Maxim MAX256 3w primary-side transformer h-bridge driver for isolated supply Datasheet

19-3748; Rev 0; 8/05
KIT
ATION
EVALU
E
L
B
A
AVAIL
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
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
Isolated Power Supplies
Industrial Process
Control
Isolated Communications
Links
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
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
PKG
CODE
MAX256ASA
-40°C to +125°C
8 SO-EP*
S8E-12
*EP = Exposed paddle.
Medical Equipment
Telecommunications
Pin Configuration
Typical Application Circuit
+5V
4.7µF
470nF
MAX256
ST1
1:2.6CT
0.1µF
MODE
ST2
CK_RS
+5V ISOLATED
CK_RS
1
VCC
2
VCC
3
MODE 4
47kΩ
GND
MAX256
EP*
8
ST1
7
GND
6
GND
5
ST2
SO-EP
*EXPOSED PAD IS CONNECTED TO GND
+5V TO ISOLATED +5V TYPICAL APPLICATION
________________________________________________________________ Maxim Integrated Products
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.
1
MAX256
General Description
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) ......±1.0A
Continuous Power Dissipation (TA = +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
(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
Supply Voltage
VCC
Supply Current
ICC
MODE = VCC, CK_RS unconnected (Note 2)
Disable Supply Current
ISD
MODE = GND,
CK_RS unconnected
External Resistance Range
RS
Driver Total Resistance
ROHL
Undervoltage Lockout Threshold
VUVLO
Undervoltage-Lockout-Threshold
Hysteresis
VUVLO_HST
Logic-Low Level
(MODE, CK_RS)
VIL
Logic-High Level
(MODE, CK_RS)
VIH
Input Leakage Current
(MODE)
ILK
Internal Pulldown Resistance on
CK_RS
Thermal Shutdown
Thermal Shutdown Hysteresis
2
MIN
TYP
3.0
1.06
MAX
5.5
V
3
mA
50
µA
10
kΩ
VCC = 4.5V (Note 3)
0.5
1.0
VCC = 3.0V (Note 3)
0.6
1.2
1.9
2.7
VCC rising
0.8
110
Ω
V
mV
VCC = 4.5V
0.8
VCC = 3.0V
0.7
2.0
V
V
1
µA
165
kΩ
TSHDN
165
°C
TSHDN_HST
10
°C
RS_INT
MODE = GND
UNITS
_______________________________________________________________________________________
3W 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
Switching Frequency
fSW
CONDITIONS
MIN
TYP
MAX
UNITS
0.75
1
1.35
MHz
MODE = VCC, CK_RS unconnected
65
100
160
kHz
2
MHz
50
51
MODE = VCC, RS = 10.5kΩ
CK_RS Input Frequency
fIN
MODE = GND
0.2
ST1 and ST2 Duty Cycle
Dtc
MODE = VCC
49
Crossover Dead Time
tDEAD
RL = 100Ω
Watchdog Timeout
tWDOG
MODE = GND
20
20
%
ns
55
µs
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 ROH is the high-side resistance,
and ROL is the low-side resistance, ROHL = ROH + ROL.
Pin Description
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.
2, 3
VCC
4
MODE
VCC Supply Voltage, +3.0V ≤ VCC ≤ +5.5V.
Bypass VCC 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.
5
ST2
Transformer Drive Output 2
6, 7
GND
Ground
8
ST1
Transformer Drive Output 1
EP
EP
Exposed Paddle. Connect the exposed paddle to ground.
_______________________________________________________________________________________
3
MAX256
TIMING CHARACTERISTICS
Typical Operating Characteristics
(VCC = +5.0V ±10%, TA = +25°C, unless otherwise noted.) (See Figure 8A)
SUPPLY CURRENT vs.
