 (文件大小：232K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0226; Rev 0; 1/94
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
The MAX253 is a monolithic oscillator/power-driver,
specifically designed to provide isolated power for an
is ola te d RS-485 or RS-232 d a ta inte rfa c e . It
drives a center-tapped transformer primary from a 5V
or 3.3V DC p owe r s up p ly. The s e c ond a ry c a n b e
wound to provide any isolated voltage needed at power
levels up to 1W.
♦ 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
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 e ns ure 50% d uty c yc le to e a c h of the s witc he s .
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.
______________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
________________________Ap p lic a t io n s
MAX253CPA
MAX253CSA
MAX253CUA
MAX253C/D
MAX253EPA
MAX253ESA
MAX253EUA
MAX253MJA
0°C to +70°C
Isolated RS-485/RS-232 Power-Supply
Transformer Driver
0°C to +70°C
8 µMAX
High Noise-Immunity Communications Interface
Isolated and/or High-Voltage Power Supplies
Bridge Ground Differentials
0°C to +70°C
Dice*
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 Plastic DIP
8 SO
8 µMAX
Medical Equipment
8 CERDIP**
Process Control
* Contact factory for dice specifications.
**Contact factory for availability and processing to MIL-STD-883.
__________________P in Co n fig u ra t io n
__________Typ ic a l Op e ra t in g Circ u it
V
IN
5V
TOP VIEW
ON / OFF
C1
4
6
OUTPUT
SD
V
CC
5V @ 200mA
1
8
D1
D1
GND1
FS
1
2
3
4
8
7
6
5
D2
C3
C2
GND2
MAX253
MAX253
V
CC
3
SD
N.C.
FS
D2
FREQUENCY
SWITCH
GND1
GND2
7
DIP/SO/µMAX
2
________________________________________________________________ Maxim Integrated Products
1
Ca ll t o ll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 fo r fre e s a m p le s o r lit e ra t u re .
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V ) ...............................................-0.3V to +7V
Operating Temperature Ranges
CC
Control Input Voltages (SD, FS) .................-0.3V to (V + 0.3V)
MAX253C_ _ ........................................................0°C to +70°C
MAX253E_ _ .....................................................-40°C to +85°C
MAX253MJA ...................................................-55°C to +125°C
Junction Temperatures
CC
Output Switch Voltage (D1, D2).............................................12V
Peak Output Switch Current (D1, D2)......................................1A
Average Output Switch Current (D1, D2) .........................200mA
Continuous Power Dissipation (T = +70°C)
MAX253C_ _/E_ _..........................................................+150°C
MAX253MJA .................................................................+175°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
A
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
MAX253
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 = 5V ±10%, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
CC
A
MIN
PARAMETER
CONDITIONS
MIN
TYP
1.5
MAX
4.0
UNITS
Switch On Resistance
D1, D2; 100mA
Ω
FS = V or open
CC
250
150
350
200
0.45
0.4
500
300
5.0
Switch Frequency
kHz
FS = 0V
Operating Supply Current (Note 1)
Shutdown Supply Current (Note 2)
No load, SD = 0V, FS low
mA
µA
V
SD = V
CC
High
Low
2.4
2.4
Shutdown Input Threshold
Shutdown Input Leakage Current
FS Input Threshold
0.8
µA
pA
10
High
V
Low
0.8
50
FS = 0V
µA
pA
V
FS Input Leakage Current
Start-Up Voltage
FS = V
10
CC
2.5
2.2
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
_______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(Circuit of Figure 6, V = 5V ±10%, T = +25°C, unless otherwise noted.)
IN
A
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = LOW)
OUTPUT RESISTANCE vs. TEMPERATURE
(FS = HIGH)
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
10.5
15
12
9
1.0
0.8
0.6
MEASURED AT TP1
MEASURED AT TP1
INCLUDES SWITCH LEAKAGE CURRENTS
10.0
9.5
9.0
8.5
V
= 4.5V
IN
V
= 4.5V
IN
8.0
7.5
7.0
6.5
6.0
V
IN
= 5.0V
0.4
0.2
0
V
IN
= 5.0V
6
-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)
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = LOW)
D1, D2 FREQUENCY vs. TEMPERATURE
(FS = HIGH)
SUPPLY CURRENT vs. TEMPERATURE
(FS = LOW)
260
240
220
480
440
400
600
550
V
IN
= 6.0V
V
= 6.0V
V = 6.0V
IN
IN
500
450
400
350
300
250
V
= 5.5V
IN
V
IN
= 5.5V
V
IN
= 5.5V
V
= 5.0V
IN
200
180
160
360
320
280
V = 5.0V
IN
V
IN
= 5.0V
V = 4.5V
IN
V
IN
= 4.5V
V
IN
= 4.5V
-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)
100
850
90
80
70
60
50
40
30
20
10
0
800
750
700
650
V
= 6.0V
= 5.5V
= 5.0V
IN
V
IN
= 5.5V
V
IN
= 4.5V
V
IN
600
550
500
450
400
V
IN
V
IN
= 4.5V
0
20 40 6080 100 120 140160 180 200
LOAD CURRENT (mA)
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
3
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 6, V = 5V ±10%, T = +25°C, unless otherwise noted.)
