19-3748; Rev 0; 8/05
KIT
ATION
EVALU
BLE
AVAILA
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
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.
Features
♦
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
♦
Undervoltage Lockout
♦
Thermal Shutdown
MAX256
Ordering Information
PART
MAX256ASA
TEMP RANGE
-40°C to +125°C
PIN-PACKAGE
8 SO-EP*
PKG
CODE
S8E-12
Applications
Isolated Power Supplies
Industrial Process
Control
Isolated Communications
Links
Medical Equipment
Telecommunications
*EP
= Exposed paddle.
Typical Application Circuit
+5V
4.7µF
470nF
Pin Configuration
MAX256
MODE
ST1
1:2.6CT
+5V ISOLATED
0.1µF
CK_RS
V
CC
V
CC
1
2
3
8
ST1
GND
GND
ST2
MAX256
7
6
CK_RS
47kΩ
GND
ST2
MODE 4
EP*
SO-EP
5
*EXPOSED PAD IS CONNECTED TO GND
+5V TO ISOLATED +5V TYPICAL APPLICATION
________________________________________________________________
Maxim Integrated Products
1
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.
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
MAX256
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
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.
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 3)
V
CC
= 3.0V (Note 3)
V
CC
rising
0.8
MODE = V
CC,
CK_RS unconnected (Note 2)
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
2
_______________________________________________________________________________________
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
MAX256
Note 2:
Minimum and maximum limits tested with ST1, ST2 unconnected.
Note 3:
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
Exposed Paddle. Connect the exposed paddle to ground.
2, 3
4
5
6, 7
8
EP
V
CC
MODE
ST2
GND
ST1
EP
_______________________________________________________________________________________
3
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
MAX256
Typical Operating Characteristics
(V
CC
= +5.0V ±10%, T
A
= +25°C, unless otherwise noted.) (See
Figure 8A)
SUPPLY CURRENT vs.
OSCILLATOR FREQUENCY
MAX256toc01
OSCILLATOR FREQUENCY vs. R
S
(+1%)
1200
OSCILLATOR FREQUENCY (kHz)
1000
800
MAX
600
400
200
0
MIN
10
10
100
R
S
(kΩ)
1000
1
TYP
R
S
(kΩ)
MAX256 toc02
R
S
vs. REQUIRED ET PRODUCT
MAX256toc03
7
6
SUPPLY CURRENT (mA)
5
4
3
2
1
0
1400
1000
+3.6V MAX SUPPLY
100
+5.5V MAX SUPPLY
100
200 300
400
500 600 700 800
900
1000
OSCILLATOR FREQUENCY (kHz)
10
REQUIRED ET PRODUCT (Vµs)
100
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8A)
MAX256 toc04
EFFICIENCY vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8A)
0.9
0.8
0.7
EFFICIENCY
0.6
0.5
0.4
0.3
0.2
0.1
4.5V
5.0V
5.5V
MAX256 toc05
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8B)
MAX256 toc06
12
10
OUTPUT VOLTAGE (V)
8
6
4
4.5V
2
0
0
200
400
600
OUTPUT CURRENT (mA)
5.0V
5.5V
1.0
12
10
OUTPUT VOLTAGE (V)
8
3.6V
6
4
3.0V
2
0
3.3V
0
800
0
200
400
600
OUTPUT CURRENT (mA)
800
0
100
200
300
400
OUTPUT CURRENT (mA)
500
EFFICIENCY vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8B)
MAX256 toc07
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8C)
MAX256 toc08
EFFICIENCY vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8C)
0.9
0.8
0.7
EFFICIENCY
0.6
0.5
0.4
0.3
0.2
0.1
4.5V
5.0V
5.5V
MAX256 toc09
1.0
0.9
0.8
0.7
EFFICIENCY
0.6
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
OUTPUT CURRENT (mA)
3.0V
3.6V
3.3V
40
35
OUTPUT VOLTAGE (V)
30
25
4.5V
20
15
10
1.0
5.0V
5.5V
0
0
20
40
60
80 100
OUTPUT CURRENT (mA)
120
140
0
20
40
60
80 100
OUTPUT CURRENT (mA)
120
140
500
4
_______________________________________________________________________________________
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
MAX256
Typical Operating Characteristics (continued)
(V
CC
= +5.0V ±10%, T
A
= +25°C, unless otherwise noted.) (See
Figure 8A)
OPERATION WITH
EXTERNAL 2MHz CLOCK
MAX256toc10
MAX256toc11
OPERATION AT 100kHz
CK_RS
5V/div
CK_RS
5V/div
ST1
5V/div
ST2
5V/div
ST1
5V/div
ST2
5V/div
1µs/div
100ns/div
Functional Diagram
V
CC
THERMAL
SHUTDOWN
V
CC
UVLO
OSC
V
UVLO
ST1
MOSFET
H-BRIDGE
DRIVER
CK_RS
165kΩ
ST2
M
U
X
FLIP-
FLOP
V
CC
MODE
WATCHDOG
_______________________________________________________________________________________
5
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