OSCILLATOR FREQUENCY
3
2
1
1000
+3.6V MAX SUPPLY
800
MAX
600
TYP
400
+5.5V MAX SUPPLY
200
0
MIN
10
0
100 200 300 400 500 600 700 800 900 1000
OSCILLATOR FREQUENCY (kHz)
100
RS (kΩ)
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8A)
EFFICIENCY vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8A)
10
0.8
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
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(TYPICAL APPLICATION FIGURE 8B)
5.5V
0.9
EFFICIENCY
8
0.2
2
100
10
REQUIRED ET PRODUCT (Vµs)
MAX256 toc05
10
1
1000
1.0
MAX256 toc04
12
100
MAX256 toc06
4
MAX256toc03
1200
RS (kΩ)
5
1000
MAX256 toc02
MAX256toc01
1400
OSCILLATOR FREQUENCY (kHz)
SUPPLY CURRENT (mA)
6
OUTPUT VOLTAGE (V)
RS vs. REQUIRED ET PRODUCT
OSCILLATOR FREQUENCY vs. RS (+1%)
7
8
3.6V
6
4
3.0V
3.3V
2
0.1
0
0
800
0
0.7
0.6
35
OUTPUT VOLTAGE (V)
3.3V
0.8
40
MAX256 toc07
3.6V
0.9
3.0V
0.5
0.4
0.3
0.2
100
200
300
400
OUTPUT CURRENT (mA)
1.0
5.5V
0.9
0.8
0.7
25
4.5V
5.0V
5.5V
20
500
EFFICIENCY vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8C)
30
4.5V
5.0V
0.6
0.5
0.4
0.3
0.2
15
0.1
0.1
0
10
0
0
4
0
800
OUTPUT VOLTAGE vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8C)
EFFICIENCY vs. OUTPUT CURRENT
(CIRCUIT OF FIGURE 8B)
1.0
200
400
600
OUTPUT CURRENT (mA)
EFFICIENCY
200
400
600
OUTPUT CURRENT (mA)
MAX256 toc08
0
MAX256 toc09
0
EFFICIENCY
MAX256
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
100
200
300
400
OUTPUT CURRENT (mA)
500
0
20
40
60
80 100
OUTPUT CURRENT (mA)
120
140
0
20
40
60
80 100
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
120
140
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
OPERATION WITH
EXTERNAL 2MHz CLOCK
MAX256toc10
MAX256toc11
OPERATION AT 100kHz
CK_RS
5V/div
CK_RS
5V/div
ST1
5V/div
ST1
5V/div
ST2
5V/div
ST2
5V/div
100ns/div
1µs/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 Characteristics (continued)
(VCC = +5.0V ±10%, TA = +25°C, unless otherwise noted.) (See Figure 8A)
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. 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.
Oscillator Modes
The MAX256 is driven by the internal programmable
oscillator or an external clock. The logic state of MODE
determines the clock source (see Table 1). Drive
MODE high to select the internal resistor programmable
oscillator. Drive MODE low to operate the MAX256 with
an external clock signal on CK_RS.
Internal Oscillator Mode
The MAX256 includes a 100kHz to 1MHz programmable oscillator. Set the oscillator frequency by connecting CK_RS to ground with a 10kΩ or larger resistor.
Leave CK_RS unconnected to set the oscillator to the
minimum default frequency of 100kHz. CK_RS is internally pulled to ground with a 165kΩ resistor.
External Clock Mode
The MAX256 provides an external clock mode. When
operating in external clock mode, an internal flip-flop
divides the external clock by two in order to generate a
switching signal with a guaranteed 50% duty cycle. As
a result, the MAX256 outputs switch at one half the
external clock frequency. The device switches on the
rising edge of the external clock signal.
flow through the primary winding of the transformer.
The MAX256 features an internal watchdog circuit to
prevent damage from this condition. The MAX256 is
disabled when the external clock signal on CK_RS
remains at the same logic level for longer than 55µs
(max). The device resumes normal operation upon the
next rising edge on CK_RS.
Disable Mode
When using the internal oscillator, drive MODE low to
disable the MAX256. The device is disabled within
55µs after MODE goes low. When operating in external
clock mode, suspend the clock signal for longer than
55µs to disable the MAX256. The device resumes normal operation when MODE is driven high or when the
external clock signal resumes.
Power-Up and Undervoltage Lockout
The MAX256 provides an undervoltage lockout feature
to ensure a controlled power-up state and prevent
operation before the oscillator has stabilized. On
power-up and during normal operation (if the supply
voltage drops below 1.8V), the undervoltage lockout
disables the device.
Thermal Shutdown
The MAX256 is protected from overtemperature damage by a thermal shutdown circuit. When the junction
temperature (TJ) exceeds +165°C, the device is disabled. The device resumes normal operation when TJ
falls below +155°C.
ESD Protection
As with all Maxim devices, ESD-protection structures
are incorporated on all pins to protect against electrostatic discharges encountered during handling and
assembly.
ESD Test Conditions
ESD performance depends on a variety of conditions.
Please contact Maxim for a reliability report documenting test setup, methodology, and results.
Watchdog
When the MAX256 is operating in external clock mode,
a stalled clock could cause excessive DC current to
6
_______________________________________________________________________________________
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
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.
Ground
The device is disabled after a maximum of
55µs following the last rising edge on
CK_RS.