IN
A
EFFICIENCY vs. LOAD CURRENT
(FS = HIGH)
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = LOW)
OUTPUT VOLTAGE vs. LOAD CURRENT
(FS = HIGH)
100
10
10
CIRCUIT OF FIGURE 7
CIRCUIT OF FIGURE 7
V = 3.3V
IN
TURNS RATIO = 1:2.1
90
80
70
60
50
40
30
20
10
0
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
V
IN
= 3.3V
5
V
= 5.5V
TURNS RATIO = 1:2.1
IN
CIRCUIT OF FIGURE 6
= 5.0V
CIRCUIT OF FIGURE 6
= 5.0V
TURNS RATIO = 1:1.3
V
IN
= 4.5V
V
IN
V
IN
TURNS RATIO = 1:1.3
CIRCUIT OF FIGURE 6
CIRCUIT OF FIGURE 6
V = 5.0V
IN
TURNS RATIO = 1:1
V
= 5.0V
IN
TURNS RATIO = 1:1
MEASURED AT TP1
MEASURED AT TP1
0
20 40 6080 100 120 140160 180 200
LOAD CURRENT (mA)
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)
SWITCHING WAVEFORMS
(BREAK BEFORE MAKE)
SWITCHING WAVEFORMS
(TWO CYCLES)
D1
D2
D1
D2
CIRCUIT OF FIGURE 1
CIRCUIT OF FIGURE 1
TIME FROM SHUTDOWN TO POWER-UP
SD
TP1 (OUTPUT VOLTAGE)
CIRCUIT OF FIGURE 6
4
_______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
_____________________P in De s c rip t io n
V
IN
PIN
NAME
FUNCTION
5V
C1
0.1µF
R1
50Ω
6
1
D1
Open drain of N-channel transfomer drive 1.
V
CC
Ground. Connect both GND1 and GND2
to ground.
2
3
4
GND1
FS
1
4
SD
FS
D1
D2
ON / OFF
Frequency switch. If FS = V or open,
CC
switch frequency = 350kHz; if FS = 0V,
switch frequency = 200kHz.
R2
50Ω
MAX253
Shutdown. Ground for normal operation,
tie high for shutdown.
3
8
SD
FREQUENCY
SWITCH
5
6
N.C.
Not internally connected.
+5V supply voltage.
GND1
GND2
2
7
V
CC
Ground. Connect both GND1 and GND2
to ground.
7
8
GND2
D2
Open drain of N-channel transformer drive 2.
Figure 1. Test Circuit
V
IN
5V
C1
5V @ 200mA
ISO OUTPUT
V
CC
D1
N
F / F
Q
Q
MAX253
OSC
C3
C2
T
FS
D2
N
FREQUENCY
SWITCH
ISO
GND
400kHz/
700kHz
SD
GND2
GND1
ON / OFF
Figure 2. Block Diagram
quency (see Figure 2). These two signals drive the
ground-referenced output switches. Internal delays
e ns ure b re a k-b e fore -ma ke a c tion b e twe e n the two
switches.
_______________De t a ile d De s c rip t io n
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
c ommunic a tions a re ob ta ine d b y c omb ining 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).
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-
e rs the s up p ly c urre nt (s e e Sup p ly Curre nt 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.
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 -
_______________________________________________________________________________________
5
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
ISOLATION
BARRIER
V
IN
5V
C1
MAX253
0.1µF
6
V
CC
1N5817
1N5817
ICT:1.3CT**
ISO 5V
C4
1
8
2
D1
IN
OUT
C3
C2
0.1µF
22µF
22µF
4
MAX667
MAX253
ON / OFF
SD
8
3
D2
FS
SET
GND SHDN
GND1 GND2
6
4
5
2
7
3.3kΩ
PC410 / 417
6
5
*74HC04
*74HC04
390Ω
1
1
DI
3.3kΩ
8
3
4
3
PC357T
4
V
CC
DI
390Ω
3.3kΩ
6
4
A
B
DE
MAX481
MAX483
MAX485
MAX487
3
1
485
I/O
DE
RO
2
PC410 / 417
6
5
*74HC04
7
390Ω
1
RO
RE
GND
2
5
4
3
*74HC04 OR EQUIVALENT
** SEE TABLE 2
Figure 3. Typical RS-485 Application Circuit, 5V Configuration
6
_______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
ISOLATION
BARRIER
V
IN
3.3V
C1
0.1µF
1N5817
ICT:2.1CT**
ISO 5V
1
8
5
8
2
D1
IN
OUT
N.C.
