MAX13256ATB

19-5847; Rev 0; 6/11 EVALUATION KIT AVAILABLE MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies General Description The MAX13256 H-bridge transformer driver provides a simple solution for making isolated power supplies up to 10W. The device drives a transformer’s primary coil with up to 300mA of current from a wide 8V to 36V DC supply. The transformer’s secondary-to-primary winding ratio defines the output voltage, allowing selection of virtually any isolated output voltage. The device features adjustable current limiting, allowing indirect limiting of secondary-side load currents. The current limit of the MAX13256 is set by an external resistor. A FAULT output asserts when the device detects an overtemperature or overcurrent condition. In addition, the device features a low-power mode to reduce the overall supply current to 0.65mA (typ) when the driver is not in use. The device can be operated using the internal oscillator or driven by an external clock to synchronize multiple MAX13256 devices and precisely set the switching frequency. Internal circuitry guarantees a fixed 50% duty cycle to prevent DC current flow through the transformer, regardless of which clock source is used. The device is available in a small 10-pin (3mm x 3mm) TDFN package and is specified over the -40NC to +125NC automotive temperature range. Benefits and Features S Simple, Flexible Design  8V to 36V Supply Range  Up to 90% Efficiency  Provides Up to 10W to the Transformer  Undervoltage Lockout  2.5V to 5V Compatible Logic Interface  Internal or External Clock Source  Adjustable Overcurrent Threshold S Integrated System Protection  Fault Detection and Indication  Overcurrent Limiting  Overtemperature Protection S Saves Space on Board  Small 10-Pin TDFN Package (3mm x 3mm) Applications Power Meters Isolated Fieldbus Interfaces 24V PLC Supply Isolation Medical Equipment Motor Controls Ordering Information appears at end of data sheet. Typical Operating Circuit +24V 1µF VDD 4.6kI FAULT EN CLK ITH RLIM GND MAX13256 0.1µF ISOLATED VOUT ST1 ST2 For related parts and recommended products to use with this part, refer to: www.maxim-ic.com/MAX13256.related ����������������������������������������������������������������� Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies ABSOLUTE MAXIMUM RATINGS (Voltages referenced to GND.) VDD, FAULT ........................................................... -0.3V to +40V ST1, ST2 ................................................... -0.3V to (VDD + 0.3V) CLK, ITH, EN ........................................................... -0.3V to +6V FAULT Continuous Current ............................................. Q50mA ST1, ST2 Continuous Current ........................................ Q850mA Continuous Power Dissipation (TA = +70NC) TDFN (Four-Layer Board) (derate 24.4mW/NC above +70NC) ......................... 1951.2mW TDFN (Single-Layer Board) (derate 18.5mW/NC above +70NC) ......................... 1481.5mW Operating Temperature Range ........................ -40NC to +125NC Junction Temperature .....................................................+150NC Storage Temperature Range............................ -65NC to +150NC Lead Temperature (soldering, 10s) ...............................+300NC Soldering Temperature (reflow) ......................................+260NC 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. PACKAGE THERMAL CHARACTERISTICS (Note 1) TDFN (Four-Layer Board) Junction-to-Ambient Thermal Resistance (BJA) .......... 41NC/W Junction-to-Case Thermal Resistance (BJC) ................. 9NC/W TDFN (Single-Layer Board) Junction-to-Ambient Thermal Resistance (BJA) .......... 54NC/W Junction-to-Case Thermal Resistance (BJC) ................. 