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19-0226; Rev 0; 1/94
Transformer Driver for Isolated RS-485 Interface
_______________General Description
The MAX253 is a monolithic oscillator/power-driver, specifically designed to provide isolated power for an isolated RS-485 or RS-232 data interface. It drives a center-tapped transformer primary from a 5V or 3.3V DC power supply. The secondary can be wound to provide any isolated voltage needed at power levels up to 1W. The MAX253 consists of a CMOS oscillator driving a pair of N-channel power switches. The oscillator runs at double the output frequency, driving a toggle flip-flop to ensure 50% duty cycle to each of the switches. Internal delays are arranged to ensure break-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.
____________________________Features
o Power-Supply Transformer Driver for Isolated RS-485/RS-232 Data-Interface Applications o Single +5V or +3.3V Supply o Low-Current Shutdown Mode: 0.4A o Pin-Selectable Frequency: 350kHz or 200kHz o 8-Pin DIP, SO, and MAX Packages
MAX253
______________Ordering Information
PART MAX253CPA MAX253CSA MAX253CUA MAX253C/D MAX253EPA MAX253ESA MAX253EUA MAX253MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO 8 MAX Dice* 8 Plastic DIP 8 SO 8 MAX 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.
__________Typical Operating Circuit
ON / OFF 4 SD 6 VCC D1 1 C3 C2 VIN 5V C1 OUTPUT 5V @ 200mA
__________________Pin Configuration
TOP VIEW
D1 GND1
1 2
8 7
D2 GND2 VCC N.C.
MAX253
3 FREQUENCY SWITCH FS GND1 2 D2 GND2 7 8
FS 3 SD 4
MAX253
6 5
DIP/SO/MAX
________________________________________________________________ Maxim Integrated Products
1
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Transformer Driver for Isolated RS-485 Interface MAX253
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 = +70C) Plastic DIP (derate 9.09mW/C above +70C) .............727mW SO (derate 5.88mW/C above +70C) ..........................471mW MAX (derate 4.10mW/C above +70C) .....................330mW CERDIP (derate 8.00mW/C above +70C) ..................640mW Operating Temperature Ranges MAX253C_ _ ........................................................0C to +70C MAX253E_ _ .....................................................-40C to +85C MAX253MJA ...................................................-55C to +125C Junction Temperatures MAX253C_ _/E_ _..........................................................+150C MAX253MJA .................................................................+175C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = +25C.) 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 FS = 0V FS = VCC 2.5 10 2.2 2.4 0.8 50 D1, D2; 100mA FS = VCC or open FS = 0V No load, SD = 0V, FS low SD = VCC 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 kHz mA A V A pA V A 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.
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Transformer Driver for Isolated RS-485 Interface
__________________________________________Typical Operating Characteristics
(Circuit of Figure 6, VIN = 5V 10%, TA = +25C, unless otherwise noted.)
OUTPUT RESISTANCE vs. TEMPERATURE (FS = LOW)
plot01
MAX253
OUTPUT RESISTANCE vs. TEMPERATURE (FS = HIGH)
plot02
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
INCLUDES SWITCH LEAKAGE CURRENTS SHUTDOWN CURRENT (A) 0.8
plot03
10.5 MEASURED AT TP1 10.0 OUTPUT RESISTANCE () 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 VIN = 5.0V VIN = 4.5V
15 MEASURED AT TP1 OUTPUT RESISTANCE ()
1.0
12 VIN = 4.5V VIN = 5.0V 9
0.6
0.4
0.2
6 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) -60 -40 -20 0 20 40 60 80 100 120 140 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)
VIN = 6.0V
plot06
260
480
600 550 SUPPLY CURRENT (A) 500 VIN = 5.5V 450 400 350 300 VIN = 5.0V
