General Description
The MAX256 evaluation kit (EV kit) is a fully assembled
and tested PCB that contains a 3W isolated H-bridge
DC-DC converter. The circuit is configured for unregu-
lated output voltages of approximately +5V and -5V
(with respect to an isolated ground). Output current is
up to 600mA from either output, or 300mA from each of
the two outputs. Input power for the circuit is provided
from a +5VDC source, or operates at any input voltage
down to +3V with a corresponding reduction in the out-
put voltages.
The EV kit provides greater than 85% overall efficiency
at +5V between 250mW and 2W output power using an
H-bridge DC-DC converter topology. Input ripple cur-
rent and radiated noise are minimized by the inherent
balanced nature of the design with no interruption in the
input current. Undervoltage lockout (UVLO) and ther-
mal shutdown provide for a robust 3W isolated supply.
The surface-mount transformer provides up to
1500VRMS galvanic isolation with each output powered
from a center-tapped full-wave rectifier circuit to reduce
output voltage ripple.
Operation at 400kHz allows the use of ceramic output
capacitors and a small transformer.
Features
♦+3V to +5VDC Input Range
♦Isolated Outputs
+VOUT: +5V Provides Up to 300mA
-VOUT: -5V Provides Up to 300mA
♦87% Overall Efficiency at +5V Input and 200mA
Load
♦Center-Tapped, Full-Wave Rectifier Output
♦400kHz Switching Frequency
♦2.7V Undervoltage Lockout (UVLO)
♦Designed for 1500VRMS Isolation
♦Low-Cost, Integrated-FET H-Bridge Design
♦Fully Assembled and Tested
Evaluates: MAX256
MAX256 Evaluation Kit
________________________________________________________________
Maxim Integrated Products
1
19-0715; Rev 0; 12/06
Component List
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.
Ordering Information
PART TEMP RANGE IC PACKAGE
MAX256EVKIT+ 0°C to +70°C* 8 SO-EP†
DESIGNATION QTY DESCRIPTION
C1 1
10µF ±10%, 10V X7R ceramic
capacitor (1206)
Murata GRM31CR71A106K
C2 1
0.47µF ±10%, 16V X7R ceramic
capacitor (0805)
Murata GRM219R71C474K
C3, C4 2
0.1µF ±10%, 25V X7R ceramic
capacitors (0805)
Murata GRM21BR71E104K
D1–D4 4 30V, 1A Schottky diodes (SOD-123)
Diodes Inc. B130LAW
JU1 1 3-pin header
DESIGNATION QTY DESCRIPTION
JU2 1 2-pin header
R1 1 39.2kΩ ±1% resistor (0603)
R2 1 100kΩ ±5% resistor (0603)
R3, R4 0 Not installed, resistors (0805)
(0Ω ±5% resistors recommended)
R5 1 0Ω ±5% resistor (0805)
T1 1
3W 1:1:2.6:2.6 turn transformer
(8-pin gull wing)
HALO Electronics TGM-H281NF
U1 1 MAX256ASA+ (8-pin SO-EP)
—2 Shunts (JU1, JU2)
—4 Rubber bumpers
—1 PCB: MAX256EVKIT+
+
Denotes a lead-free and RoHS-compliant EV kit.
*
This limited temperature range applies to the EV kit PCB only.
The MAX256 IC temperature range is -40°C to +125°C.
†
EP = Exposed paddle.
Evaluates: MAX256
Quick Start
Required Equipment
•One 5V, 1A current-limited power supply with a
built-in current meter
•Two voltmeters
•Two 1A meters
•Two 100mA loads (or 51Ωresistors)
Procedure
The MAX256 EV kit is fully assembled and tested.
Follow the steps below to verify board operation.
Caution: Do not turn on the power supply until all con-
nections are completed.
1) Connect a voltmeter to the +VOUT pad and SGND.
2) Connect the second voltmeter to the -VOUT pad
and SGND.
3) Connect a 1A meter in series with the 100mA load
to +VOUT. Connect the load ground to the SGND
PC pad.
4) Connect the second 1A meter in series with the
100mA load to -VOUT. Connect the load ground to
the SGND PC pads.
5) Verify that a shunt is installed across pins 2-3 of
jumper JU1 (400kHz internal oscillator) and that a
shunt is not installed across the pins of jumper JU2
(mode = enabled).
6) Connect the +5V power supply to the VCC pad.
Connect the power supply’s ground to the GND
pad.
7) Turn on the power supply and verify that the volt-
meter at +VOUT reads approximately +6V.