9NC/W Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. ELECTRICAL CHARACTERISTICS (VDD = 8V to 36V, VEN = 0V, TA = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER DC CHARACTERISTICS Supply Voltage Range Supply Current Disable Supply Current VDD IDD IDIS ROH Driver Output Resistance ROL Undervoltage-Lockout Threshold VUVLO (Note 3) VEN = 0V, VCLK = 0V, RLIM = 1000I, ST1/ST2 not connected VEN = 3.3V, VCLK = 0V ST1 = ST2 = high, IST1, ST2 = +300mA, RLIM = 1000I ST1 = ST2 = low, IST1, ST2 = -300mA, RLIM = 1000I VDD rising 5.9 8 6 0.65 1 0.6 6.3 300 36 9 1.1 1.5 I 1.0 6.9 V mV V mA mA SYMBOL CONDITIONS MIN TYP MAX UNITS Undervoltage-Lockout Threshold VUVLO_HYST Hysteresis ����������������������������������������������������������������� Maxim Integrated Products 2 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies ELECTRICAL CHARACTERISTICS (continued) (VDD = 8V to 36V, VEN = 0V, TA = -40NC to +125NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER ST1, ST2 Current Limit ST1, ST2 Leakage Current LOGIC SIGNALS (CLK, EN, FAULT) Input Logic-High Voltage Input Logic-Low Voltage Input Leakage Current FAULT Output Logic-Low Voltage FAULT Leakage Current AC CHARACTERISTICS Switching Frequency CLK Input Frequency ST1/ST2 Duty Cycle ST1/ST2 Rise Time ST1/ST2 Fall Time Crossover Dead Time Watchdog Timeout Current-Limit Blanking Time Current-Limit Autoretry Time PROTECTION Thermal-Shutdown Threshold Thermal-Shutdown Hysteresis TSHDN TSHDN_HYS +160 10 NC NC fSW fEXT DTC tRISE tFALL tDEAD tWDOG tBLANK tRETRY Figure 2 Figure 2 VCLK = 0V, measured at ST1/ST2 outputs External clocking Internal or external clocking ST1/ ST2 = 20% to 80% of VDD, RL = 1kI, CL = 50pF, Figure 1a ST1/ST2 = 80% to 20% of VDD, RL = 1kI, CL = 50pF, Figure 1a RL = 200I, Figure 1b 20 0.73 23.4 30 32 1.2 38.4 55 2.0 64.0 255 200 49 50 425 700 2000 51 100 100 kHz kHz % ns ns ns Fs ms ms VIH VIL IIL VOL ILKGF VCLK = VEN = 5.5V or 0V IFAULT = 10mA VFAULT = 36V, FAULT deasserted -1 2 0.8 +1 1 10 V V FA V FA SYMBOL ILIM ILKG RLIM = 1000I RLIM = 3010I VEN = 3.3V, VCLK = 0V, VST1 = VST2 = 0V or VDD CONDITIONS MIN 500 165 -1 TYP 650 215 MAX 800 265 +1 UNITS mA FA Note 2: All units are production tested at TA = +25NC. Specifications over temperature are guaranteed by design. Note 3: If VDD is greater than 27V, see the Snubber section. ����������������������������������������������������������������� Maxim Integrated Products 3 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Test Circuits/Timing Diagrams ST1/ST2 RL CL ST2 (A) VDD 80% ST1 20% 0V VDD ST2 0V (C) tRISE tFALL ST1 RL (B) 80% 20% tDEAD Figure 1. Test Circuits (A and B) and Timing Diagram (C) for Rise, Fall, and Dead Times ILIM IST1, ST2 0mA 50% tBLANK 50% tRETRY 50% Figure 2. Timing Diagram for Current Limiting ����������������������������������������������������������������� Maxim Integrated Products 4 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Typical Operating Characteristics (VDD = 24V, TA = +25NC, unless otherwise noted.) SUPPLY CURRENT vs. EXTERNAL CLOCK FREQUENCY 9 8 7 fSW (kHz) IDD (mA) 6 5 4 3 2 1 0 200 500 800 1100 1400 1700 2000 EXTERNAL CLOCK FREQUENCY (kHz) MAX13256 toc01 ST1/ST2 SWITCHING FREQUENCY vs. TEMPERATURE MAX13256 toc02 CURRENT-LIMIT THRESHOLD vs. RLIM MAX13256 toc03 10 600 550 500 450 400 350 700 600 500 ILIM (mA) 400 300 200 NO LOAD 300 CLK = GND 100 -40 -25 -10 5 20 35 50 65 80 95 110 125 TA (°C) 1000 1400 1800 2200 2600 3000 RLIM (I) NORMALIZED CURRENT-LIMIT THRESHOLD vs. TEMPERATURE MAX13256 toc04 ST1/ST2 OUTPUT-VOLTAGE LOW vs. SINK CURRENT MAX13256 toc05 ST1/ST2 OUTPUT-VOLTAGE HIGH vs. SOURCE CURRENT 24.0 23.9 23.8 VOH (V) 23.7 23.6 23.5 23.4 23.3 23.2 23.1 MAX13256 toc06 1.10 1.08 1.06 1.04 1.02 ILIM 1.00 0.98 0.96 0.94 0.92 0.90 600 500 400 VOL (mV) 300 200 100 0 24.1 -40 -25 -10 5 20 35 50 65 80 95 110 125 TA (°C) 0 100 200 300 400 500 600 700 800 ISINK (mA) 0 100 200 300 400 500 600 700 800 ISOURCE (mA) FAULT OUTPUT-VOLTAGE LOW vs. SINK CURRENT MAX13256 toc07 ISOLATED OUTPUT VOLTAGE vs. LOAD CURRENT 29 28 27 VOUT (V) 26 25 24 23 22 21 20 350 300 VOL (mV) 250 200 150 100 50 0 0 2 4 6 8 VDD = 24V 1:1 TRANSFORMER FULL-WAVE RECTIFIER NO SNUBBER 10 0 100 200 300 400 500 ISINK (mA) ILOAD (mA) ����������������������������������������������������������������� Maxim Integrated Products 5 MAX13256 