SD
C3
0.1µF
C2
22µF
C4
22µF
4
6
MAX667
MAX253
ON / OFF
V
CC
D2
FS
3
1N5817
SET
GND SHDN
GND1 GND2
6
4
5
2
7
1N5817
1N5817
C5
0.1µF
PC410 / 417
3.3kΩ
6
5
*74HC04
*74HC04
390Ω
1
1
DI
3.3kΩ
8
3
4
PC357T
4
V
CC
DI
390Ω
3.3kΩ
6
4
A
B
DE
MAX481
MAX483
MAX485
MAX487
3
1
485
I/O
DE
RO
2
3
PC410 / 417
6
5
*74HC04
7
390Ω
1
RO
RE
GND
2
5
*74HC04 OR EQUIVALENT
** SEE TABLE 2
4
3
Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration
_______________________________________________________________________________________
7
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
V
IN
ISOLATION
BARRIER
1N5817
6
5V
C1
0.1µF
V
ICT:1.3CT**
CC
ISO 5V
1
5
4
8
2
D1
IN
OUT
N.C.
SD
C3
0.1µF
C2
22µF
C4
22µF
MAX667
MAX253
ON / OFF
8
D2
FS
SET GND SHDN
3
1N5817
6
4
5
MAX253
GND1 GND2
2
7
5 x 3.3kΩ
10 x PC417
*74HC04
V
GND
CC
6
390Ω
390Ω
390Ω
390Ω
390Ω
1
2
8
7
3
T1
T1
OUT
IN
5
4
T1
IN
74HC04
4
T2
T2
OUT
IN
T2
IN
74HC04
15
16
22
2
T3
T3
OUT
IN
T3
IN
74HC04
1
T4
T4
OUT
IN
T4
IN
74HC04
19
T5
T5
OUT
IN
T5
IN
5 X 3.3kΩ
MAX205
74HC04
6
390Ω
390Ω
390Ω
390Ω
390Ω
9
6
10
5
1
2
R1
R1
IN
OUT
5
4
R1
OUT
74HC04
R2
OUT
R2
IN
R2
OUT
74HC04
23
17
14
24
18
13
R3
OUT
R3
IN
R3
OUT
74HC04
R4
OUT
R4
IN
R4
OUT
74HC04
R5
OUT
R5
IN
R5
OUT
SD
EN
20
*74HC04 OR EQUIVALENT
** SEE TABLE 2
21
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps)
V
CC
V
CC
1N5711
4N25
6
1N5711
6
4N25
3.3kΩ
74HCO4
3.3kΩ
390Ω
ISO
R
OUT
1
1
R
OUT
T
IN
ISO
5
5
4
T
390Ω
ISO
IN
*74HC04
2
2
ISO
4
GND
GND
Figure 5. Typical RS-232 Application Circuit
_______________________________________________________________________________________
8
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
appropriate Maxim interface device for data-transfer
rates up to 2.5Mbps.
__________Ap p lic a t io n s In fo rm a t io n
Figures 3–5 are typical isolated RS-485/RS-232 data-inter-
Refer to the MAX1480 data sheet for a complete isolat-
ed RS-485 solution in one package.
face circuits. These circuits withstand 1800V
(1sec)
RMS
and are intended for industrial communications and control
applications where very high voltage transients, differential
ground potentials, or high noise may be encountered.
Is o la t e d RS -2 3 2 Da t a In t e rfa c e
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.
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.
For 3.3V operation, substitute the primary portion of
Figure 5 with the circuit of Figure 7.
♦ 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.
For applications requiring two transceivers or fewer,
refer to the MAX250/MAX251 or MAX252 data sheet.
Is o la t e d P o w e r S u p p lie s
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 (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 sup-
ply, 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.
Refer to the µMAX package information for pin spacing
and physical dimensions.
Is o la t e d RS -4 8 5 Da t a In t e rfa c e
The MAX253 p owe r-s up p ly tra ns forme r d rive r 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 s ub s tituting the MAX483/MAX487 with the
MAX488/MAX489 (for data rates up to 250kbps).
In Figure 7, the MAX253 is configured to operate from a
3.3V supply, deriving a “boost” V for the MAX253 by
CC
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.
