19-2830; Rev 0; 4/03 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies The MAX8540/MAX8541 pulse-width-modulated (PWM) controllers for forward/flyback isolated DC-to-DC converters provide maximum flexibility to power-supply designers and reduce external component count. These controllers allow common PC board layout for currentmode (MAX8540) and voltage-mode (MAX8541) designs. Both controllers feature adjustable switching frequency and external synchronization from 200kHz to 1MHz. Active-high and active-low enable, undervoltage protection (UVP), and overvoltage protection (OVP) reduce external component count. Maximum duty cycle is adjustable, and the feed-forward function scales the maximum duty cycle with input voltage to limit the maximum volt-seconds applied to the transformer primary. The MAX8540 allows the user to select the value of slope compensation to further optimize magnetics design. The MAX8541 features useradjustable ramp magnitude for the PWM comparator. A cycle-by-cycle current-limit function controls the peak primary current during overload and short circuit. Both controllers can be set to latch off or to hiccup when a short circuit is detected. The number of current-limited cycles to initiate the hiccup mode and number of cycles skipped are user selectable to allow startup with high-capacitance loads and protect the converter against sustained short circuits. The MAX8540/MAX8541 feature 9Ω internal gate drivers for low-power applications and are compatible with external gate drivers for high-power applications. Both devices are available in the space-saving 16-pin QSOP package. The MAX8541 EV kit evaluates the MAX8540 and the MAX8541 to speed designs. Design examples are also available in application notes: 50W Voltage-Mode Forward Converter Design with the MAX8541 and 50W Current-Mode Forward Converter Design with the MAX8540. Applications Isolated DC-to-DC Modules (Bricks) Cellular Base Stations Telecom and Network Systems High-Performance Off-Line AC/DC Converters Features ♦ 200kHz to 1MHz Adjustable Switching Frequency ♦ Synchronization to External Clock ♦ Programmable, Constant Maximum Volt-Second Simplifies Transformer Design ♦ Programmable Hiccup/Skip Cycles or Latch-Mode Protection ♦ Clean Startups with High-Capacitance Loads ♦ Programmable Under/Overvoltage Protection ♦ Current-Mode, Adjustable Slope Compensation (MAX8540) ♦ Voltage-Mode, Adjustable Ramp Magnitude (MAX8541) ♦ 10mA, 5V Regulator ♦ 75mV to 1.25mV Adjustable Current Limit Reduces External Components ♦ Internal Gate Driver for Low-Power Applications ♦ External Gate Driver for High-Power Applications Ordering Information PART TEMP RANGE PIN-PACKAGE MAX8540EEE -40°C to +85°C 16 QSOP MAX8541EEE -40°C to +85°C 16 QSOP Pin Configurations TOP VIEW REF 1 16 VCC OV 2 15 DRV UV 3 14 GND FREQ/SYNC 4 MAX8540EEE SS 5 13 SKTON 12 CS MAXDTY 6 11 SKTOFF EN 7 10 ILIM SCOMP 8 9 OPTO QSOP Functional Diagrams and Typical Operating Circuit appear at end of data sheet. Pin Configurations continued at end of data sheet. ________________________________________________________________ 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 MAX8540/MAX8541 General Description MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies ABSOLUTE MAXIMUM RATINGS VCC to GND ............................................................-0.3V to +20V OPTO, UV, OV, EN, PRAMP, ILIM, REF to GND ......-0.3V to +6V SCOMP, FREQ, CS, SKTON, SKTOFF, SS, MAXDTY to GND .....................................-0.3V to VREF + 0.3V DRV to GND ................................................