toc08 400 30 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Typical Operating Characteristics (continued) (VDD = 24V, TA = +25NC, unless otherwise noted.) ISOLATED OUTPUT VOLTAGE vs. LOAD CURRENT MAX13256 toc09 ISOLATED OUTPUT VOLTAGE vs. LOAD CURRENT MAX13256 toc10 EFFICIENCY vs. LOAD CURRENT 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 50 MAX13256 toc11 10 9 8 7 VOUT (V) 6 5 4 3 2 1 0 0 400 800 1200 1600 20 18 16 14 VOUT (V) 12 10 8 6 4 2 0 100 VDD = 24V 4:1 TRANSFORMER FULL-WAVE RECTIFIER NO SNUBBER VDD = 24V 4:1 TRANSFORMER VOLTAGE DOUBLER NO SNUBBER VDD = 12V VDD = 8V VDD = 16V VDD = 24V 1:1 TRANSFORMER FULL-WAVE RECTIFIER NO SNUBBER 100 150 200 250 300 350 400 ILOAD (mA) 2000 0 100 200 300 400 500 ILOAD (mA) ILOAD (mA) EFFICIENCY vs. LOAD CURRENT MAX13256 toc12 EFFICIENCY vs. LOAD CURRENT 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 EFFICIENCY vs. LOAD CURRENT MAX13256 toc13 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 VDD = 16V VDD = 24V VDD = 28V 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 VDD = 36V VDD = 36V VDD = 32V VDD = 28V VDD = 32V VDD = 8V VDD = 12V 4:1 TRANSFORMER FULL-WAVE RECTIFIER NO SNUBBER 250 500 750 ILOAD (mA) 1000 1250 1500 1:1 TRANSFORMER FULL-WAVE RECTIFIER WITH SNUBBER 0 50 100 150 200 250 300 350 400 ILOAD (mA) 4:1 TRANSFORMER FULL-WAVE RECTIFIER WITH SNUBBER 250 500 750 ILOAD (mA) 1000 1250 1500 MAXIMUM OUTPUT CURRENT vs. TEMPERATURE VDD = 16V 900 800 700 600 500 MAXIMUM OUTPUT CURRENT vs. TEMPERATURE MAX13256 toc15 VDD = 8V VDD = 16V 800 700 VDD = 8V IST1, ST2 (mA) VDD = 24V VDD = 36V IST1, ST2 (mA) VDD = 24V 600 500 400 VDD = 36V MULTILAYER BOARD 400 -40 -25 -10 5 20 35 50 65 80 95 110 125 TA (°C) 300 SINGLE-LAYER BOARD -40 -25 -10 5 20 35 50 65 80 95 110 125 TA (°C) ����������������������������������������������������������������� Maxim Integrated Products 6 MAX13256 toc16 1000 900 MAX13256 toc14 100 100 100 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Pin Configuration TOP VIEW ST1 10 GND ST2 9 8 GND FAULT 7 6 MAX13256 + 1 VDD 2 VDD 3 CLK 4 EN *EP 5 ITH TDFN *EXPOSED PAD—CONNECT TO GND Pin Description PIN 1, 2 3 4 5 6 7, 9 8 10 — NAME VDD CLK EN ITH FAULT GND ST2 ST1 EP FUNCTION Power Supply. Bypass VDD to ground with a 1FF capacitor as close as possible to the device. Clock Input. Connect CLK to GND to enable internal clocking. Apply a clock signal to CLK to enable external clocking. Enable Input. Drive EN low to enable the device. Drive EN high to disable the device. Overcurrent Threshold Adjustment Input. Connect a resistor (RLIM) from ITH to GND to set the overcurrent threshold for the ST1 and ST2 outputs. Do not exceed 10pF of capacitance to GND on ITH. Fault Open-Drain Output. The fault open-drain transistor turns on when there is either an overtemperature or overcurrent condition. Ground Transformer Drive Output 2 Transformer Drive Output 1 Exposed Pad. Internally connected to GND. Connect EP to a large ground plane to maximize thermal performance; not intended as an electrical connection point. ����������������������������������������������������������������� Maxim Integrated Products 7 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Functional Diagram VDD VDD MAX13256 UVLO P ST1 OSC VUVLO N MOSFET H-BRIDGE DRIVER CLK MUX FLIPFLOP VDD P WATCHDOG EN ITH FAULT CURRENT LIMIT ST2 N GND Detailed Description The MAX13256 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 MOSFETs to provide up to 300mA of current to the primary winding of a transformer. The device can be operated using the internal oscillator or driven by an external clock to synchronize multiple MAX13256 devices and control EMI behavior. 