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
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates
FULL DUPLEX
RS-485 IC
HALF DUPLEX
RS-485 IC
OPTOCOUPLER
FOR DI / RO
OPTOCOUPLER
FOR DE
DATA RATE
250kbps
2.5Mbps
MAX488/MAX489
MAX490/MAX491
MAX483/MAX487
MAX481/MAX485
PC417*
PC410*
PC357T*
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
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
V
IN
5V
C1
6
0.1µF
5V @ 200mA
ISO OUTPUT
V
CC
1N5817
1N5817
TP1
MAX253
ICT:1.3CT*
1
8
4
3
D1
D2
SD
FS
ON / OFF
C3
0.1µF
C2
22µF
MAX253
FREQUENCY
SWITCH
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
GND1
GND2
L2
25µH
FILTER
OUTPUT
2
7
OUTPUT
C7
2.2µF
*SEE TABLE 2
Figure 6. 5V to Isolated 5V Application Circuit
V
IN
3.3V
C1
0.1µF
5V @ 100mA
ISO OUTPUT
1N5817
1N5817
TP1
ICT:2.1CT*
1
8
4
3
D1
SD
FS
ON / OFF
C3
0.1µF
C2
22µF
MAX253
D2
FREQUENCY
SWITCH
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
V
CC
GND1 GND2
L2
25µH
FILTER
OUTPUT
2
7
6
1N5817
1N5817
OUTPUT
C7
2.2µF
C4
0.1µF
*SEE TABLE 2
Figure 7. 3.3V to Isolated 5V Application Circuit
10 ______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
V
IN
ISOLATION
BARRIER
6
5V
24V UNREGULATED
10µF
V
1CT:5CT*
CC
1
8
D1
D2
1N5817
1N5817
MAX253
4
SD
78L05
GND1
GND2
7
2
R
L
1
2
3
5V
IL300
0kΩ to 1kΩ
3
2
7
0.1V to 0.5V
6
MAX480
4
6
5
ISO
5V
3
2
4
6
7
49.9kΩ
2N3904
49.9kΩ
MAX480
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.
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.
Is o la t e d 4 m A t o 2 0 m A An a lo g In t e rfa c e
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.
The s e s ig na ls a re c ommonly re fe rre d to a d is ta nt
ground that may be at a considerably higher voltage
with respect to the local ground.
Is o la t e d 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
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-
______________________________________________________________________________________ 11
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
V
IN
INPUT
6
V
1N5817
CC
1CT:1CT*
1
8
+V
OUT
OUTPUT
D1
≈ 2V
IN
+
R
L
MAX253
MAX253
+
R
R -
L
D2
L
GND1
GND2
2
7
R -
L
-V
OUT
OUTPUT
≈ -2V
IN
*SEE TABLE 2
1N5817
Figure 9a. Half-Wave Rectifier—Bipolar
V
IN
INPUT
6
V
4 x 1N5817
CC
1CT:1CT*
1
8
D1
D2
V
OUT
≈ +V
IN
MAX253
OUTPUT
GND1
GND2
2
7
V
≈ -V
IN
OUT
*SEE TABLE 2
OUTPUT
Figure 9b. Full-Wave Rectifier—Bipolar
V
IN
INPUT
6
4 x 1N5817
1CT:1CT*
V
1
8
CC
D1
D2
V
OUT
≈ 2 x V
IN
MAX253
OUTPUT
GND1
GND2
2
7
*SEE TABLE 2
Figure 9c. Full-Wave Rectifier—Unipolar
12 ______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
V
IN
5V
ISOLATION
BARRIER
1CT : 1.5CT : 3CT*
1
D1
78L05
4 x 1N5817
10µF
6
8
ISO
5V
MAX253
V
CC
4
ON/OFF
D2
SD
79 L12
ISO
-12V
GND1
GND2
10µF
2
7
+5V
74HC04
START
6N136
INPUT CLOCK
8
7
6
5
1
2
3
4
200Ω
7
QH
3k
14
11
12
10
6
QG
SER
5
74HC595 QF
10µF
4
3
QE
QD
QC
QB
QA
SCK
RCK
SCLR
MAX176
D11(MSB)
D10
0.1µF
1
2
8
6N136
6N136
2
V
V
SS
DD
8
7
6
5
1
2
3
4
1
D9
7
6
5
200Ω
ANALOG
INPUT
AIN
CONVST
CLOCK
DATA
15
16
+5V
D8
3k
3
4
+5V
VREF
GND
0.1µF
13
8
470Ω
74HC04
0.1µF
1
8
7
6
5
10µF
0.1µF 10µF
2
3
4
8
QH′
8.2k
7
6
D7
D6
QH
QG
14
11
12
10
SER
SIGNAL
GROUND
5
D5
74HC595 QF
4
D4
QE
QD
QC
QB
QA
SCK
RCK
SCLR
3
D3
2
D2
1
D1
15
16
+5V
D0(LSB)
+5V
0.1µF
13
8
*SEE TABLE 2
Figure 10. Typical Isolated ADC Application
______________________________________________________________________________________ 13
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
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.
