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19-3748; Rev 0; 8/05
KIT ATION EVALU ABLE AVAIL
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 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 (-40C to +125C) 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 -40C to +125C 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 4.7F 470nF
Pin Configuration
MAX256
MODE
ST1
1:2.6CT
+5V ISOLATED 0.1F
CK_RS VCC VCC
1 2 3
8
ST1 GND GND ST2
MAX256
7 6
CK_RS 47k 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 VCC..................................................-0.3V to +6V ST1, ST2, CK_RS, MODE (Note 1)................-0.3V to VCC + 0.3V ST1, ST2 Maximum Continuous Current (TA < +125C) ....0.6A ST1, ST2 Maximum Continuous Current (TA < +100C) ....0.9A ST1, ST2 Maximum Continuous Current (TA < +85C) ......1.0A Continuous Power Dissipation (TA = +70C) 8-Pin SO (derate 18.9mW/C above +70C)..............1509mW Operating Temperature Range .........................-40C to +125C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C Lead Temperature (soldering, 10s) .................................+300C
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 = +25C, 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 VCC ICC ISD RS ROHL VUVLO VUVLO_HST VIL VIH ILK RS_INT TSHDN TSHDN_HST MODE = GND 165 165 10 VCC = 4.5V VCC = 3.0V 2.0 1 VCC = 4.5V (Note 3) VCC = 3.0V (Note 3) VCC rising 0.8 MODE = VCC, 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 A k V mV V V A k C C
2
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies
TIMING CHARACTERISTICS
(VCC = +3.0V to +5.5V, TA = TMIN to TMAX. Typical values are at VCC = +5.0V and TA = +25C, unless otherwise noted.)
PARAMETER Switching Frequency CK_RS Input Frequency ST1 and ST2 Duty Cycle Crossover Dead Time Watchdog Timeout SYMBOL fSW fIN Dtc tDEAD tWDOG CONDITIONS MODE = VCC, RS = 10.5k MODE = VCC, CK_RS unconnected MODE = GND MODE = VCC RL = 100 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 s
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 ROH is the high-side resistance, and ROL is the low-side resistance, ROHL = ROH + ROL.
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 10k 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. VCC Supply Voltage, +3.0V VCC +5.5V. Bypass VCC to ground with a 4.7F 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
VCC MODE ST2 GND ST1 EP
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
Typical Operating Characteristics
(VCC = +5.0V 10%, TA = +25C, unless otherwise noted.) (See Figure 8A)
SUPPLY CURRENT vs. OSCILLATOR FREQUENCY
MAX256toc01
OSCILLATOR FREQUENCY vs. RS (+1%)
1200 OSCILLATOR FREQUENCY (kHz) 1000 800 MAX 600 400 200 0 MIN 10 10 100 RS (k) 1000 1 TYP RS (k)
MAX256 toc02
RS 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 (Vs)
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
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
Typical Operating Characteristics (continued)
(VCC = +5.0V 10%, TA = +25C, 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
1s/div
100ns/div
Functional Diagram
VCC
THERMAL SHUTDOWN
VCC
UVLO OSC
VUVLO ST1
MOSFET H-BRIDGE DRIVER CK_RS 165k
ST2
M U X
FLIPFLOP
VCC
MODE
WATCHDOG
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
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. 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 55s (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 55s after MODE goes low. When operating in external clock mode, suspend the clock signal for longer than 55s 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.
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.
Thermal Shutdown
The MAX256 is protected from overtemperature damage by a thermal shutdown circuit. When the junction temperature (TJ) exceeds +165C, the device is disabled. The device resumes normal operation when TJ falls below +155C.
Internal Oscillator Mode
The MAX256 includes a 100kHz to 1MHz programmable oscillator. Set the oscillator frequency by connecting CK_RS to ground with a 10k 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 165k resistor.
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.
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.
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
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies
Table 1. Oscillator Modes
OSCILLATOR MODE Internal Programmable Frequency CK_RS Unconnected or pulled to ground by RS. RS must be greater than 10k. MODE OPERATION 100kHz to 1MHz (typ). Leave CK_RS unconnected for minimum switching frequency. CK_RS is pulled to ground by an internal 165k resistor. The device switches at one half the external clock frequency. The device is disabled after a maximum of 55s following the last rising edge on CK_RS.
MAX256
VCC
External Clock
Digital input. Drive CK_RS with an external clock signal. Connected to VCC or GND (external clock mode)
Ground
Disable Unconnected or pulled to ground with RS (internal clock mode)
Ground
Applications Information
Available Output Power
With a supply voltage of +5V over the extended -40C to +85C 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 +85C to +125C 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: PD = ROHL x 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.