8) Verify that the voltmeter at -VOUT reads approxi-
mately -6V.
The +5V supply powering the MAX256 EV kit must be
current-limited at 1A and the output load current for
each output limited to less than 300mA.
Detailed Description
The MAX256 EV kit is a 3W, isolated H-bridge DC-DC
converter that provides approximately +5V and -5V with
respect to an isolated ground. Either of the outputs
provides up to approximately 600mA when operated
from a +5V supply. The total current is limited to
approximately 600mA. The EV kit circuit should be
powered from a +5V source capable of at least 800mA,
but can also be operated at lower voltages consistent
with the 2.7V UVLO limit.
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 H-bridge FET drivers to provide
up to 3W of power to the primary winding of a trans-
former. The MAX256 operates using the internal pro-
grammable oscillator, or can be driven by an external
clock for improved EMI performance. Regardless of the
clock source 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 and
includes UVLO for controlled startup. The device pre-
vents cross-conduction 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 IC’s UVLO provides controlled turn-on
during power-up and during brownouts. If the input
voltage at VCC falls below 1.9V (typ), the MAX256 IC
shuts down. The MAX256 driver switches nominally at
400kHz frequency, set by resistor R1. The switching-
frequency duty cycle is fixed at 50% to control energy
transfer to the isolated outputs.
The PCB is designed for 1500VRMS isolation, with 300
mils spacing between the GND and SGND planes. The
bottom PCB GND plane under U1 is utilized as a ther-
mal heatsink for power dissipation of the MAX256 IC’s
MAX256 Evaluation Kit
2 _______________________________________________________________________________________
Component Suppliers
SUPPLIER PHONE WEBSITE
Diodes Inc. 805-446-4800 www.diodes.com
HALO Electronics 650-903-3800 www.haloelectronics.com
Murata Mfg. Co., Ltd. 770-436-1300 www.murata.com
Note: Indicate that you are using the MAX256 when contacting these component suppliers.
thermally enhanced SO package. Test points TP1
(GND) and TP2 (SGND) are provided on the PCB for
probing the respective ground plane, or to connect the
GND to SGND planes for nonisolated evaluation of the
circuit.
Jumper Configurations
External/Internal Oscillator, Clock Modes, and
Shutdown Control
The MAX256 EV kit features two multifunction jumpers
that set the MAX256 IC’s modes of operation. Jumpers
JU1 and JU2 configure the circuit for one of several
modes: an externally programmable mode, a MAX256
internal oscillator mode, or a clock mode using an
externally supplied TTL/CMOS clock. Each mode
demonstrates the MAX256 shutdown feature to reduce
current.
Table 1 lists the externally programmable (default)
mode configuration, Table 2 lists the internal oscillator-
mode configuration, and Table 3 lists the configuration
for a clock mode. See the respective table for the
selectable jumper options to configure a specific mode
of operation.
For the clock mode, connect an external square-wave
clock to the MAX256 EV kit CLOCK and GND pads.
The TTL/CMOS square-wave clock source must pro-
vide the following signal qualities:
•Output voltage:
Logic-low = 0 to 0.8V
Logic-high = 2.0V to VCC
•Output frequency:
200kHz to 2MHz*
•Duty cycle:
40% to 60%
*
Observe transformer ET product limitations. Refer to the
MAX256 data sheet for details.
Evaluating Other Transformer
Configurations
Transformer T1 Configuration and Output Voltages
The MAX256 EV kit PCB layout provides an easy
method to reconfigure transformer T1 primary windings
for series (default) or parallel configuration. Changing
the primary winding series/parallel configuration
changes the circuit output voltages and available maxi-
mum currents. Surface-mount (0805 case) resistors R3,
R4, and R5 configure the primary windings. Two extra
resistors for R3 and R4 pads are included with the EV
kit in a sealed bag. See Table 4 for configuring the pri-
mary windings
H-Bridge Converter Output and
Documentation
If none of these configurations are suitable for your
needs contact your transformer vendor. A custom
design may be less expensive than you expect, espe-
cially if based on an existing design.
Evaluates: MAX256
MAX256 Evaluation Kit
_______________________________________________________________________________________ 3
Evaluates: MAX256
MAX256 Evaluation Kit
4 _______________________________________________________________________________________
Table 4. Transformer Primary Configuration
PRIMARY
CONFIGURATION R3 R4 R5 +VOUT AT 1.5W -VOUT AT 1.5W
Series* Open Open 0Ω+6V -6V
——————
Parallel 0Ω0ΩOpen +12V -12V
*
Default configuration.
