19-0226; Rev 2; 4/10 Transformer Driver for Isolated RS-485 Interface ____________________________Features 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. ♦ Power-Supply Transformer Driver for Isolated RS-485/RS-232 Data-Interface Applications 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. ♦ 8-Pin DIP, SO, and µMAX® Packages The SD pin puts the entire device into a low-power shutdown state, disabling both the power switches and oscillator. ________________________Applications Isolated RS-485/RS-232 Power-Supply Transformer Driver High Noise-Immunity Communications Interface Isolated and/or High-Voltage Power Supplies Bridge Ground Differentials Medical Equipment Process Control ♦ Single 5V or 3.3V Supply ♦ Low-Current Shutdown Mode: 0.4µA ♦ Pin-Selectable Frequency: 350kHz or 200kHz ______________Ordering Information PART MAX253CPA TEMP RANGE 0°C to +70°C PIN-PACKAGE 8 Plastic DIP MAX253CSA 0°C to +70°C 8 SO MAX253CUA 0°C to +70°C 8 μMAX MAX253C/D 0°C to +70°C Dice* MAX253EPA -40°C to +85°C 8 Plastic DIP MAX253ESA -40°C to +85°C 8 SO MAX253ESA/V -40°C to +85°C 8 SO MAX253MJA -55°C to +125°C 8 CERDIP** *Contact factory for dice specifications. **Contact factory for availability and processing to MIL-STD-883. Devices are also available in a lead(Pb)-free/RoHS-compliant package. Specify lead-free by adding a (+) to the part number when ordering. /V Denotes an automotive qualified part. __________Typical Operating Circuit VIN ON / OFF 4 6 SD VCC D1 5V C1 OUTPUT 5V @ 200mA 1 C3 C2 MAX253 3 FREQUENCY SWITCH FS D2 GND1 GND2 2 7 8 µMAX is a registered trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products 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. 1 MAX253 _______________General Description 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, 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 (VCC = 5V ±10%, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) CONDITIONS MIN TYP FS = VCC or open 250 VFS = 0V 150 200 300 5.0 PARAMETER Switch On-Resistance Switch Frequency D1, D2; 100mA MAX UNITS 1.5 4.0 Ω 350 500 Operating Supply Current (Note 1) No load, VSD = 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 0.8 10 High 2.4 0.8 VFS = 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 Low FS = VCC mA µA 2.4 Low Shutdown Input Leakage Current kHz _______________________________________________________________________________________ Transformer Driver for Isolated RS-485 Interface MEASURED AT TP1 1.0 plot02 15 plot01 10.5 SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE 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 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 7.0 0.8 0.6 0.4 0.2 6.5 6.0 0 6 -60 -40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) D1, D2 FREQUENCY vs. TEMPERATURE (FS = LOW) D1, D2 FREQUENCY vs. TEMPERATURE (FS = HIGH) SUPPLY CURRENT vs. TEMPERATURE (FS = LOW) plot06 600 plot05 480 plot04 260 VIN = 6.0V 550 VIN = 5.5V 200 VIN = 5.0V FREQUENCY (kHz) 220 400 VIN = 5.5V 360 VIN = 5.0V 320 VIN = 5.5V 450 VIN = 5.0V 400 350 VIN = 4.5V VIN = 4.5V 160 250 280 -60 -40 -20 0 500 300 VIN = 4.5V 20 40 60 80 100 120 140 -60 -40 -20 0 TEMPERATURE (°C) 20 40 60 80 100 120 140 -60 -40 -20 0 EFFICIENCY vs. LOAD CURRENT (FS = LOW) plot08 100 plot07 850 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) SUPPLY CURRENT vs. TEMPERATURE (FS = HIGH) 90 800 VIN = 6.0V 700 VIN = 5.5V 650 600 VIN = 5.0V 550 VIN = 5.5V 80 750 EFFICIENCY (%) SUPPLY CURRENT (μA) FREQUENCY (kHz) VIN = 6.0V 440 SUPPLY CURRENT (μA) VIN = 6.0V 240 180 20 40 60 80 100 120 140 TEMPERATURE (°C) 70 VIN = 4.5V 60 50 40 30 500 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 80 100 120 140 160 180 200 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.) OUTPUT VOLTAGE vs. LOAD CURRENT (FS = LOW) 9 90 VIN = 5.5V 70 VIN = 4.5V 60 50 40 30 8 OUTPUT VOLTAGE (V) 80 8 5 4 3 2 10 1 0 0 20 40 60 80 100 120 140 160 180 200 9 CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1.3 6 20 0 CIRCUIT OF FIGURE 6 VIN = 5.0V TURNS RATIO = 1:1 plot11 CIRCUIT OF FIGURE 7 VIN = 3.3V TURNS RATIO = 1:2.1 7 10 plot10 10 plot09 100 OUTPUT VOLTAGE vs. LOAD CURRENT (FS = HIGH) OUTPUT VOLTAGE (V) 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 0 20 40 60 80 100 120 140 160 180 200 220 LOAD CURRENT (mA) 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 TOP VIEW + D1 1 GND1 2 FS 3 MAX253 SD 4 DIP/SO/μMAX 8 D2 7 GND2 6 VCC 5 N.C. _____________________Pin Description PIN NAME FUNCTION 1 D1 Open drain of N-channel transformer drive 1. 2 GND1 3 FS 4 SD 5 N.C. Not internally connected. 6 VCC 5V supply voltage. 7 GND2 8 D2 Ground. Connect both GND1 and GND2 to ground. Frequency switch. If FS = VCC or open, switch frequency = 350kHz; if VFS = 0V, switch frequency = 200kHz. Shutdown. Ground for normal operation, connect high for shutdown. Ground. Connect both GND1 and GND2 to ground. Open drain of N-channel transformer drive 2. _______________________________________________________________________________________ 5 MAX253 __________________Pin Configuration MAX253 Transformer Driver for Isolated RS-485 Interface VIN 5V C1 0.1µF R1 50Ω 6 VCC 4 SD D1 1 ON / OFF R2 50Ω MAX253 3 FS FREQUENCY SWITCH D2 GND1 GND2 2 7 8 Figure 1. Test Circuit VIN 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- 6 quency (Figure 2). These two signals drive the groundreferenced 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 be set to the highfrequency state if not connected. _______________________________________________________________________________________ Transformer Driver for Isolated RS-485 Interface MAX253 ISOLATION BARRIER VIN 5V C1 0.1μF 6 VCC D1 ON / OFF 4 SD 2 8 C3 0.1μF MAX253 D2 GND1 1CT: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 1 B 1 RO RE 4 7 GND 2 *74HC04 OR EQUIVALENT ** SEE TABLE 2 485 I/O 6 390Ω 6 MAX481 MAX483 MAX485 MAX487 DE 2 3 PC410 / 417 5 RO VCC DI 5 3 Figure 3. Typical RS-485 Application Circuit, 5V Configuration _______________________________________________________________________________________ 7 MAX253 Transformer Driver for Isolated RS-485 Interface ISOLATION BARRIER VIN 3.3V C1 0.1µF 5 ON / OFF 4 6 D1 N.C. SD 1CT: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 A 3 *74HC04 3.3kΩ MAX481 MAX483 MAX485 MAX487 DE 2 3 PC410 / 417 1 390Ω B 1 RO RE GND 2 *74HC04 OR EQUIVALENT ** SEE TABLE 2 4 3 Figure 4. Typical RS-485 Application Circuit, 3.3V Configuration 8 _______________________________________________________________________________________ 6 485 I/O 6 5 RO VCC DI 4 5 7 Transformer Driver for Isolated RS-485 Interface 5V C1 0.1µF 5 ON / OFF 4 ISOLATION BARRIER 1CT: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 MAX253 VIN 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 R5OUT R5IN SD EN 20 10 2 R2OUT 74HC04 19 MAX205 5 X 3.3kΩ 74HC04 1 390Ω 14 21 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 T 390Ω IN 74HCO4 *74HC04 2 2 ISO ISO 4 4 GND GND 5 24 18 13 VCC ISO ROUT Figure 5. Typical RS-232 Application Circuit _______________________________________________________________________________________ 9 MAX253 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: ♦ 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. 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 (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 10 ______________________________________________________________________________________ Transformer Driver for Isolated RS-485 Interface MAX253 VIN 5V C1 0.1µF 6 VCC 4 D1 SD ON / OFF 1 1CT:1.3CT* 1N5817 5V @ 200mA ISO OUTPUT TP1 C3 0.1µF 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 1CT: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 ______________________________________________________________________________________ 11 MAX253 Transformer Driver for Isolated RS-485 Interface VIN ISOLATION BARRIER 6 5V VCC D1 