+ VIN
-
1:N CT + VOUT = N / 2 * VIN - VD
-
VD = DIODE FORWARD VOLTAGE FIGURE 1A. PUSH-PULL RECTIFICATION
1:N + VIN
-
+ VOUT = 2(NVIN - VD)
-
FIGURE 1B. VOLTAGE DOUBLER
1:N + VIN
-
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
+ 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
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
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.
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.
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. +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.
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 (-40C to +85C). The MAX256 produces up to 2.5W over the +85C to +125C temperature range. For a supply voltage of +3.3V, the MAX256 delivers 2W of power to the transformer over the -40C to +85C temperature range, and 1.4W between +85C and +125C. 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 0.47F ceramic capacitor as close to the device as possible. Additionally, place a 4.7F 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.
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
+3.3V 4.7F 470nF VCC
TGM-H281NF ST1
MBRS140 x 4
MAX256
MODE
MAX1659
15V
CK_RS 300k
0.1F ST2 GND
10F
+ 5V -
Figure 2. +3.3V to Isolated Regulated +5V
+5V 4.7F 470nF VCC TGM-H281NF ST1
MBRS140 0.1F
15V
MAX256
MODE
MAX1658
10F
+ 3.3V -
CK_RS 100k
ST2 GND
MBRS140
Figure 3. +5V to Isolated Regulated +3.3V
+5V 4.7F 470nF VCC TGM-H281NF ST1 0.1F
MBRS140 0.1F
MAX256
MODE
7812
10F
+ 12V -
CK_RS 100k
ST2 GND
MBRS140
Figure 4. +5V to Isolated Regulated +12V _______________________________________________________________________________________ 9
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
MBRS140
TGM-280NS +5V 0.1F
+15V
7815
10F
4.7F
470nF
0.1F VCC
MODE ST1 MBRS140 COMMON ST2 GND TGM-280NS 0.1F MBRS140
MAX256
CK_RS 47k
7815
0.1F 10F
-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 x 1 2 x fSW
When CK_RS is unconnected, the internal oscillator is programmed for the minimum frequency. The default required ET product for the MAX256 is 42.3Vs, (assuming +5.5V maximum VCC), or 27.7Vs 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 x 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.
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.
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
VCC +15V
MAX256
COMMON
-15V VCC
RS485 MPU M U X
DAC/ADC
OPTOISOLATORS Figure 6. Isolated Power Supply for Process Control Applications
OPTOISOLATORS
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.
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
+5V
L1 25H
FILTER OUTPUT C1 2.2F
4.7F
470nF
MAX256
MODE
ST1
1:1.75 0.1F
+5V ISOLATED 0.1F
CK_RS
ST2 0.1F
Figure 7. Output Ripple Filter
47k
GND 0.1F
-15V ISOLATED ALL DIODES MBRS140
+5V 4.7F 470nF
Figure 8c. +5V to Isolated 15V
MAX256
MODE ST2 ST1 1:2.6CT +5V ISOLATED 0.1F
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.
CK_RS 47k
GND
Figure 8a. +5V to Isolated +5V
+3.3V 4.7F 470nF
Capacitor Selection
ST1 1:2 +5V ISOLATED ST2 0.1F ALL DIODES MBRS140
MAX256
MODE
CK_RS 47k
GND
Input Bypass Capacitor Bypass the supply voltage to GND with a 0.47F ceramic capacitor as close to the device as possible. Additionally, connect a 4.7F 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 0C to help reduce output voltage ripple in sensitive applications. In applications where low outputvoltage ripple is not critical, standard ceramic 0.1F capacitors are sufficient.
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.
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3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies
Table 2. Suggested External Component Manufacturers
MANUFACTURER Central Semiconductor Halo Electronics Kemet Sanyo Taiyo Yuden TDK COMPONENT diodes transformers capacitors capacitors capacitors capacitors WEBSITE www.centralsemi.com www.haloelectronics.com www.kemet.com www.sanyo.com www.t-yuden.com www.component.tdk.com PHONE 631-435-1110 650-903-3800 864-963-6300 619-661-6835 408-573-4150 888-835-6646
MAX256
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 = 1 2 LC PROCESS: BiCMOS
Chip Information
SUBSTRATE CONNECTED TO GND
Use an inductor with low DC resistance and sufficient saturation current rating to minimize filter power dissipation.
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13
3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies MAX256
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
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 (c) 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
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