-0.3V to VVCC + 0.3V Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C 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. ELECTRICAL CHARACTERISTICS (VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS 18.0 V VCC Operating Voltage Range Undervoltage Lockout (Note 1) 7.6 VCC rising 7.6 8.0 8.4 VCC falling 6.6 7.0 7.4 0.9 1.0 Undervoltage Lockout Hysteresis Supply Current VREF is active, IREF = 0 Shutdown Current V EN = 3V No switching Switching with no load V V 3.5 4.2 5 6.3 0.5 0.9 5.0 5.1 V 20 50 mV mA mA REF Output Voltage IREF = 0 Load Regulation IREF = 0 to 10 mA 4.9 Line Regulation VVCC = 8.4V to 18V 5 20 mV Pulldown Resistor in Shutdown V EN = 3V 10 30 Ω UNDERVOLTAGE/OVERVOLTAGE (UV/OV) UVLO Threshold VUV rising, 120mV typical hysteresis 1.200 1.250 1.300 VUV falling 1.083 1.128 1.173 -0.5 +0.01 +0.5 Input Bias Current OVLO Threshold VOV rising, 130mV typical hysteresis VOV falling 2.9 3.021 3.1 2.778 2.894 3.010 V µA V EN Input Low Threshold VIL Input High Threshold VIH Pullup Current V EN = 2.6V 1.2 2.6 5 V V 17 µA CURRENT LIMIT VILIM Range 0.075 CS Current-Limit to DRV Turn-Off Includes leading-edge blanking time Delay VCS Cycle-by-Cycle Threshold Voltage 2 110 Accuracy when VILIM = 0.075V VILIM = 1.250V 1.250 V 150 ns ±10 1.175 1.250 _______________________________________________________________________________________ % 1.330 V Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies (VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS ILIM Leakage Current MIN TYP -1 Hiccup Skip Off-Time Hiccup Skip Off-Charging CSKTOFF = 0.1µF (Note 3) Hiccup Skip On-Time CSKTON = 0.01µF (Note 3) MAX +1 90 0.9 1.5 UNITS µA ms 2.1 9 µA ms Hiccup Skip On-Charging Current 1.5 2.5 3.5 µA Hiccup SKTOFF Voltage Threshold 2.3 2.5 2.7 V OPTO Input Bias Current VOPTO = 3.5V 30 µA Minimum Input Voltage Minimum voltage for duty-cycle control 0.6 V Maximum Input Voltage Maximum voltage for duty-cycle control 2.6 V RMAXDTY = 24.3kΩ, VUV = 1.3V 15 DUTY CYCLE Maximum Duty Cycle RMAXDTY = 130kΩ, VUV = 1.3V 80 VUV = 1.3V, RMAXDTY = 97.6kΩ 60 % Internally programmed limit for maximum duty cycle at all frequencies 79 80 82 VSS = 1V 4.5 5.5 6.5 Pulldown Resistance in Shutdown ISS = 10mA, V E N = 3V 10 30 Time 440 SOFT-START Source Current µA Ω ms/µF FREQUENCY RFREQ = 32.4kΩ Initial Accuracy Switching Frequency Range 280 300 RFREQ = 48.7kΩ 200 RFREQ = 8.87kΩ 1000 320 kHz kHz SCOMP (MAX8540) Slope Compensation RSCOMP = 14kΩ 1.5 RSCOMP = 204kΩ 0.1 RPRAMP = 12.4kΩ 2.5 V PRAMP (MAX8541) Ramp Voltage Amplitude RPRAMP = 21kΩ 1 RPRAMP = 42kΩ 0.5 CDRV = 500pF, 10% to 90% of VVCC 30 CDRV = 2000pF, 10% to 90% of VVCC 57 V DRV (MOSFET DRIVER) Rise Time ns _______________________________________________________________________________________ 3 MAX8540/MAX8541 ELECTRICAL CHARACTERISTICS (continued) MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies ELECTRICAL CHARACTERISTICS (continued) (VVCC = 12V, CREF = 1µF, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VCS = 0.6V, CDRV = 10pF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Fall Time CONDITIONS MIN TYP CDRV = 500pF, 90% to 10% of VVCC 27 CDRV = 2000pF, 90% to 10% of VVCC 53 MAX UNITS ns On-Resistance Pullup 9 Ω On-Resistance Pulldown 7 Ω SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION) Frequency Range Frequency syncs ±30% of frequency set by external R on FREQ pin FREQ Input Threshold (Note 4) 0.2 1.0 MHz 2 3 V THERMAL SHUTDOWN Thermal Shutdown TA rising, 20°C hysteresis °C 170 ELECTRICAL CHARACTERISTICS (VVCC = 12V, CREF = 1µF, VUV = VOV = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40°C to +85°C, unless otherwise noted.) (Note 5) PARAMETER CONDITIONS MIN TYP MAX UNITS V VCC Operating Voltage Range Undervoltage Lockout (Note 1) 7.6 18.0 VCC rising 7.6 8.4 VCC falling 6.6 7.4 Undervoltage Lockout Hysteresis 0.9 Supply Current VREF = is active, IREF = 0 Shutdown Current V EN = 3V V V No switching 4.2 Switching with no load 6.3 0.9 mA mA REF Output Voltage IREF = 0 5.1 V Load Regulation IREF = 0 to 10mA 4.9 50 mV Line Regulation VVCC = 8.4V to 18V 20 mV Shutdown Resistor V EN = 3V 30 Ω UNDERVOLTAGE/OVERVOLTAGE (UV/OV) UVLO Threshold VUV falling, 120mV typical hysteresis 1.200 1.300 VUV rising 1.083 1.173 -0.5 +0.5 VOV falling, 130 mV typical hysteresis 