OPERATION
100kHz to 1MHz (typ).
Leave CK_RS unconnected for minimum
switching frequency.
Connected to VCC or GND
(external clock mode)
Disable
Unconnected or pulled to ground with RS
(internal clock mode)
Applications Information
1:N CT
Available Output Power
+
With a supply voltage of +5V over the extended -40°C to
+85°C temperature range, the MAX256 is specified to
provide up to 3W of power to the primary side of a transformer in an isolated power supply. The device provides
up to 2.5W of power to the primary winding over the
+85°C to +125°C temperature range. The output power
is specified at ST1 and ST2 since losses in the transformer and rectification network are dependent upon
component selection and topology. The power dissipation of the MAX256 is approximated by:
VIN
PD = ROHL × IPRI2
where ROHL is the total high-side and low-side on-resistance of the internal FET drivers, and IPRI is the load
current flowing through the transformer primary between
ST1 and ST2. For low output load currents, include the
contribution to PD from the quiescent supply current:
ICC x VCC.
PC Board Layout Guidelines
As with all power-supply circuits, careful PC board layout is important to achieve low switching losses and stable operation. For thermal performance, connect the
exposed paddle to a solid copper ground plane.
The traces from ST1 and ST2 to the transformer must be
low-resistance and inductance paths. Place the transformer as close as possible to the MAX256 using short,
wide traces.
When the device is operating with the internal oscillator,
it is possible for high-frequency switching components
on ST1 and ST2 to couple into the CK_RS circuitry
through PC board parasitic capacitance. This capacitive
MAX256
Table 1. Oscillator Modes
+
VOUT = N / 2 * VIN - VD
-
-
VD = DIODE FORWARD VOLTAGE
FIGURE 1A. PUSH-PULL RECTIFICATION
1:N
+
+
VOUT = 2(NVIN - VD)
VIN
-
-
FIGURE 1B. VOLTAGE DOUBLER
1:N
+
VIN
-
+
VOUT = NVIN - 2VD
-
FIGURE 1C. FULL-WAVE RECTIFIER
Figure 1. Secondary-Side Rectification Topologies
coupling can induce duty-cycle errors in the oscillator,
resulting in a DC current through the transformer. To
ensure proper operation, shield the CK_RS circuitry
from ST1 and ST2 by placing a grounded trace between
_______________________________________________________________________________________
7
MAX256
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
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.
For applications requiring a regulated output voltage,
Maxim provides several solutions. In the following
examples, assume a tolerance of ±10% variation for the
input voltage.
When a full-bridge power supply is operated under
maximum input voltage and low output load current, the
voltage at the output of the rectifier network can exceed
the absolute maximum input voltage of the low dropout
regulator (LDO). If the minimum output load current is
less than approximately 5mA, connect a zener diode
from the output voltage to ground (as shown in Figure
2) to limit the output to a safe value.
+3.3V to Isolated, Regulated +5.0V
In the circuit of Figure 2, the MAX1659 LDO regulates
the output of the MAX256 to +5V. The Halo TGMH281NF provides a center-tapped 1:2.6 turns ratio, and
the secondary circuit implements a 4-diode bridge rectifier (Figure 1C).
For a minimum input voltage of +3.0V, the output voltage of the bridge rectifier is approximately +5.5V at a
current of 200mA. A 15V zener diode protects the LDO
from high input voltages, but adds a few microamps to
the no-load input current of the MAX256.
+5V to Isolated, Regulated +3.3V
In Figure 3, the MAX1658 LDO is used with the TGMH281NF transformer and a 2-diode push-pull rectifier
(Figure 1A). This topology produces approximately
+4.5V at a current of 350mA. The MAX1658 produces a
regulated +3.3V output voltage.
+5V to Isolated, Regulated +12V
In Figure 4, the 7812 LDO is used with the TGMH281NF transformer and the voltage doubler network
(Figure 1B). This circuit produces approximately
+12.5V at a load current of 150mA. The 7812 produces
a regulated +12V output.
Isolated DAC/ADC Interface for Industrial
Process Control
The MAX256 provides isolated power for data converters in industrial process control applications (Figure 6).
The 3W isolated power output capability allows for data
converters operating across multiple isolation barriers.
The power output capability also supports circuitry for
signal conditioning and multiplexing.
Isolated RS-485/RS-232 Data Interfaces
The MAX256 provides power for multiple transceivers in
isolated RS-485/RS-232 data interface applications. The
3W isolated power output capability of the MAX256
allows more than ten RS-485 transceivers simultaneously.
