Datasheet

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General Description

The MAX256 is an integrated primary-side controller

and H-bridge driver for isolated power-supply circuits.

The device contains an on-board oscillator, protection

circuitry and internal FET drivers to provide up to 3W of

power to the primary winding of a transformer. The

MAX256 can be operated using the internal program-

mable oscillator or can be driven by an external clock

for improved EMI performance. Regardless of the clock

source being used, an internal flip-flop stage guaran-

tees a fixed 50% duty cycle to prevent DC current flow

in the transformer.

The MAX256 operates from a single-supply voltage of

+5V or +3.3V, and includes undervoltage lockout for

controlled startup. The device prevents cross-conduc-

tion of the H-bridge MOSFETs by implementing break-

before-make switching. Thermal shutdown circuitry

provides additional protection against damage due to

overtemperature conditions.

The MAX256 is available in the 8-pin thermally-enhanced

SO package. The device is specified for the automotive

(-40°C to +125°C) temperature range.

Applications

Features

o Provides Up to 3W to the Transformer in Isolated

Power Supplies

o Single Supply +5V or +3.3V Operation

o Internal Resistor-Programmable Oscillator Mode

o External Clock Mode with Watchdog

o Disable Mode

o Undervoltage Lockout

o Thermal Shutdown

MAX256

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

________________________________________________________________

Maxim Integrated Products

19-3748; Rev 1; 3/12

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

GND

ST2MODE

ST1

GND

CK_RS

*EP

SO-EP

*CONNECT EXPOSED PAD TO GND.

MAX256

Pin Configuration

Ordering Information

PART TEMP RANGE PIN-PACKAGE

MAX256ASA+ - 40°C to +125°C 8 SO-EP*

MAX256ASA/V+T - 40°C to +125°C 8 SO-EP*

EP = Exposed paddle.

/V denotes an automotive qualified part.

Denotes a lead(Pb)-free/RoHS-compliant package.

T = Tape and reel.

Isolated Power Supplies

Industrial Process

Control

Isolated Communications

Links

Medical Equipment

Telecommunications

Typical Application Circuit

GND

ST2

+5V ISOLATED

MAX256

47kΩ

4.7µF

ST1

MODE

CK_RS

0.1

µF

+5V

470nF

1:2.6CT

+5V TO ISOLATED +5V TYPICAL APPLICATION

Summary of content (14 pages)