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 as long as the period of the clock is constant. The device operates from a wide single-supply voltage of 8V to 36V, and includes undervoltage lockout for controlled startup. The device features break-before-make switching to prevent cross conduction of the H-bridge MOSFETs. An external resistor sets an overcurrent limit, allowing primary-side limiting of load currents on the transformer’s secondary side. Thermal-shutdown circuitry provides additional protection against excessive power dissipation. The MAX13256 allows a versatile range of secondaryside rectification circuits (see Figure 3). The primary-tosecondary transformer winding ratio can be chosen to adjust the isolated output voltage. The device delivers up to 300mA of current to the transformer with a supply up to +36V. The MAX13256 provides the advantages of the H-bridge converter topology, including multiple isolated outputs, step-up/step-down or inverted output, relaxed filtering requirements, and low output ripple. Isolated Power Supply ����������������������������������������������������������������� Maxim Integrated Products 8 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Either the internal oscillator or an external clock provides the switching signal for the MAX13256. Connect CLK to ground to select the internal oscillator. Provide a clock signal to CLK to automatically select external clocking. Internal Oscillator Mode The MAX13256 includes an internal oscillator that drives the H-bridge when a watchdog timeout is detected on CLK. The outputs switch at 425kHz (typ) with a guaranteed 50% duty cycle in the internal oscillator mode. External Clock Mode The MAX13256 provides an external clock mode. When an external clock source is applied to CLK, the external clock drives the H-bridge. 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 device outputs switch at one-half of the external clock frequency. The device switches on the rising edge of the external clock signal. A stalled clock could cause excessive DC current to flow through the primary winding of the transformer. The MAX13256 features an internal watchdog circuit to prevent damage from this condition. The internal oscillator provides the switching signal to the H-bridge whenever the period between edges on CLK exceeds the watchdog timeout period of 20Fs (min). When the MAX13256 switches, there is a period of time when both ST1 and ST2 are high impedance to ensure that there are no shoot-through currents in the H-bridge. During this dead time, the voltage at these pins may temporarily exceed the absolute maximum ratings due to the inductive load presented by the transformer. This transient voltage will not damage the device. The MAX13256 provides a disable mode to reduce current consumption. The ST1 and ST2 outputs are high impedance in disable mode. Clock Source The MAX13256 provides an undervoltage-lockout feature to both ensure a controlled power-up state and prevent operation before the oscillator has stabilized. On powerup and during normal operation if the supply voltage drops below VUVLO, the undervoltage-lockout circuit forces the device into disable mode. The ST1 and ST2 outputs are high impedance in disable mode. The MAX13256 limits the ST1/ST2 output current. Connect an external resistor (RLIM) to ITH to set the current limit. When the current reaches the limit for longer than the blanking time of 1.2ms (typ), the drivers are disabled and FAULT is asserted low. The drivers are reenabled after the autoretry time of 38.4ms (typ). If a continuous fault condition is present, the duty cycle of the fault current is approximately 3%. To set the current-limit threshold, use the following equation: RLIM (I) = 650mV/IST1, ST2 (mA) where IST1, ST2 is the desired current threshold in the range of 215mA < IST1, ST2 < 650mA (typ). For example, a 1kI resistor sets the current limit to 650mA. Use a 1% resistor for RLIM for increased accuracy. Ensure that the overcurrent threshold is set to at least twice the expected maximum operating current. For an expected maximum operating current of 300mA, set the ILIM to 650mA. For an expected operating current of 100mA, set the ILIM to 215mA. The FAULT output is asserted low whenever the device is disabled due to a fault condition. FAULT is automatically deasserted when the device is enabled after the autoretry time following an