2.901 3.142 VOV rising 2.778 3.010 Input Bias Current OVLO Threshold 4 _______________________________________________________________________________________ V µA V Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies (VVCC = 12V, CREF = 1µF, VUV = VOV = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, V EN = 0, RSCOMP = 68kΩ, RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40°C to +85°C, unless otherwise noted.) (Note 5) PARAMETER CONDITIONS MIN TYP MAX UNITS EN VIL Input Threshold VIH Pullup Current V EN = 2.6V 1.2 2.6 5 V µA CURRENT LIMIT VILIM Range 0.075 CS Current Limit to DRV Delay Includes leading-edge blanking time VCS Cycle-by-Cycle Threshold Voltage Accuracy when VILIM = 0.075V VILIM = 1.250V 1.25 V 150 ns ±10 % 1.130 1.375 V ILIM Leakage Current -1 +1 µA Hiccup Skip Off-Charging Current 0.9 2.1 µA Hiccup Skip On-Charging Current 1.5 3.5 µA Hiccup SKTOFF Voltage Threshold 2.3 2.7 V DUTY CYCLE Duty-Cycle Range VUV = 1.3V 15 80 % Maximum Duty Cycle Internal programmed max duty at all frequencies 79 82 % Source Current VSS = 1V 4.5 6.5 µA Pulldown Resistance in Shutdown ISS = 10mA, V EN = 3V 30 Ω 280 320 kHz 0.2 1.0 MHz 2 3 V SOFT-START FREQUENCY Initial Accuracy RFREQ = 32.4kΩ SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION) Frequency Range Frequency syncs ±30% of frequency set by external R on FREQ pin FREQ Input Threshold (Note 4) This is the VCC operating range after clearing the rising UVLO. Guaranteed by design. See the SKTON and SKTOFF section. The minimum FREQ pulse must be 200ns in duration with a minimum magnitude of 3V plus VF of the external diode and have a maximum duty cycle of 55%. Note 5: Specifications to -40°C are guaranteed by design and not production tested. Note 1: Note 2: Note 3: Note 4: _______________________________________________________________________________________ 5 MAX8540/MAX8541 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VCC = 12V, TA = +25°C, unless otherwise specified.) 580 560 540 5.2 520 5.0 4.8 500 460 4.4 440 4.95 420 VEN = 0 4.0 VEN = 5V 400 -20 0 20 40 60 100 80 4.90 -40 -20 0 20 40 60 80 100 -20 0 20 40 60 80 TA (°C) TA (°C) MAXIMUM DUTY CYCLE vs. TEMPERATURE FREQUENCY vs. TEMPERATURE CURRENT-SENSE THRESHOLD vs. TEMPERATURE FREQUENCY (kHz) 80 RFREQ = NONE 75 70 65 130 800 125 RFREQ = 10kΩ 120 VCS (mV) 85 1000 600 400 RFREQ = 36.5kΩ 115 110 RFREQ = 20kΩ 60 100 MAX8540 toc06 MAX8540 toc04 90 200 105 55 RFREQ = 50kΩ 100 0 50 -40 -20 0 20 40 60 -40 100 80 -20 0 20 40 60 80 -20 0 20 4.0 180 MAX8540 toc08 190 60 OV/UV THRESHOLD VOLTAGE vs. TEMPERATURE MAX8540 toc07 200 40 TA (°C) CURRENT-LIMIT DELAY TIME vs. TEMPERATURE 3.5 3.0 170 OV/UV VOLTAGE (V) DELAY TIME (ns) -40 100 TA (°C) TA (°C) 160 150 140 130 VOV 2.5 2.0 1.5 1.0 120 VUV 0.5 110 100 0 -40 -20 0 20 40 TA (°C) 6 -40 TA (°C) MAX8540 toc05 -40 5.00 480 4.6 4.2 5.05 VREF (V) 5.4 IVCC (µA) IVCC (mA) 5.6 5.10 MAX8540 toc02 5.8 REFERENCE VOLTAGE vs. TEMPERATURE 600 MAX8540 toc01 6.0 SHUTDOWN VCC CURRENT vs. TEMPERATURE MAX8540 toc03 VCC NO-LOAD SWITCHING CURRENT vs. TEMPERATURE DMAX (%) MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies 60 80 100 -40 -20 0 20 40 60 TA (°C) _______________________________________________________________________________________ 80 100 80 100 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies SKTON/SKTOFF TIME vs. CAPACITANCE FREQUENCY vs. INPUT VOLTAGE PROGRAMABLE LATCH-OFF DELAY 800 1000 VDRV 800 TIME (ms) RFREQ = 10kΩ 600 400 600 400 2V/div VCS RFREQ = 20kΩ 200 200 RFREQ = 50kΩ CSKTON = 0.01µF 0 0 9.7 11.8 13.8 15.9 0 18.0 0.2 0.4 0.6 1.0 0.8 4ms/div CAPACITANCE (µF) INPUT VOLTAGE (V) REFERENCE VOLTAGE vs. LOAD CURRENT REFERENCE VOLTAGE vs. VCC 5.06 5.08 5.06 MAX8540 toc14 5.10 MAX8540 toc13 5.04 5.04 5.02 5.02 VREF (V) 7.6 VREF (V) FREQUENCY (kHz) 1000 MAX8540 toc11 MAX8540 toc10 MAX8540 toc12 5.00 5.00 4.98 4.98 4.96 4.94 4.96 4.92 4.90 4.94 0 2 4 6 8 10 12 7.6 LOAD CURRENT (mA) 9.7 11.8 