PAGE 1
9-3748; Rev 1; 3/12 KIT ATION EVALU E L B AVAILA 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies The MAX256 is available in the 8-pin thermally-enhanced SO package. The device is specified for the automotive (-40°C to +125°C) temperature range. Applications Isolated Power Supplies Industrial Process Control Isolated Communications Links Medical Equipment Telecommunications Features o Provides Up to 3W to the Transformer in Isolated Power Supplies o Single Supply +5V or +3.
PAGE 2
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies ABSOLUTE MAXIMUM RATINGS (All voltages referenced to GND, unless otherwise noted.) Supply Voltage VCC..................................................-0.3V to +6V ST1, ST2, CK_RS, MODE (Note 1)................-0.3V to VCC + 0.3V ST1, ST2 Maximum Continuous Current (TA < +125°C) ....±0.6A ST1, ST2 Maximum Continuous Current (TA < +100°C) ....±0.9A ST1, ST2 Maximum Continuous Current (TA < +85°C) ......±1.
PAGE 3
W Primary-Side Transformer H-Bridge Driver for Isolated Supplies (VCC = +3.0V to +5.5V, TA = TMIN to TMAX. Typical values are at VCC = +5.0V and TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MODE = VCC, RS = 10.5kΩ MIN TYP MAX UNITS 0.75 1 1.35 MHz 100 160 kHz 2 MHz 51 % Switching Frequency fSW MODE = VCC, CK_RS unconnected 65 CK_RS Input Frequency fIN MODE = GND 0.
PAGE 4
Typical Operating Characteristics (VCC = +5.0V ±10%, TA = +25°C, unless otherwise noted.) (See Figure 8) 1200 3 2 1 800 MAX TYP 400 200 0 +5.5V MAX SUPPLY MIN 10 0 10 OSCILLATOR FREQUENCY (kHz) 100 RS (kΩ) OUTPUT VOLTAGE vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 8) EFFICIENCY vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 8) 0.8 OUTPUT VOLTAGE vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 9) 0.7 5.5V 6 4 4.5V 0.6 5.0V 0.5 0.4 0.3 4.5V 12 10 OUTPUT VOLTAGE (V) 5.0V 5.5V 0.
PAGE 5
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies OPERATION WITH EXTERNAL 2MHz CLOCK MAX256toc11 MAX256toc10 OPERATION AT 100kHz CK_RS 5V/div CK_RS 5V/div ST1 5V/div ST1 5V/div ST2 5V/div ST2 5V/div 1µs/div 100ns/div Functional Diagram VCC THERMAL SHUTDOWN VCC UVLO OSC VUVLO ST1 MOSFET H-BRIDGE DRIVER M U X CK_RS FLIPFLOP VCC 165kΩ ST2 MODE WATCHDOG _______________________________________________________________________________________ 5 MAX256 Typical Operating Cha
PAGE 6
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies Detailed Description The MAX256 is an integrated primary-side controller and H-bridge driver for isolated power-supply circuits. The device contains an on-board oscillator, protection circuitry, and internal FET drivers to provide up to 3W of power to the primary winding of a transformer. The MAX256 can be operated using the internal programmable oscillator, or can be driven by an external clock for improved EMI performance.
PAGE 7
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256 Table 1. Oscillator Modes OSCILLATOR MODE CK_RS MODE Internal Programmable Frequency Unconnected or pulled to ground by RS. RS must be greater than 10kΩ. VCC External Clock Digital input. Drive CK_RS with an external clock signal. Ground CK_RS is pulled to ground by an internal 165kΩ resistor. The device switches at one half the external clock frequency.
PAGE 8
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies from ST1 and ST2 by placing a grounded trace between these circuits. Place RS as close as possible to the CK_RS pin. An additional capacitance of 100nF from CK_RS to GND may be required in some applications. Output Voltage Regulation For many applications, the unregulated output of the MAX256 meets the supply voltage tolerances. This configuration represents the highest efficiency possible with the MAX256.
PAGE 9
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256 +3.3V 4.7µF 470nF VCC MAX256 MBRS140 x 4 TGM-H281NF ST1 MODE MAX1659 15V 10µF 0.1µF CK_RS ST2 300kΩ + 5V - GND Figure 2. +3.3V to Isolated Regulated +5V +5V 4.7µF 470nF VCC MAX256 MBRS140 TGM-H281NF ST1 0.1µF MODE MAX1658 10µF 15V CK_RS 100kΩ + 3.3V - MBRS140 ST2 GND Figure 3. +5V to Isolated Regulated +3.3V +5V 4.7µF 470nF VCC MAX256 MBRS140 TGM-H281NF ST1 0.1µF 0.
PAGE 10
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MBRS140 TGM-280NS +5V +15V 0.1µF 7815 4.7µF 10µF 0.1µF 470nF VCC MODE ST1 MBRS140 MAX256 CK_RS 47kΩ COMMON ST2 MBRS140 TGM-280NS GND 0.1µF 7815 10µF 0.1µF -15V MBRS140 Figure 5. +5V to Isolated Regulated ±15V Component Selection Transformer Selection Transformer selection for the MAX256 can be simplified by the use of a design metric, the ET product.
PAGE 11
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256 VCC +15V MAX256 COMMON -15V VCC RS485 MPU OPTOISOLATORS M U X DAC/ADC OPTOISOLATORS Figure 6. Isolated Power Supply for Process Control Applications As with all power-supply designs, it is important to optimize efficiency. In designs incorporating small transformers, the possibility of thermal runaway makes low transformer efficiencies problematic.
PAGE 12
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies +5V FILTER OUTPUT L1 25µH 4.7µF 470nF C1 2.2µF MAX256 ST1 1:1.75 +5V ISOLATED 0.1µF 0.1µF MODE ST2 CK_RS Figure 7. Output Ripple Filter 0.1µF 47kΩ GND 0.1µF ALL DIODES MBRS140 +5V 4.7µF 470nF MAX256 ST1 1:2.6CT +5V ISOLATED Diode Selection The high switching speed of the MAX256 necessitates high-speed rectifiers.
PAGE 13
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256 Table 2. Suggested External Component Manufacturers MANUFACTURER COMPONENT Central Semiconductor diodes Halo Electronics transformers WEBSITE PHONE www.centralsemi.com 631-435-1110 www.haloelectronics.com 650-903-3800 Kemet capacitors www.kemet.com 864-963-6300 Sanyo capacitors www.sanyo.com 619-661-6835 Taiyo Yuden capacitors www.t-yuden.com 408-573-4150 TDK capacitors www.component.tdk.
PAGE 14
MAX256 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies Revision History REVISION NUMBER REVISION DATE 0 8/05 Initial release 3/12 Added automotive-qualified part information. Added lead-free packaging information 1 DESCRIPTION PAGES CHANGED — 1–4, 8, 12 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.