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MAX253

Transformer Driver for

Isolated RS-485 Interface

General Description

The MAX253 monolithic oscillator/power-driver is

specifically designed to provide isolated power for an

isolated RS-485 or RS-232 data interface. The device

drives a center-tapped transformer primary from a 5V

or 3.3V DC power supply. The secondary can be

wound to provide any isolated voltage needed at power

levels up to 1W.

The MAX253 consists of a CMOS oscillator driving a

pair of N-channel power switches. The oscillator runs

at double the output frequency, driving a toggle flip-

flop to ensure 50% duty cycle to each of the switches.

Internal delays are arranged to ensure break-before-

make action between the two switches.

The SD pin puts the entire device into a low-power

shutdown state, disabling both the power switches and

oscillator.

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

Benefits and Features

• Simple Power-Supply Transformer Driver Design 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 Take Minimal

Board Space

Ordering Information

MAX253

D2FS

GND1 GND2

FREQUENCY

SWITCH

5V @ 200mA

OUTPUT

ON / OFF

Typical Operating Circuit

PART TEMP RANGE PIN-PACKAGE

MAX253CPA 0°C to +70°C 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.

µMAX is a registered trademark of Maxim Integrated Products, Inc.

19-0226; Rev 3; 2/15

Summary of content (18 pages)

PAGE 1
Transformer Driver for Isolated RS-485 Interface MAX253 General Description Benefits and 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.
PAGE 2
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) .........................
PAGE 3
Transformer Driver for Isolated RS-485 Interface MAX253 Typical Operating Characteristics (Circuit of Figure 6, VIN = 5V ±10%, TA = +25°C, unless otherwise noted.) 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.
PAGE 4
Transformer Driver for Isolated RS-485 Interface MAX253 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) EFFICIENCY (%) 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.
PAGE 5
Transformer Driver for Isolated RS-485 Interface MAX253 Pin Configuration Pin Description TOP VIEW + D1 1 GND1 2 FS 3 MAX253 SD 4 DIP/SO/μMAX www.maximintegrated.com 8 D2 7 GND2 6 VCC 5 N.C. 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.
PAGE 6
Transformer Driver for Isolated RS-485 Interface MAX253 VIN 5V C1 0.1μF R1 50Ω 6 VCC 4 SD 1 D1 ON / OFF R2 50Ω MAX253 3 FS FREQUENCY SWITCH 8 D2 GND1 2 GND2 7 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.
PAGE 7
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.
PAGE 8
Transformer Driver for Isolated RS-485 Interface MAX253 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 4 A 3 *74HC04 3.
PAGE 9
Transformer Driver for Isolated RS-485 Interface MAX253 VIN 5V C1 0.1μF 5 ON / OFF 4 ISOLATION BARRIER 1CT:1.3CT** 1N5817 6 VCC N.C. D1 1 MAX253 SD D2 FS GND1 GND2 2 7 8 2 IN OUT C2 MAX667 22μF C3 0.1μF ISO 5V C4 22μF 8 3 SET GND SHDN 6 4 5 1N5817 5 x 3.
PAGE 10
Transformer Driver for Isolated RS-485 Interface 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.
PAGE 11
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 C3 0.1μF MAX253 3 FS FREQUENCY SWITCH D2 5V @ 200mA ISO OUTPUT TP1 C2 22μF 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 C3 0.1μF MAX253 3 FREQUENCY SWITCH 5V @ 100mA TP1 ISO OUTPUT 1CT:2.
PAGE 12
Transformer Driver for Isolated RS-485 Interface MAX253 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 RL 0kΩ to 1kΩ IL300 7 0.1V to 0.5V MAX480 2 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.
PAGE 13
Transformer Driver for Isolated RS-485 Interface MAX253 VIN INPUT 6 VCC D1 1 1CT:1CT* 1N5817 +VOUT ≈ 2V IN OUTPUT MAX253 RL+ 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.
PAGE 14
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 3kΩ 10μF MAX176 0.1μF ANALOG INPUT 1 2 3 0.1μF VDD AIN VREF VSS CONVST CLOCK 4 GND 10μF DATA 14 6 3 5 4 11 1 12 8 8 7 3kΩ 6 5 0.
PAGE 15
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.
PAGE 16
Transformer Driver for Isolated RS-485 Interface 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).
PAGE 17
Transformer Driver for Isolated RS-485 Interface MAX253 Chip Information Package Information PROCESS: CMOS For the latest package outline information and land patterns (footprints), go to www.maximintegrated.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. www.maximintegrated.com PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
PAGE 18
Transformer Driver for Isolated RS-485 Interface MAX253 Revision History REVISION NUMBER REVISION DATE PAGES CHANGED 0 2/94 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 3 2/15 Added the Benefits and Features section 1 DESCRIPTION For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s we