13.8 15.9 18.0 VCC (V) HICCUP-MODE OVERCURRENT WAVEFORMS LATCH-OFF OVERCURRENT WAVEFORMS MAX8540 toc16 MAX8540 toc15 VDRV VDRV VSKTON 1V/div VSKTOFF 5V CSKTON = 0.01µF 10ms/div 1V/div VSKTON VSKTOFF 1V/div CSKTON , CSKTFOF = 0.01µF 4ms/div _______________________________________________________________________________________ 7 MAX8540/MAX8541 Typical Operating Characteristics (continued) (VCC = 12V, TA = +25°C, unless otherwise specified.) MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies Pin Description PIN MAX8540 MAX8541 8 NAME FUNCTION 1 1 REF 5V Reference Voltage Output. Bypass to ground with a 1µF capacitor. REF is pulled to GND during shutdown. 2 2 OV System Input Overvoltage Monitoring. The OV threshold is 3.021V with 130mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to set the overvoltage trip point (see the Overvoltage Threshold section). System Input Undervoltage Monitoring and Feed-Forward Input. The UV threshold is 1.25V with 120mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to set the undervoltage trip point (see the Undervoltage Threshold section). This input can be used as a disable input by pulling it below 1.2V. UV remains active during shutdown. 3 3 UV 4 4 FREQ/ SYNC Programmable Frequency and Sync Input. Connect a resistor in parallel with RC (10kΩ, 0.01µF) from FREQ/SYNC to GND to set the switching frequency or apply an external clock signal through a diode to synchronize to an external frequency. 5 5 SS Soft-Start Capacitor Connection. Connect a capacitor from SS to GND to set the soft-start time period. Programmable Maximum Duty-Cycle Input. Connect a resistor from MAXDTY to GND to set the maximum duty cycle for minimum system input voltages. The maximum duty cycle is inversely MAXDTY proportional to the system input voltage. The higher the input voltage, the less the maximum duty cycle. The maximum duty cycle is internally limited to 80% at any switching frequency. MAXDTY is high impedance during shutdown. Bypass MAXDTY with a 2200pF capacitor. 6 6 7 7 EN 8 — SCOMP Programmable Slope Compensation Input. Connect a resistor from SCOMP to GND to set the slope compensation. — 8 PRAMP Programmable Voltage-Mode Control-Ramp Input. Connect a resistor from PRAMP to GND to set the magnitude of the voltage ramp. 9 9 OPTO Isolated Feedback Input. Connect to an optocoupler for an isolated power converter or connect to the output of a voltage-error amplifier for a nonisolated power converter for a feedback-error signal. 10 10 ILIM Current-Limit Threshold Input. Connect to the center of a resistor-divider from REF to GND to set the current-limit threshold voltage. 11 11 SKTOFF Current-Limit Off-Time Input. Connect a capacitor from SKTOFF to GND to set the current-limit off-time. Pull SKTOFF to REF through a 10kΩ resistor to latch off the IC after an overcurrent event. 12 12 CS Current-Sense Input. Provides current-sense feedback for cycle-by-cycle current limit and is also the current-mode input for the MAX8540. Connect a current-sense resistor from CS to GND. 13 13 SKTON 14 14 GND Ground 15 15 DRV Gate Drive for External N-Channel Power MOSFET. Connect to the gate of an external MOSFET for low-power applications. Connect to the input of an external gate driver for high-power applications. 16 16 VCC Voltage Supply for the IC. Operating input voltage range is 7.6V to 18V. Bypass VCC with a ceramic capacitor to GND. Active-Low Enable Input. Drive EN to GND to enable the IC. Drive high to place the IC in shutdown mode. Current-Limit On-Time Input. Connect a capacitor to ground to set the current-limit on-time. _______________________________________________________________________________________ Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies Overvoltage Threshold The MAX8540/MAX8541 include an overvoltage protection (OVP) feature that turns off the external MOSFET when the input voltage exceeds the user-set threshold. Connect a