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.