MAXIM MAX256 User Manual

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 program­mable 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 guaran­tees 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-conduc­tion of the H-bridge MOSFETs by implementing break­before-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

1

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

1

+

2

87ST1

GND

V

CC

V

CC

CK_RS

3

4

6

5

*EP

SO-EP

*CONNECT EXPOSED PAD TO GND.

MAX256

Pin Configuration

Ordering Information

*

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

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*

4.7µF

47kΩ

+5V

470nF

MAX256

MODE

CK_RS

GND

+5V TO ISOLATED +5V TYPICAL APPLICATION

ST2

ST1

1:2.6CT

+5V ISOLATED

µF

0.1

MAX256

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

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.

(All voltages referenced to GND, unless otherwise noted.)
Supply Voltage V

CC

..................................................-0.3V to +6V

ST1, ST2, CK_RS, MODE (Note 1)................-0.3V to V

CC

+ 0.3V

ST1, ST2 Maximum Continuous Current (T

A

< +125°C) ....±0.6A

ST1, ST2 Maximum Continuous Current (T

A

< +100°C) ....±0.9A

ST1, ST2 Maximum Continuous Current (T

A

< +85°C) ......±1.0A

Continuous Power Dissipation (T

A

= +70°C)

8-Pin SO (derate 18.9mW/°C above +70°C)..............1509mW

Operating Temperature Range .........................-40°C to +125°C

Storage Temperature Range .............................-65°C to +150°C

Junction Temperature......................................................+150°C

Lead Temperature (soldering, 10s) .................................+300°C

Soldering Temperature (reflow) .......................................+260°C

DC ELECTRICAL CHARACTERISTICS

(VCC= +3.0V to +5.5V, TA= T

MIN

to T

MAX

. Typical values are at VCC= +5.0V and TA= +25°C, unless otherwise noted.)

Note 1: ST1 and ST2 are not protected against short circuits. Damage to the device may result from a short-circuit fault.

SO-EP

Junction-to-Ambient Thermal Resistance (θ

JA

)...............53°C/W

Junction-to-Case Thermal Resistance (θ

JC

)......................5°C/W

Note 2: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-

layer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial

.

PACKAGE THERMAL CHARACTERISTICS (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Voltage V

Supply Current I

Disable Supply Current I

External Resistance Range R

Driver Total Resistance R

Undervoltage Lockout Threshold V

Undervoltage-Lockout-Threshold
Hysteresis

Logic-Low Level
(MODE, CK_RS)

Logic-High Level
(MODE, CK_RS)

Input Leakage Current
(MODE)

Internal Pulldown Resistance on
CK_RS

Thermal Shutdown T

Thermal Shutdown Hysteresis T

CC

CC

SD

OHL

UVLO

V

UVLO_HST

V

V

I

LK

R

S_INT

SHDN

SHDN_HST

M OD E = V

MODE = GND,
CK_RS unconnected

S

VCC = 4.5V (Note 4) 0.5 1.0

VCC = 3.0V (Note 4) 0.6 1.2

VCC rising 0.8 1.9 2.7 V

VCC = 4.5V 0.8

IL

VCC = 3.0V 0.7

IH

MODE = GND 165 kΩ

3.0 5.5 V

C K_RS unconnected ( N ote 3) 1.06 3 mA

C C ,

50 µA

10 kΩ

110 mV

2.0 V

1µA

165 °C

10 °C

Ω

V

MAX256

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

_______________________________________________________________________________________ 3

TIMING CHARACTERISTICS

(VCC= +3.0V to +5.5V, TA= T

MIN

to T

MAX

. Typical values are at VCC= +5.0V and TA= +25°C, unless otherwise noted.)

Note 3: Minimum and maximum limits tested with ST1, ST2 unconnected.
Note 4: Total driver resistance includes the on-resistance of the top and the bottom internal FETs. If R

OH

is the high-side resistance,

and R

OL

is the low-side resistance, R

OHL

= ROH+ ROL.

Pin Description

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Switching Frequency f

CK_RS Input Frequency f

ST1 and ST2 Duty Cycle Dtc MODE = V

Crossover Dead Time t

Watchdog Timeout t

PIN NAME FUNCTION

1 CK_RS

Clock Input/Oscillator Frequency Adjust. When MODE is HIGH, set the internal oscillator frequency by
connecting a 10kΩ or greater resistor from CK_RS to ground. When MODE is LOW, apply an external
clock signal to CK_RS. The MAX256 outputs switch at one half the external clock frequency.