resistor-divider from the system input to GND with OV connected to the center to set the OVP trip point. The threshold voltage for OV is 3.021V (typ): ⎛ R1 + R2 ⎞ VIN(MAX) = ⎜ ⎟ × VOV ⎝ R2 ⎠ where VOV is the OV threshold, VIN(MAX) is the overvoltage trip point, R1 is the resistor from the system input to OV, and R2 is the resistor from OV to GND. Undervoltage Threshold The MAX8540/MAX8541 also include an undervoltage (UV) sensing input. The IC holds the external MOSFET low until UV reaches its threshold (1.25V typ). Once the threshold has been reached, the circuit enters soft-start and brings the output into regulation. Connect a resistordivider from the system input to GND with UV at the center to set the undervoltage protection (UVP) trip point. ⎛ R3 + R4 ⎞ VIN(MIN) = ⎜ ⎟ × VUV ⎝ R4 ⎠ where VUV is the UV threshold, VIN(MIN) is the UV trip point, R3 is the resistor from the system input to UV, and R4 is the resistor from UV to GND. An alternate method used in the application circuits for setting the OV and UV trip points is demonstrated in Figure 1. Use 36.5kΩ for the bottom resistor (RC). RA and RB are calculated as follows: ⎛ VOV × VIN(MIN) RB = RC × ⎜ ⎝ VUV × VIN(MAX) RA = RC × VIN(MIN) VUV − RB − − ⎞ 1⎟ ⎠ RC where VIN(MIN) is the UV trip point, VIN(MAX) is the OV trip point, VUV is the UV threshold (1.25V typ), and VOV is the OV threshold (3.021V typ). RA should consist of two equal-value resistors in series to protect against single-point failure. Table 1. Typical Specifications of UV/OV and the Actual On/Off Hysteresis of Input Voltage UV VOLTAGE SPECIFICATIONS (V) VIN OFF WINDOW (LTP) (V) MIN 1.083 29.76 TYP 1.128 31.00 MAX 1.173 32.24 MIN 1.200 TYP 1.250 MAX 1.300 OV VOLTAGE SPECIFICATIONS (V) 32.97 VIN OFF WINDOW (UTP) (V) 2.901 79.70 3.021 83.00 MAX 3.142 86.32 2.778 2.894 MAX 3.010 34.34 36.00 TYP TYP — — MIN MIN VIN ON WINDOW (UTP) (V) VIN ON WINDOW (LTP) (V) — 76.32 — 79.50 82.68 Assume that 1.250V at UV/OV is scaled to 34.34V for a 48V bus. Table 2. Switching Frequency Selection RFREQ/SYNC (kΩ) SWITCHING FREQUENCY (kHz) 48.7 200 19.1 500 15.8 600 11.0 800 8.87 1000 Switching Frequency and Synchronization The MAX8540/MAX8541 oscillator operates in two modes: stand-alone or synchronized (sync). A single input, FREQ/SYNC, doubles as the attachment point for the frequency-programming resistor and as the synchronization input. The mode recognition is automatic, based on the signal applied to FREQ/SYNC. In stand-alone mode, an external resistor connected from FREQ/SYNC to GND sets the operating frequency. A 1.25V source is internally applied to FREQ/SYNC and the oscillator frequency is proportional to the current out of FREQ/SYNC through the programming resistor. _______________________________________________________________________________________ 9 MAX8540/MAX8541 Detailed Description MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies The MAX8540/MAX8541 also synchronize with an external oscillator. Drive FREQ/SYNC with a square wave through a series diode with a positive pulse width of at least 200ns and a minimum pulse amplitude of 3V, plus the VF of the external diode. Alternatively, a 1.5V level-shifted external clock can be applied without the series diode. The maximum duty cycle of the external signal allowed is 55%. The MAX8540/MAX8541 synchronize to frequencies between 200kHz and 1MHz; however, the signal must be within ±30% of the frequency set by the external resistor at FREQ/SYNC. VIN 36V TO 76V C1 RMAXDTY = DMAX × (97.6 × 103 ) 60 where DMAX is the desired maximum duty cycle. The range of valid resistor values for RMAXDTY is from 24.3kΩ to 130kΩ. N-Channel MOSFET Driver The DRV output drives an N-channel MOSFET in lowpower applications. In high-power applications, the gate driver internal to the MAX8540/MAX8541 may not be capable of driving the external MOSFET efficiently and an external gate driver may be required. In this situation, connect DRV to