240 FREQUENCY (kHz)
VIN = 6.0V FREQUENCY (kHz)
440
VIN = 6.0V
220
VIN = 5.5V
400 VIN = 5.5V 360 VIN = 5.0V VIN = 4.5V 280
200
180
VIN = 5.0V VIN = 4.5V
320
VIN = 4.5V 250 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
160 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
SUPPLY CURRENT vs. TEMPERATURE (FS = HIGH)
plot07
EFFICIENCY vs. LOAD CURRENT (FS = LOW)
90
plot08
850 800 SUPPLY CURRENT (A) 750 700 650 600 550 500 450 400 VIN = 4.5V VIN = 5.0V VIN = 5.5V VIN = 6.0V
100 VIN = 5.5V VIN = 4.5V
80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
0
200 20 40 60 80 100120 140 160 180 LOAD CURRENT (mA)
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Transformer Driver for Isolated RS-485 Interface MAX253
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 6, VIN = 5V 10%, TA = +25C, 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) 7 6 5 4 3 2 1 0
CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1 CIRCUIT OF FIGURE 7 VIN = 3.3V TURNS RATIO = 1:2.1 CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1.3
plot11
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 200 20 40 60 80 100120 140 160 180 LOAD CURRENT (mA) VIN = 4.5V VIN = 5.5V
10 9 8 OUTPUT VOLTAGE (V) 7 6 5 4 3 2 1 0
CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1 CIRCUIT OF FIGURE 7 VIN = 3.3V TURNS RATIO = 1:2.1 CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1.3
10
MEASURED AT TP1 0 20 40 60 80 100 120 140 160 180 200 220 LOAD CURRENT (mA)
MEASURED AT TP1 0 20 40 60 80 100 120 140 160 180 200 220 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
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Transformer Driver for Isolated RS-485 Interface
_____________________Pin Description
PIN 1 2 3 4 5 6 7 8 NAME D1 GND1 FS SD N.C. VCC GND2 D2 FUNCTION Open drain of N-channel transfomer drive 1. Ground. Connect both GND1 and GND2 to ground. Frequency switch. If FS = VCC or open, switch frequency = 350kHz; if FS = 0V, switch frequency = 200kHz. Shutdown. Ground for normal operation, tie high for shutdown. Not internally connected. +5V supply voltage. Ground. Connect both GND1 and GND2 to ground. Open drain of N-channel transformer drive 2.
MAX253
VIN 5V C1 0.1F 4 ON / OFF
6 VCC SD D1
R1 50 1 R2 50 D2 8
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7
Figure 1. Test Circuit
VIN C1 F/F VCC Q T OSC FREQUENCY SWITCH 400kHz/ 700kHz Q
5V
D1 N C3
5V @ 200mA ISO OUTPUT C2
MAX253
FS
D2 N SD GND2 GND1 ISO GND
ON / OFF
Figure 2. Block Diagram
_______________Detailed Description
The MAX253 is an isolated power-supply transformer driver specifically designed to form the heart of a fully isolated RS-485 data interface. Completely isolated communications are obtained by combining the MAX253 with a linear regulator, a center-tapped 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.
_______________________________________________________________________________________
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Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1F
ISOLATION BARRIER
6 VCC D1 1
ICT:1.3CT** 1N5817 C3 0.1F C2 22F
8
IN
OUT
2
ISO 5V C4 22F
ON / OFF
4
SD
MAX253
D2 8 3 1N5817
MAX667
GND1 2
GND2 7
FS
SET 6
GND 4
SHDN 5
PC410 / 417
3.3k 6
*74HC04
390 DI 1
5
3.3k 3 4 PC357T 390 DE 1 4 4 DI 8 VCC A 6
*74HC04
3 3.3k 2 3 PC410 / 417 6 5 1 390 1
DE
MAX481 MAX483 MAX485 MAX487
B 7
485 I/O
*74HC04
RO
RO
RE 2
GND 5
*74HC04 OR EQUIVALENT ** SEE TABLE 2
4
3
Figure 3. Typical RS-485 Application Circuit, 5V Configuration
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Transformer Driver for Isolated RS-485 Interface MAX253
VIN 3.3V C1 0.1F 5 4 6 1
ISOLATION BARRIER
ICT:2.1CT** 1N5817 C3 0.1F C2 22F
N.C. SD VCC GND1 2 GND2 7
D1
8
IN
OUT
2
ISO 5V C4 22F
ON / OFF
MAX253
D2 FS 8 3 1N5817
MAX667
SET 6
GND 4
SHDN 5
1N5817