SW

IN

DEAD

WDOG

MODE = VCC, RS = 10.5kΩ 0.75 1 1.35 MHz

MODE = V

MODE = GND 0.2 2 MHz

RL = 100Ω 20 ns

MODE = GND 20 55 µs

CK_RS unconnected 65 100 160 kHz

CC,

CC

49 50 51 %

2, 3 V

4 MODE

5 ST2 Transformer Drive Output 2

6, 7 GND Ground

8 ST1 Transformer Drive Output 1

—EP

CC

VCC Supply Voltage, +3.0V ≤ VCC ≤ +5.5V.
Bypass V

Mode Control Input. Drive MODE high to enable internal oscillator. Drive MODE low and supply a valid
clock signal on CK_RS for external clock mode.

EP is internally connected to GND. Connect to a large ground plane to maximize thermal
performance; not intended as an electrical connection point.

to ground with a 4.7µF capacitor and a 470nF ceramic capacitor.

CC

MAX256

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

4 _______________________________________________________________________________________

Typical Operating Characteristics

(VCC= +5.0V ±10%, TA= +25°C, unless otherwise noted.) (See

Figure 8

)

SUPPLY CURRENT vs.

OSCILLATOR FREQUENCY

MAX256toc01

OSCILLATOR FREQUENCY (kHz)

SUPPLY CURRENT (mA)

900

400

2

3

4

5

6

7

1

100

0

200 300 500 600 700 800 1000

OSCILLATOR FREQUENCY vs. RS (+1%)

RS (kΩ)

OSCILLATOR FREQUENCY (kHz)

MAX256 toc02

0

200

400

600

800

1000

1200

1400

10 100 1000

MAX

TYP

MIN

OUTPUT VOLTAGE vs. OUTPUT CURRENT

(TYPICAL APPLICATION FIGURE 9)

OUTPUT CURRENT (mA)

OUTPUT VOLTAGE (V)

MAX256 toc06

0 100 200 300 400 500

0

2

4

6

8

10

12

3.0V

3.6V

3.3V

OUTPUT VOLTAGE vs. OUTPUT CURRENT

(TYPICAL APPLICATION FIGURE 8)

12

10

8

6

4

OUTPUT VOLTAGE (V)

2

0

0 200 400 600 800

OUTPUT CURRENT (mA)

4.5V

5.0V

5.5V

MAX256 toc04

EFFICIENCY vs. OUTPUT CURRENT

(TYPICAL APPLICATION FIGURE 8)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

EFFICIENCY

0.3

0.2

0.1

0

0 200 400 600 800

4.5V

OUTPUT CURRENT (mA)

5.0V

5.5V

MAX256 toc05

(kΩ)

S

R

1000

100

RS vs. REQUIRED ET PRODUCT

+3.6V MAX SUPPLY

10

REQUIRED ET PRODUCT (Vµs)

+5.5V MAX SUPPLY

10

MAX256toc03

1001

EFFICIENCY vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 9)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

EFFICIENCY

0.3

0.2

0.1

0

0 100 200 300 400 500

3.6V

3.0V

OUTPUT CURRENT (mA)

3.3V

MAX256 toc07

OUTPUT VOLTAGE (V)

OUTPUT VOLTAGE vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 10)

40

35

30

25

20

15

10

4.5V

0 20 40 60 80 100 120 140

OUTPUT CURRENT (mA)

5.0V

5.5V

MAX256 toc08

EFFICIENCY vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 10)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

EFFICIENCY

0.3

0.2

0.1

0

0 20 40 60 80 100 120 140

OUTPUT CURRENT (mA)

4.5V

5.5V

MAX256 toc09

5.0V

MAX256

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

_______________________________________________________________________________________

5

Typical Operating Characteristics (continued)

(VCC= +5.0V ±10%, TA= +25°C, unless otherwise noted.) (See

Figure 8

)

Functional Diagram

OPERATION AT 100kHz

MAX256toc10

CK_RS
5V/div

ST1
5V/div

ST2
5V/div

1µs/div

V

CC

OPERATION WITH

EXTERNAL 2MHz CLOCK

MAX256toc11

CK_RS

5V/div

ST1
5V/div

ST2
5V/div

100ns/div

CK_RS

MODE

OSC

165kΩ

M

U
X

UVLO

V

FLIP­FLOP

WATCHDOG

UVLO

MOSFET

H-BRIDGE

DRIVER

THERMAL

SHUTDOWN

V

CC

ST1

V

CC

ST2