the input of the external gate driver. Slope Compensation (MAX8540) The MAX8540 is a current-mode device and requires slope compensation for proper operation. To provide slope compensation, connect a resistor from SCOMP to GND (RSCOMP). The value of RSCOMP is determined as follows. For applications using a synchronous rectifier in the output, set the slope compensation equal to the negative slope of the output inductor. RSCOMP is equal to: SCF = N RSCOMP = 10 VOUT RCS L1 dVRAMP R × SUM 2SCF dt C3 RA Maximum Duty Cycle Set the maximum duty cycle at the minimum system input voltage (V IN(MIN) ) connecting a resistor from MAXDTY to GND. The maximum duty cycle is inversely proportional to the voltage at UV. As the voltage on UV increases, the duty cycle decreases. The maximum duty cycle is internally limited to 80% at all switching frequencies. The MAXDTY resistor is determined as: C2 RB OV RC UV MAX8541 Figure 1. A Method to Set UV and OV Thresholds where dVRAMP/dt = 2.5V(fS), RSUM = 25kΩ, N is the turns ratio of the primary to secondary, L1 is the output inductance, VOUT is the output voltage, fS is the switching frequency, and RCS is the current-sense resistance. For applications where a diode is used in the output instead of the synchronous rectifier, the slope compensation resistor is then equal to: SCF = N RSCOMP = VOUT + VF RCS L1 dVRAMP R × SUM 2SCF dt where dVRAMP/dt = 2.5V(fS),RSUM = 25kΩ, N is the turns ratio of the primary to secondary, L1 is the output inductance, VOUT is the output voltage, VF is the diode voltage, fS is the switching frequency, and RCS is the currentsense resistance. Voltage-Ramp Amplitude (MAX8541) The MAX8541 is a voltage-mode device and features adjustable voltage ramp. Connect a resistor from PRAMP to GND (R PRAMP ) to set the voltage-ramp magnitude, VM. VM = 1.25V 25kΩ RPRAMP ______________________________________________________________________________________ Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies Soft-Start The soft-start feature allows converters built using the MAX8540/MAX8541 to apply power to the load in a controllable soft ramp, thus reducing startup surges and stresses. It also determines power-up sequencing when several converters are used. Upon power turn-on, the soft-start pin acts as a current sink to discharge any capacitance connected to it. Once the voltage at VCC has exceeded its lockout value, softstart then charges the external capacitor (CSS), allowing the converter output voltage to ramp up. Full output voltage is reached in approximately 440ms/µF. Current Limit The MAX8540/MAX8541 utilize two current-limit schemes: cycle-by-cycle current limit and short-circuit current limit. Set the current-limit threshold using a resistor-divider from REF to GND with ILIM connected to the center. The current-limit threshold is determined as: VILIM = R5 × VREF R5 + R6 where R5 is the resistor from ILIM to GND and R6 is the resistor from REF to ILIM. Use 240kΩ for R16 and vary R23 to change the threshold. The CS signal provides feedback on the current ramp through the main external MOSFET. The voltage on CS is monitored by the IC. The cycle-by-cycle current limit abbreviates the on-time of the external MOSFET in the event that the voltage at CS is greater than the threshold voltage set by ILIM. The current-limit feature protects against a hard short or overcurrent fault at the output by one of two selected protection modes: by latching off the output, or pulsing the output to reduce the average output current (hiccup mode). To select latched mode, connect SKTOFF to REF. In this mode, if the hard short exists for the time period set by the capacitance at SKTON, the output is latched off. To unlatch the output, toggle EN or cycle the input power to VCC. To select hiccup mode, connect capacitors to SKTON and SKTOFF to program the hiccup mode on- and offtimes. When a cycle-by-cycle event is detected, the IC charges the capacitor