1N5817
C5 0.1F PC410 / 417
*74HC04
390 DI 1
6 5
3.3k
3.3k
3
4 PC357T 4 4 DI
8 VCC A 6
*74HC04
390 DE 1
3 3.3k 2 3 PC410 / 417 6 5 1 390 1
DE
MAX481 MAX483 MAX485 MAX487
B
485 I/O 7
*74HC04
RO
RO
RE 2
GND 5
*74HC04 OR EQUIVALENT ** SEE TABLE 2
4
3
Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration
_______________________________________________________________________________________
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Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1F 5 6 N.C. VCC D1 1 ISOLATION BARRIER ICT:1.3CT**1N5817 C3 0.1F 8 3 1N5817 5 x 3.3k 10 x PC417 *74HC04 T1IN 74HC04 T2IN 74HC04 T3IN 74HC04 T4IN 74HC04 T5IN 5 X 3.3k 74HC04 R1OUT 74HC04 R2OUT 74HC04 R3OUT 74HC04 R4OUT 74HC04 R5OUT *74HC04 OR EQUIVALENT 390 14 390 17 390 23 6 5 4 1 2 390 6 R2OUT R2IN 5 390 9 390 22 390 16 T4IN T4OUT 1 390 15 390 390 1 2 6 5 4 7 8 VCC T1IN GND 3 T1OUT SET GND SHDN 6 4 5 8 2 IN OUT C2 22F MAX667 ISO 5V C4 22F
ON / OFF
4
SD
MAX253
D2
FS GND1 GND2 2 7
T2IN
T2OUT
4
T3IN
T3OUT
2
T5IN
T5OUT
19
MAX205
R1OUT R1IN 10
R3OUT
R3IN
24
R4OUT
R4IN
18
R5OUT SD 21
R5IN EN 20
13
** SEE TABLE 2
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps) VCC 1N5711 4N25 6 1N5711 6 4N25 3.3k 3.3k 390 1 1 TIN ISO ROUT 5 5 TIN 390 74HCO4 *74HC04 2 2 ISO ISO 4 4 GND GND
VCC ISO ROUT
Figure 5. Typical RS-232 Application Circuit
8 _______________________________________________________________________________________
Transformer Driver for Isolated RS-485 Interface
__________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: o 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. o 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 MAX package information for pin spacing and physical dimensions. 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.
MAX253
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.
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
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates
DATA RATE 250kbps 2.5Mbps FULL DUPLEX RS-485 IC MAX488/MAX489 MAX490/MAX491 HALF DUPLEX RS-485 IC MAX483/MAX487 MAX481/MAX485 OPTOCOUPLER FOR DI / RO PC417* PC410* OPTOCOUPLER FOR DE 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
Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V C1 0.1F 4 ON / OFF
6 VCC SD D1 1 ICT:1.3CT*1N5817 C3 0.1F TP1 C2 22F 5V @ 200mA ISO OUTPUT
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7
OUTPUT
D2
8 1N5817
OPTIONAL 21kHz LOWPASS OUTPUT FILTER L2 25H FILTER OUTPUT C7 2.2F
*SEE TABLE 2
Figure 6. 5V to Isolated 5V Application Circuit
VIN 3.3V C1 0.1F 4 ON / OFF 1 ICT:2.1CT* 1N5817 C3 0.1F 5V @ 100mA TP1 ISO OUTPUT C2 22F
SD
D1
MAX253
3 FREQUENCY SWITCH FS GND1 2 GND2 7 D2 VCC 6 1N5817 1N5817
OUTPUT
8 1N5817
OPTIONAL 21kHz LOWPASS OUTPUT FILTER L2 25H FILTER OUTPUT C7 2.2F
*SEE TABLE 2
C4 0.1F
Figure 7. 3.3V to Isolated 5V Application Circuit
10
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Transformer Driver for Isolated RS-485 Interface MAX253
VIN 5V 6 VCC D1 1 ISOLATION BARRIER 1CT:5CT* 1N5817 24V UNREGULATED 10F
MAX253
4 SD GND1 2 GND2 7 D2 8 1N5817 78L05
5V 0.1V to 0.5V
1 3 7
IL300
RL 0k to 1k
MAX480
2 4
6
2
3
6
ISO 5V 3 49.9k 2 6
4 49.9k
5
MAX480
4
7
2N3904 2N3904 10k
*SEE TABLE 2
24.9
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 20A to 100A. 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 20 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
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Transformer Driver for Isolated RS-485 Interface MAX253
VIN INPUT 6 VCC D1 1 1CT:1CT* 1N5817 +VOUT 2VIN OUTPUT RL+ 8 RL+ RLRL-VOUT -2V IN OUTPUT
MAX253
D2 GND2 7
GND1 2
*SEE TABLE 2
1N5817
Figure 9a. Half-Wave Rectifier--Bipolar
VIN INPUT
6 VCC D1 1 1CT:1CT* 4 x 1N5817
MAX253
D2 GND1 2 *SEE TABLE 2 GND2 7 8
VOUT +VIN OUTPUT
VOUT -VIN OUTPUT
Figure 9b. Full-Wave Rectifier--Bipolar