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 s e c ond a ry wind ing ma y or ma y not b e c e nte r
tapped, depending on the rectifier topology used. The
phasing of the secondary winding is not critical. In
s ome a p p lic a tions , multip le s e c ond a rie s mig ht b e
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.
MAX253
______________Co m p o n e n t S e le c t io n
Tra n s fo rm e r S e le c t io n
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
The transformer turns ratio must be set to provide the
minimum re q uire d outp ut volta g e a t the ma ximum
anticipated load with the minimum expected input volt-
Table 2. Typical Transformer Characteristics
CHARACTERISTIC
+5V to ±10V
9a
+5V to +5V
2, 3, 5, 6
+3.3V to +5V
4, 7
+5V to +24V
+5V to ±5V; ±12V
10
Figure
8
Turns Ratio
1CT*:1
44CT
44
1CT:1.3CT
44CT
1CT:2.1CT
28CT
1CT:5CT
44CT
1CT:1.5CT:3CT
44CT
Primary
Typical
Windings
Secondary
FS Low
56CT
56CT
220CT
18.3V-µs
11V-µs
66CT, 132CT
18.3V-µs
18.3V-µs
11V-µs
18.3V-µs
11V-µs
12V-µs
7.2V-µs
Primary ET
Product
FS High
11V-µs
*CT = Center Tapped
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
TRANSFORMERS
BH Electronics
TRANSFORMER CORES
Philips Components
OPTOCOUPLERS
Quality Technology
Phone: (507) 532-3211
FAX: (507) 532-3705
Phone: (407) 881-3200
FAX: (407) 881-3300
Phone: (408) 720-1440
FAX: (408) 720-0848
Coilcraft
Magnetics Inc.
Sharp Electronics
Phone: (708) 639-6400
FAX: (708) 639-1469
Phone: (412) 282-8282
FAX: (412) 282-6955
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 ______________________________________________________________________________________
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
MAX253
a g e . In a d d ition, inc lud e in the c a lc ula tions a n
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).
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-
p ut filte r c a p a c itor C2 s hould ha ve a low e ffe c tive
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.
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 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.
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
inp ut b yp a s s c a p a c itor. A low-c os t 0.1µF c hip or
c e ra mic c a p a c itor is norma lly s uffic ie nt for inp ut
Dio d e S e le c t io n
The MAX253’s hig h s witc hing fre q ue nc y d e ma nd s
hig h-s p e e d re c tifie rs . Sc hottky d iod e s a re re c om-
mended. Ensure that the Schottky diode average cur-
rent rating exceeds the load-current level. The 1N5817
Table 4. Suggested Capacitor Suppliers
PRODUCTION METHOD
CAPACITORS
Matsuo
267 series (low ESR)
USA Phone: (714) 969-2491, FAX: (714) 960-6492
Sprague Electric Co.
Surface Mount
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
Sanyo
High-Performance
Through Hole
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
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
Through Hole
* 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
Tra n s fo rm e r Drive r fo r
Is o la t e d RS -4 8 5 In t e rfa c e
___________________Ch ip To p o g ra p h y
D1
D2
MAX253
0. 085"
(2. 159mm)
GND1
GND2
FS
V
CC
SD
0. 058"
(1. 4732mm)
TRANSISTOR COUNT: 31;
SUBSTRATE CONNECTED TO V
.
CC
________________________________________________________P a c k a g e In fo rm a t io n
INCHES
MILLIMETERS
DIM
MIN
0.032
MAX
0.036
0.008
0.014
0.007
0.120
0.120
MIN
0.81
0.10
0.25
0.13
2.95
2.95
MAX
0.91
0.20
0.46
0.18
3.04
3.04
A
A1 0.004
B
C
D
E
e
0.010
0.005
0.116
0.116
E
H
0.0256
0.65
H
L
0.188
0.016
0°
0.198
0.026
6°
4.90
0.55
0°
– –
– –
α
6°
21-0036
D
C
α
A
8-PIN µMAX
.127mm
.004 in
PACKAGE
e
B
A1
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 __________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 (4 0 8 ) 7 3 7 -7 6 0 0
© 1994 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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