Table 1. Externally Programmable (Default) Mode (400kHz) and Shutdown Control
*
Refer to the MAX256 IC data sheet for selecting a different programmable oscillator frequency and choosing resistor R1.
JU1 SHUNT
LOCATION MAX256 CK_RS PIN JU2 SHUNT
LOCATION MAX256 MODE PIN OSCILLATOR MODE
2-3 Connected to GND through
R1* Not installed Connected to VCC
through R2
Internal oscillator switching
at 400kHz
2-3 Connected to GND through
R1* Installed Connected to GND Shutdown
Table 2. Internal Oscillator Mode (100kHz*) and Shutdown Control
JU1 SHUNT
LOCATION MAX256 CK_RS PIN JU2 SHUNT
LOCATION MAX256 MODE PIN OSCILLATOR MODE
Not installed* Internally connects to GND Not installed Connected to VCC
through R2
Internal oscillator switching
at 100kHz*
Not installed Internally connects to GND Installed Connected to GND Shutdown
*
Internal oscillator switching at 100kHz is NOT ALLOWED with a TGM-H281 transformer.
Table 3. Clock Mode and Shutdown Control
JU1 SHUNT
LOCATION MAX256 CK_RS PIN JU2 SHUNT
LOCATION MAX256 MODE PIN CLOCK MODE
1-2 Connected to CLOCK
PCB pad Installed* Connected to GND U1 switches at 1/2 external
clock frequency
1-2 Connected to CLOCK
PCB pad Installed* Connected to GND Shutdown when external
clock stops (0Hz)
*
JU2 must be installed when operating in clock mode; otherwise U1 may be damaged or destroyed.
Evaluates: MAX256
MAX256 Evaluation Kit
_______________________________________________________________________________________ 5
SERIES PRIMARY, +VOUT TO SGND
MAX256EV toc01
OUTPUT CURRENT (mA)
OUTPUT CURRENT (V)
300
4
8
3
7
2
6
1
5
0
6000 400100 500200
5.0V
3.6V
5.5V
4.5V
3.0V
3.3V
Figure 1. T1 Series Primary: +VOUT Voltage vs. Output Current
(+VOUT to SGND Loading)
SERIES PRIMARY, +VOUT TO -VOUT
MAX256EV toc02
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
150
8
16
6
14
4
12
2
10
0
3000 20050 250100
5.0V
3.6V
5.5V
4.5V
3.0V
3.3V
Figure 2. T1 Series Primary: ±VOUT Voltage vs. Output Current
(+VOUT to -VOUT Loading)
PARALLEL PRIMARY, +VOUT TO SGND
MAX256EV toc03
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
150
8
16
6
14
4
12
2
10
0
3000 20050 250100
5.0V
3.6V
5.5V
4.5V
3.0V
3.3V
Figure 3. T1 Parallel Primary: +VOUT Voltage vs. Output
Current (+VOUT to SGND Loading)
PARALLEL PRIMARY, +VOUT TO -VOUT
MAX256EV toc04
OUTPUT CURRENT (mA)
OUTPUT CURRENT (V)
60
30
25
10
20
5
15
0
14008020 12010040
5.5V
3.6V
5.0V
4.5V
3.0V
3.3V
Figure 4. T1 Parallel Primary: ±VOUT Voltage vs. Output
Current (+VOUT to -VOUT Loading)
Evaluates: MAX256
MAX256 Evaluation Kit
6 _______________________________________________________________________________________
Figure 6. MAX256 EV Kit Component Placement Guide—
Component Side
Figure 7. MAX256 EV Kit PCB Layout—Component Side
MAX256
U1
CK_RS
VCC
VCC
MODE
8
JU1
C1
10µF
R1
39.2kΩ
1%
7
6
5
ST1
GND
GND
ST2
ST2
11
D3
3
2
4
8
6
7
5
1
2
3
2
3
4
JU2
1
2
CLOCK
VCC
GND
GND
R2
100kΩ
R3
OPEN
R4
OPEN
C3
0.1µF
C4
0.1µF
R5
0Ω
C2
0.47µF
T1-8
ST2
T1-8
D4
D1
D2
+VOUT
-VOUT
T1
SGND
TP1 TP2
Figure 5. MAX256 EV Kit Schematic
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________
7
© 2006 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Evaluates: MAX256
MAX256 Evaluation Kit
Figure 8. MAX256 EV Kit PCB Layout—Solder Side Figure 9. MAX256 EV Kit Component Placement Guide—
Solder Side