VIN INPUT
6 VCC D1 1 1CT:1CT* 4 x 1N5817
MAX253
D2 GND1 2 GND2 7 8
VOUT 2 x VIN OUTPUT
*SEE TABLE 2
Figure 9c. Full-Wave Rectifier--Unipolar
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Transformer Driver for Isolated RS-485 Interface MAX253
ISOLATION BARRIER 1CT : 1.5CT : 3CT* 78L05 ISO 5V 10F 4 x 1N5817
VIN 5V
1 6 8 79 L12 ISO -12V 10F +5V
D1 VCC D2 GND1 2 GND2 7
MAX253
SD 4 ON/OFF
74HC04
START INPUT CLOCK
8 7 3k 6 10F 5 VSS CONVST CLOCK DATA 8 8 7 6 5 3k 6 470 5 1 2 3 SIGNAL GROUND 4 7
6N136
1 2 3 4 11 12 200 +5V 10 200 QH 14 SER SCK RCK SCLR QG QE QD 1 2 3 4 13 8 QC QB QA 7 6 5 4 3 2 1 15 16
74HC595 QF
MAX176
0.1F ANALOG INPUT 1 2 3 4 GND VDD AIN VREF
6N136
D11(MSB) D10 D9 D8 +5V 0.1F
0.1F 10F
74HC04
6N136
0.1F 10F
8 7 6 5 8.2k 14 11 12 10 SER SCK RCK SCLR 8 QH
QH QG QE QD QC QB QA
7 6 5 4 3 2 1 15 16
D7 D6 D5 D4 D3 D2 D1 D0(LSB) +5V 0.1F
74HC595 QF
+5V
13
8
*SEE TABLE 2
Figure 10. Typical Isolated ADC Application
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Transformer Driver for Isolated RS-485 Interface MAX253
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. 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 4s. 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-
______________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 6.67s. The required ET product
Table 2. Typical Transformer Characteristics
CHARACTERISTIC Figure Turns Ratio Typical Windings Primary ET Product Primary Secondary FS Low FS High +5V to 10V 9a 1CT*:1 44CT 44 18.3V-s 11V-s +5V to +5V 2, 3, 5, 6 1CT:1.3CT 44CT 56CT 18.3V-s 11V-s +3.3V to +5V 4, 7 1CT:2.1CT 28CT 56CT 12V-s 7.2V-s 8 1CT:5CT 44CT 220CT 18.3V-s 11V-s +5V to +24V +5V to 5V; 12V 10 1CT:1.5CT:3CT 44CT 66CT, 132CT 18.3V-s 11V-s
*CT = Center Tapped
Table 3. Transformer, Transformer Core, and Optocoupler Suppliers
TRANSFORMERS BH Electronics Phone: (507) 532-3211 FAX: (507) 532-3705 Coilcraft Phone: (708) 639-6400 FAX: (708) 639-1469 Coiltronics Phone: (407) 241-7876 FAX: (407) 241-9339 TRANSFORMER CORES Philips Components Phone: (407) 881-3200 FAX: (407) 881-3300 Magnetics Inc. Phone: (412) 282-8282 FAX: (412) 282-6955 Fair-Rite Products Phone: (914) 895-2055 FAX: (914) 895-2629 OPTOCOUPLERS Quality Technology Phone: (408) 720-1440 FAX: (408) 720-0848 Sharp Electronics Phone: (206) 834-2500 FAX: (206) 834-8903 Siemens Components Phone: (408) 777-4500 FAX: (408) 777-4983
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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 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 832H. The secondary can be scaled to produce the required DC output. 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.
MAX253
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 0C. In applications where output ripple is not critical, a 0.1F 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 0.1F chip or ceramic capacitor is normally sufficient for input
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
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. 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 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
Surface Mount
High-Performance Through Hole
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
Transformer Driver for Isolated RS-485 Interface MAX253
___________________Chip Topography
D1 D2
0.085" (2.159mm) GND1
GND2
FS SD 0.058" (1.4732mm)
V CC
TRANSISTOR COUNT: 31; SUBSTRATE CONNECTED TO VCC.
________________________________________________________Package Information
DIM A A1 B C D E e H L 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
E
H
D
C A .127mm .004 in B A1 L
8-PIN MAX PACKAGE
e
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 (c) 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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