overcurrent fault, resulting in FAULT toggling during a continuous overcurrent condition. FAULT is asserted for the entire duration of an overtemperature fault. FAULT is an open-drain output. The MAX13256 is protected from overtemperature damage by a thermal-shutdown circuit. When the junction temperature (TJ) exceeds +160NC, the device is disabled and FAULT is asserted low. FAULT stays low for the duration of an overtemperature fault. The device resumes normal operation when TJ falls below +150NC. Power-Up and Undervoltage Lockout Overcurrent Limiting Watchdog Transients on ST1/ST2 During tDEAD FAULT Output Disable Mode Thermal Shutdown ����������������������������������������������������������������� Maxim Integrated Products 9 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies N:1 CT + VIN - Applications Information + VOUT = 1/(2 x N) x VIN - VD - VD = DIODE FORWARD VOLTAGE FIGURE 3A. PUSH-PULL RECTIFICATION For VDD greater than 27V, use a simple RC snubber circuit on ST1 and ST2 to ensure that the peak voltage is less than 40V during switching (Figure 4). Recommended values for the snubber are R = 91I and C = 330pF. The power dissipation of the device is approximated by: Snubber Power Dissipation N:1 + VIN - PD = (ROHL x IPRI2) + (IDD x VDD) + VOUT = 2(VIN/N - VD) - where ROHL is the combined high-side and low-side onresistance of the internal FET drivers, and IPRI is the load current flowing through ST1 and ST2. When the MAX13256 is operated under high ambient temperatures, the power dissipated in the package can raise the junction temperature close to thermal shutdown. Under such temperature conditions, the power dissipation should be held low enough so that that junction temperature observes a factor of safety margin. The maximum junction temperature should be held below +140°C. Use the package’s thermal resistances to calculate the junction temperature. Alternatively use the Maximum Output Current vs. Temperature curves shown in the Typical Operating Characteristics section to determine the maximum ST1/ST2 load currents. If the MAX13256 is inserted into a live backplane, it is possible to damage the device. Damage is caused by overshoot on VDD exceeding the absolute maximum rating. Limit the transient input voltage to the MAX13256 with an external protection device. FIGURE 3B. VOLTAGE DOUBLER High-Temperature Operation N:1 + VIN - + VOUT = VIN/N - 2VD - FIGURE 3C. FULL-WAVE RECTIFIER Figure 3. Secondary-Side Rectification Topologies ST1 Hot Insertion ST2 R 91I C 330pF R 91I C 330pF Output-voltage ripple can be reduced with a lowpass LC filter (see Figure 5). The component values shown give a cutoff frequency of 21.5kHz by the equation: f3dB = 1 2π LC Output-Ripple Filtering Figure 4. Output Snubber L 25µH FILTER OUTPUT C 2.2µF Use an inductor with low DC resistance and sufficient saturation current rating to minimize filter power dissipation. Bypass VDD to ground with a 1FF ceramic capacitor as close as possible to the device. Power-Supply Decoupling Figure 5. Output Ripple Filtering ���������������������������������������������������������������� Maxim Integrated Products 10 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies For many applications, the unregulated output of the MAX13256 meets the output-voltage tolerances. This configuration represents the highest efficiency possible with the device. Output-Voltage Regulation connect a zener diode from the output node to ground as shown in Figure 6 to limit the output voltage to a safe value. Example 1: +24V to Isolated, Regulated +3.3V In Figure 6, the MAX13256 feeds approximately +4.4V to the input of an LDO through a TGMR-502V6LF 4:1 transformer and 4-diode bridge rectifier (see Figure 3C). From this, a MAX604 LDO produces a regulated +3.3V output at up to 500mA. Example 2: +24V to Isolated, Regulated +12V In the circuit of Figure 7, the MAX13256 feeds approximately +14.2V through a 1.5:1 transformer and a 4-diode bridge rectifier (see Figure 3C). From this, a MAX1659 LDO produces a regulated +12V