Isolated Power Supply
The MAX256 allows a versatile range of secondary-side
rectification circuits (see Figure 1). The secondary
transformer winding can be wound to provide a wide
range of isolated voltages. The MAX256 delivers 3W of
power to the transformer with a +5V supply (-40°C to
+85°C). The MAX256 produces up to 2.5W over the
+85°C to +125°C temperature range. For a supply voltage of +3.3V, the MAX256 delivers 2W of power to the
transformer over the -40°C to +85°C temperature
range, and 1.4W between +85°C and +125°C. Figure
8A shows a +5V to isolated +5V application that delivers up to 500mA. In Figure 8B, the MAX256 is configured to provide +5V from a +3.3V supply at 350mA,
and in Figure 8C, the MAX256 provides isolated +15V
and -15V at a total current up to 75mA.
The MAX256 provides the advantages of the full-bridge
converter topology, including multiple isolated outputs,
step-up/step-down or inverted output, relaxed filtering
requirements, and low output ripple.
Power-Supply Decoupling
Bypass VCC to ground with a 0.47µF ceramic capacitor
as close to the device as possible. Additionally, place a
4.7µF capacitor from VCC to ground.
Exposed Paddle
Ensure that the exposed paddle is soldered to the bottom layer ground for best thermal performance. Failure
to provide a low thermal impedance path to the ground
plane will result in excessive junction temperatures
when delivering maximum output power.
+5V to Isolated, Regulated ±15V
In Figure 5, the MAX256 is used with two TGM-280NS
transformers and voltage doubler networks (Figure 1B)
to supply 20V to a pair of 7815 regulators. The circuit
produces a regulated ±15V at 50mA.
8
_______________________________________________________________________________________
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
0.1µF
MODE
CK_RS
100kΩ
ST2
7812
10µF
+
12V
-
MBRS140
GND
Figure 4. +5V to Isolated Regulated +12V
_______________________________________________________________________________________
9
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. The ET
product relates the maximum allowable magnetic flux
density in a transformer core to the voltage across a
winding and switching period. Inductor current in the
primary linearly increases with time in the operating
region of the MAX256. Transformer manufacturers
specify a minimum ET product for each transformer. For
the MAX256, the requirement on ET product is calculated as:
ET = VCC ×
1
2 × fSW
By choosing a transformer with sufficient ET product in
the primary winding, it is ensured that the transformer
will not saturate during operation. Saturation of the
magnetic core results in significantly reduced inductance of the primary, and therefore a large increase in
current flow. Excessive transformer current results in a
temperature rise and possible damage to the transformer and/or the MAX256.
10
When CK_RS is unconnected, the internal oscillator is
programmed for the minimum frequency. The default
required ET product for the MAX256 is 42.3Vµs, (assuming +5.5V maximum VCC), or 27.7Vµs for +3.3V operation (assuming +3.6V maximum VCC). Both of these ET
products assume the minimum oscillator frequency of
65kHz. See the Typical Operating Characteristics plot,
RS vs. Required ET Product to determine the required
ET product for a given value of RS.
In addition to the constraint on ET product, choose a
transformer with a low DC-winding resistance. Power
dissipation of the transformer due to the copper loss is
approximated as:
PD _ TX = ILOAD2 × ⎛⎝ N2 RPRI + RSEC ⎞⎠
where RPRI is the DC-winding resistance of the primary,
and R SEC is the DC-winding resistance of the secondary. In most cases, an optimum is reached when:
RSEC = N2 RPRI
For this condition, the power dissipation is equal for the
primary and secondary windings.
______________________________________________________________________________________
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. Transformer losses produce a temperature rise that reduces the efficiency of the transformer. The lower efficiency, in turn,
produces an even larger temperature rise.
To ensure that the transformer meets these requirements under all operating conditions, the design should
focus on the worst-case conditions. The most stringent
demands on ET product arise for minimum switching
frequency, maximum input voltage, maximum temperature, and load current. Additionally, the worst-case values for transformer and rectifier losses should be
considered.
The primary should be a single winding; however, the
secondary can be center-tapped, depending on the
desired rectifier topology. In most applications, the
phasing between primary and secondary windings is
not significant. Half-wave rectification architectures are
possible with the MAX256; however, these are discouraged. If a net DC current results due to an imbalanced
load, the magnetic flux in the core is increased. This
reduces the effective ET product and can lead to saturation of the transformer core.
Transformers for use with the MAX256 are typically
wound on a high-permeability magnetic core. To minimize radiated electromagnetic emissions, select a
toroid, pot core, E/I/U core, or equivalent.
