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.
Benefits and Features
•
Simple, Flexible Design Provides Up to 3W to the
Transformer in Isolated Power Supplies
•
Single Supply +5V or +3.3V Operation
•
Internal Resistor-Programmable Oscillator Mode
•
External Clock Mode with Watchdog
•
Disable Mode
•
Integrated System Protection
•
Undervoltage Lockout
•
Thermal Shutdown
•
Saves Space on Board
•
8-Pin Thermally-Enhanced SO Package
Ordering Information
PART
MAX256ASA+
MAX256ASA/V+T
TEMP RANGE
-40°C to +125°C
-40°C to +125°C
PIN-PACKAGE
8 SO-EP*
8 SO-EP*
Applications
Isolated Power Supplies
Industrial Process
Control
Isolated Communications
Links
Medical Equipment
Telecommunications
*EP
= Exposed paddle.
*/V
denotes an automotive qualified part.
+Denotes
a lead(Pb)-free/RoHS-compliant package.
T = Tape and reel.
Typical Application Circuit
+5V
4.7μF
470nF
Pin Configuration
+
MAX256
MODE
ST2
ST1
1:2.6CT
+5V ISOLATED
0.1μF
CK_RS 1
V
CC
V
CC
2
3
8
ST1
GND
GND
ST2
MAX256
*EP
SO-EP
7
6
5
CK_RS
47kΩ
GND
MODE 4
*CONNECT EXPOSED PAD TO GND.
+5V TO ISOLATED +5V TYPICAL APPLICATION
19-3748; Rev 2; 2/15
MAX256
3W Primary-Side Transformer
H-Bridge Driver for Isolated Supplies
Absolute Maximum Ratings
(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
Note 1:
ST1 and ST2 are not protected against short circuits. Damage to the device may result from a short-circuit fault.
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.
Package Thermal Characteristics (Note 2)
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.maximintegrated.com/thermal-tutorial.
DC Electrical Characteristics
(V
CC
= +3.0V to +5.5V, T
A
= T
MIN
to T
MAX
. Typical values are at V
CC
= +5.0V and T
A
= +25°C, unless otherwise noted.)
PARAMETER
Supply Voltage
Supply Current
Disable Supply Current
External Resistance Range
Driver Total Resistance
Undervoltage Lockout Threshold
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
Thermal Shutdown Hysteresis
SYMBOL
V
CC
I
CC
I
SD
R
S
R
OHL
V
UVLO
V
UVLO_HST
V
IL
V
IH
I
LK
R
S_INT
T
SHDN
T
SHDN_HST
MODE = GND
165
165
10
V
CC
= 4.5V
V
CC
= 3.0V
2.0
1
V
CC
= 4.5V (Note 4)
V
CC
= 3.0V (Note 4)
V
CC
rising
0.8
MODE = V
CC,
CK_RS unconnected (Note 3)
MODE = GND,
CK_RS unconnected
10
0.5
0.6
1.9
110
0.8
0.7
1.0
1.2
2.7
CONDITIONS
MIN
3.0
1.06
TYP
MAX
5.5
3
50
UNITS
V
mA
µA
kΩ
Ω
V
mV
V
V
µA
kΩ
°C
°C
www.maximintegrated.com
Maxim Integrated | 2
MAX256
3W Primary-Side Transformer
H-Bridge Driver for Isolated Supplies
Timing Characteristics
(V
CC
= +3.0V to +5.5V, T
A
= T
MIN
to T
MAX
. Typical values are at V
CC
= +5.0V and T
A
= +25°C, unless otherwise noted.)
PARAMETER
Switching Frequency
CK_RS Input Frequency
ST1 and ST2 Duty Cycle
Crossover Dead Time
Watchdog Timeout
SYMBOL
f
SW
f
IN
Dtc
t
DEAD
t
WDOG
CONDITIONS
MODE = V
CC
, R
S
= 10.5kΩ
MODE = V
CC,
CK_RS unconnected
MODE = GND
MODE = V
CC
R
L
= 100Ω
MODE = GND
20
MIN
0.75
65
0.2
49
50
20
55
TYP
1
100
MAX
1.35
160
2
51
UNITS
MHz
kHz
MHz
%
ns
µs
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
= R
OH
+ R
OL
.
Pin Description
PIN
1
NAME
CK_RS
FUNCTION
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.
V
CC
Supply Voltage, +3.0V
≤
V
CC
≤
+5.5V.
Bypass V
CC
to ground with a 4.7µF capacitor and a 470nF ceramic capacitor.
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.
Transformer Drive Output 2
Ground
Transformer Drive Output 1
EP is internally connected to GND. Connect to a large ground plane to maximize thermal
performance; not intended as an electrical connection point.
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