output at up to 350mA. For applications requiring a regulated output voltage, Maxim provides several solutions. In the following examples, assume a tolerance of Q10% for the input voltage. When the load currents on the transformer’s secondary side are low, the output voltage can strongly increase. If operation under low load currents is expected, outputvoltage limiting should be used to keep the voltage within the tolerance range of the subsequent circuitry. If the minimum output load current is less than approximately 5mA, +24V 1µF VDD MAX13256 TGMR-502V6LF ST1 MBRS140 x 4 MAX604 10µF + 10µF +3.3V - EN CLK ST2 GND 4:1 Figure 6. +24V to Isolated, Regulated +3.3V +24V 1µF VDD ST1 EN CLK GND MAX13256 MBRS140 x 4 MAX1659 1.0µF ST2 1.5:1 + 10µF +12V - Figure 7. +24V to Isolated, Regulated +12V ���������������������������������������������������������������� Maxim Integrated Products 11 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Example 3: +24V to Isolated, Regulated ±15V In Figure 8, the MAX13256 is used with a 1:1.5 center tapped transformer and a 4-diode bridge rectifier network (see Figure 3C) to supply Q17.1V to a MAX8719 LDO and a 7915 LDO. The circuit produces regulated Q15V outputs at up to 100mA. The MAX13256 provides isolated power for data converters in industrial process control applications (see Figure 9). The 300mA output current capability allows for multiple data converters operating across an isolation barrier. The power output capability also supports circuitry for signal conditioning and multiplexing. Isolated DAC/ADC Interface for Industrial Process Control +24V 1µF 1:1.5 CT 4 x MBRS140 EN ST1 MAX13256 CLK GND ST2 0.1µF +15V MAX8719 R1 10µF R2 COMMON 0.1µF 7915 10µF -15V Figure 8. +24V to Isolated, Regulated ±15V ���������������������������������������������������������������� Maxim Integrated Products 12 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies VDD MAX13256 +15V COMMON VDD -15V RS485 MPU M U X DAC/ADC OPTOISOLATORS OPTOISOLATORS Figure 9. Isolated Power Supply for Industrial Control Applications ���������������������������������������������������������������� Maxim Integrated Products 13 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies The MAX13256 provides power for multiple transceivers in isolated RS-485/RS-232 data interface applications. The 300mA output current capability of the MAX13256 allows multiple RS-485/RS-232 transceivers to operate simultaneously. As with all power-supply circuits, careful PCB layout is important to achieve low switching losses and stable operation. For thermal performance, connect the exposed pad 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 MAX13256 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 CLK circuitry through PCB parasitic capacitance. This capacitive coupling can induce duty-cycle errors in the oscillator, resulting in a DC current through the transformer. To ensure proper operation, ensure that CLK has a solid ground connection. Ensure that the exposed pad has a solid connection to the ground plane for best thermal performance. Failure to provide a low thermal impedance path to the ground plane results in excessive junction temperatures when delivering maximum output power. Isolated RS-485/RS-232 Data Interfaces PCB Layout Guidelines Choose a transformer with sufficient ET product in the primary winding to ensure that the transformer does 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. This can cause the current limit to be reached even when the load is not high. For example, when the internal oscillator is used to drive the H-bridge, the required transformer ET product for an application with VDD(max) = 36V is 70.6VFs. An application with VDD(max) = 8.8V has a transformer ET product requirement of 17.3VFs. 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 (RPRI/N2 + RSEC) where RPRI is the DC winding resistance of the primary, and RSEC is the DC winding resistance of the secondary. In most cases, an optimum is reached when RSEC = RPRI/N2. For this condition, the power dissipation is equal for the primary and secondary windings. 