+3.3V Operation
The MAX256 can be operated from a +3.3V supply by
increasing the turns ratio of the transformer, or by
designing a voltage-doubler or voltage-tripler circuit as
shown in Figure 1B.
Optimum performance at +3.3V is obtained with fewer
turns on the primary winding, since the ET product
is lower than for a +5V supply. However, any of the
transformers for use with a +5V supply will operate
properly with a +3.3V supply. For a given power level,
the transformer currents are higher with a +3.3V supply
than with a +5V supply. Therefore, the DC resistance
of the transformer windings has a larger impact on the
circuit efficiency.
______________________________________________________________________________________
11
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
0.1µF
Figure 7. Output Ripple Filter
47kΩ
GND
0.1µF
-15V ISOLATED
ALL DIODES
MBRS140
+5V
4.7µF
Figure 8c. +5V to Isolated ±15V
470nF
MAX256
ST1
1:2.6CT
+5V ISOLATED
0.1µF
MODE
ST2
CK_RS
47kΩ
GND
Figure 8a. +5V to Isolated +5V
Capacitor Selection
+3.3V
4.7µF
470nF
MAX256
ST1
47kΩ
1:2
+5V
ISOLATED
MODE
ST2
CK_RS
GND
0.1µF
ALL DIODES
MBRS140
Figure 8b. +3.3V to Isolated +5V
Low-Power Applications and Multiple Transformers
For more information about transformer selection, please
refer to the MAX3535E data sheet. The MAX3535E uses a
transformer in a similar topology. See Tables 3, 4, and 5
in the MAX3535E data sheet for a list of commercially
available transformers. These transformers are preferred
for lower power applications and are suitable for use with
the MAX256 up to the power limits of the transformers.
Alternatively, the MAX256 can drive the primaries of two
or more low-power transformers to provide multiple isolated outputs. One or more of the manufacturers listed in the
MAX3535E data sheet may produce a custom transformer for specific applications. Contact the individual
transformer suppliers for details.
12
Diode Selection
The high switching speed of the MAX256 necessitates
high-speed rectifiers. Ordinary silicon signal diodes
such as 1N914 or 1N4148 may be used for low-output
current levels (less than 50mA). At higher output currents, select low forward-voltage Schottky diodes to
improve efficiency. Ensure that the average forward
current rating for the rectifier diodes exceeds the maximum load current of the circuit. For surface-mount
applications, Schottky diodes such as the BAT54,
MBRS140 and MBRS340 are recommended.
Input Bypass Capacitor
Bypass the supply voltage to GND with a 0.47µF
ceramic capacitor as close to the device as possible.
Additionally, connect a 4.7µF or greater capacitor to
provide input voltage filtering. The equivalent series
resistance (ESR) of the input capacitors is not as critical as for the output capacitors. Typically, ceramic X7R
capacitors are adequate.
Output Filter Capacitor
In most applications, the actual capacitance rating of the
output filter capacitor is less critical than the capacitor's
ESR. In applications sensitive to output voltage ripple,
the output filter capacitor must have low ESR. For optimal
performance, the capacitance should meet or exceed
the specified value over the entire operating temperature
range. Capacitor ESR typically rises at low temperatures;
however, OS-CON capacitors can be used at temperatures below 0°C to help reduce output voltage ripple in
sensitive applications. In applications where low outputvoltage ripple is not critical, standard ceramic 0.1µF
capacitors are sufficient.
______________________________________________________________________________________
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
MANUFACTURER
COMPONENT
Central Semiconductor
diodes
Halo Electronics
transformers
Kemet
capacitors
WEBSITE
PHONE
www.centralsemi.com
631-435-1110
www.haloelectronics.com
650-903-3800
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.com
888-835-6646
Output-Ripple Filtering
Output voltage ripple can be reduced with a lowpass
LC pi-filter (Figure 7). The component values shown
give a cutoff frequency of 21.5kHz by the equation:
f3dB =
MAX256
Table 2. Suggested External Component Manufacturers
Chip Information
PROCESS: BiCMOS
SUBSTRATE CONNECTED TO GND
1
2π LC
Use an inductor with low DC resistance and sufficient saturation current rating to minimize filter power dissipation.
______________________________________________________________________________________
13
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
8L, SOIC EXP. PAD.EPS
MAX256
3W Primary-Side Transformer H-Bridge Driver
for Isolated Supplies
PACKAGE OUTLINE
8L SOIC, .150" EXPOSED PAD
21-0111
C
1
1
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2005 Maxim Integrated Products
Campillo
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
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