24V UNREGULATED 1CT:5CT* 1 1N5817 10µF MAX253 4 SD D2 GND1 78L05 1N5817 GND2 2 5V 8 7 1 3 7 0.1V to 0.5V MAX480 2 RL 0kΩ to 1kΩ IL300 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 approximately 10mVp-p. The cutoff frequency shown is 21kHz. Since the filter inductor is in series with the circuit output, minimize its resistance so the voltage drop across it is not excessive. Isolated 4mA to 20mA Analog Interface The 4mA to 20mA current loop is a standard analog signal range that is widely used in the process-control industry for transducer and actuator control signals. These signals are commonly referred to a distant ground that may be at a considerably higher voltage with respect to the local ground. An analog signal in the range of 0.1V to 0.5V is applied to the first MAX480 to generate a signal current in the range of 20µA to 100µA. This low-level signal is transferred across the barrier by the Siemens IL300 linear optocoupler. This device is unique in that it corrects the dominant nonlinearity present in most optocou12 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 ______________________________________________________________________________________ Transformer Driver for Isolated RS-485 Interface MAX253 VIN INPUT 6 VCC D1 1 1CT:1CT* 1N5817 +VOUT ≈ 2V IN 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 ______________________________________________________________________________________ 13 MAX253 Transformer Driver for Isolated RS-485 Interface 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 3kΩ 10μF MAX176 0.1μF ANALOG INPUT 1 2 3 0.1μF 10μF VDD AIN VREF VSS CONVST CLOCK 4 GND DATA 14 6 3 5 4 11 1 12 8 8 7 3kΩ 6 5 0.1μF 10μF SIGNAL GROUND 6N136 7 470Ω QE SCK QC RCK QB 200Ω 5V 10 QA SCLR 13 4 6N136 6 5 4 3 2 1 15 16 3 5 1 74HC595 QF QD 2 6 QG SER 7 D10 D9 D8 5V 0.1μF 8 74HC04 8 2 7 3 6 4 5 8 QH′ 8.2kΩ 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 *SEE TABLE 2 Figure 10. Typical Isolated ADC Application 14 D11(MSB) ______________________________________________________________________________________ 8 D7 D6 D5 D4 D3 D2 D1 D0(LSB) 5V 0.1μF Transformer Driver for Isolated RS-485 Interface ______________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 connected 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: (516) 241-7876 FAX: (516) 241-9339 Fair-Rite Products Phone: (914) 895-2055 FAX: (914) 895-2629 Siemens Components Phone: (408) 777-4500 FAX: (408) 777-4983 ______________________________________________________________________________________ 15 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. MAX253 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 approximately 22 turns on each side of the center tap, or 44 turns total. This will result in a nominal primary inductance of approximately 832µH. The secondary can be scaled to produce the required DC output. Diode Selection The MAX253’s high switching frequency demands high-speed rectifiers. Schottky diodes are 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 bypassing. 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 16 ______________________________________________________________________________________ Transformer Driver for Isolated RS-485 Interface PROCESS: CMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 8 µMAX U8+1 21-0036 8 PDIP P8+1 21-0043 8 SO S8+4 21-0041 8 CDIP J8-2 21-0045 ______________________________________________________________________________________ 17 MAX253 ___________________Chip Information MAX253 Transformer Driver for Isolated RS-485 Interface Revision History PAGES CHANGED REVISION NUMBER REVISION DATE 0 1/94 Initial release — 1 8/09 Deleted the MAX253EUA part number from the Ordering Information table 1 2 4/10 Added automotive qualified part number to the Ordering Information table 1 DESCRIPTION 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. 18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.