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 input voltage, switching frequency, and 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 MAX13256; however, these are discouraged. If a net DC current results due to an imbalanced load, the average magnetic flux in the core is increased. This reduces the effective ET product and can lead to saturation of the transformer core. Exposed Pad Component Selection Transformer selection for the MAX13256 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 magnetizing current in the primary winding changes linearly with time during the switching period of the device. Transformer manufacturers specify a minimum ET product for each transformer. The transformer’s ET product must be larger than: ET = VDD/(2 x fSW) where fSW is the minimum switching frequency of the ST1/ST2 outputs (255kHz (min)) when the internal oscillator is used or one-half of the clock frequency when an external clock source is used. Transformer Selection ���������������������������������������������������������������� Maxim Integrated Products 14 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Transformers for use with the device 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. The MAX13256 can be operated from an +8V supply by decreasing the turns ratio of the transformer, or by designing a voltage doubler circuit as shown in Figure 3B. 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. Low-Voltage Operation Capacitor Selection Input Bypass Capacitor Bypass the supply pin to GND with a 1FF ceramic capacitor as close as possible to the device. The equivalent series resistance (ESR) of the input capacitors is not as critical as for the output filter capacitors. Typically ceramic X7R capacitors are adequate. Output Filter Capacitor In most applications, the actual capacitance rating of the output filter capacitors 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 0NC to help reduce output-voltage ripple in sensitive applications. In applications where low outputvoltage ripple is not critical, standard ceramic 0.1FF capacitors are sufficient. Optimum performance at +8V is obtained with fewer turns on the primary winding since the ET product requirement is lower than for a +24V supply. However, any of the transformers for use with a +24V supply can operate properly with a +8V supply. For a given power level, the transformer currents are higher with a +8V supply than with a +24V supply. Therefore, the DC resistance of the transformer windings has a larger impact on the circuit efficiency. The high switching speed of the MAX13256 necessitates high-speed rectifiers. Ordinary silicon signal diodes such as 1N914 or 1N4148 can be used for low-output current levels (less than 50mA.) But at higher output current levels, their reverse recovery times might degrade efficiency. At higher output currents, select low forwardvoltage Schottky diodes to improve efficiency. Ensure Diode Selection Suggested External Component Manufacturers Table 1. 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 ���������������������������������������������������������������� Maxim Integrated Products 15 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Ordering Information PART MAX13256ATB+ TEMP RANGE -40NC to +125NC PIN-PACKAGE 10 TDFN-EP* Package Information For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE 10 TDFN-EP PACKAGE CODE T1033+1 OUTLINE NO. 21-0137 LAND PATTERN NO. 90-0003 +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed Pad Chip Information PROCESS: BiCMOS ���������������������������������������������������������������� Maxim Integrated Products 16 MAX13256 36V H-Bridge Transformer Driver for Isolated Supplies Revision History REVISION NUMBER 0 REVISION DATE 6/11 Initial release DESCRIPTION PAGES CHANGED — 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. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. 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