at SKTON. The capacitor continues to charge as long as the CS voltage is greater than the ILIM threshold voltage. Once the voltage on SKTON reaches its threshold voltage, the MAX8540/MAX8541 begin skipping switching cycles for a time determined by the capacitance connected to SKTOFF. Once this time period has elapsed, the IC begins to switch for the time period set by the capacitance connected to SKTON. This process continues until the output short is removed. See the SKTON and SKTOFF section for details on setting the hiccup-mode periods. Connect SKTOFF to REF and SKTON to GND to disable the latched-mode and hiccup-mode protection, and operate continuously in cycle-by-cycle current-limit. PMW Comparator The PWM comparator of the MAX8540 transforms the optocoupled error voltage (VOPTO) into a duty cycle by comparing the opto feedback-error voltage with a summed voltage. The summed voltage is the sum of the programmable slope compensation and the current-sense voltages. When the summed voltage exceeds the opto feedback error voltage, the gatedrive logic turns off the external MOSFET. The PWM comparator in the MAX8541 compares VOPTO with the programmable voltage ramp. When the voltage ramp exceeds V OPTO, the gate-driver logic turns off the external MOSFET. SKTON and SKTOFF The capacitance, CSKTON, determines the time period allowed before the short-circuit current limit initiates. Once the CS voltage exceeds the ILIM threshold, the capacitor at SKTON begins to charge. The capacitor continues to charge until the SKTON threshold voltage is reached or the overcurrent event is removed. This feature allows for the higher currents required during startup with high-capacitance loads. Set CSKTON to allow sufficient time for startup. The required capacitance at SKTON is determined as: CSKTON = tON / 103 where tON is in ms and CSKTON is in µF. The allowable range for CSKTON is 100pF to 0.01µF. The capacitance at SKTOFF determines the time period that the external MOSFET is turned off during an overcurrent event. Once the SKTON time period is exceeded, the SKTOFF capacitor charges. Once VSKTOFF reaches its threshold, the IC begins to switch again. CSKTOFF is determined as: CSKTOFF = tOFF / 103 where tOFF is in ms and CSKTOFF is in µF. The allowable range for CSKTOFF is 1000pF to 1µF. ______________________________________________________________________________________________________ 11 MAX8540/MAX8541 where RPRAMP is in kΩ. The ramp-voltage magnitude is independent of frequency. The range of values for RPRAMP is from 12.4kΩ to 42kΩ. MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies Pull VSKTOFF to VREF through a 10kΩ pullup resistor to enable the latch-off feature. In this mode, once the SKTON time has elapsed, the IC is latched off. The circuit remains off until EN is toggled, or the input power is toggled. Pin Configurations (continued) TOP VIEW Soft-Start Capacitor Selection REF 1 16 VCC During startup, the capacitor at soft-start is charged using a 5µA current source. Once the voltage at softstart reaches the threshold voltage (2V typ), the IC switches normally. Use a low-ESR ceramic capacitor placed as close as possible to the IC at soft-start. The value is determined as follows: OV 2 15 DRV CSS = tSS 530 where tSS is the desired soft-start period in ms and CSS is in µF. Applications Information Refer to the following application notes for the MAX8540/MAX8541 application circuits and applications information: 50W Current-Mode Forward Converter Design with the MAX8540 and 50W VoltageMode Forward Converter Design with the MAX8541. 12 UV 3 FREQ/SYNC 4 14 GND MAX8541EEE SS 5 13 SKTON 12 CS MAXDTY 6 11 SKTOFF EN 7 10 ILIM PRAMP 8 9 OPTO QSOP Chip Information TRANSISTOR COUNT: 2704 PROCESS: BiCMOS ______________________________________________________________________________________ Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies VCC UVLO COMPARATOR VCC VCC TEMP OK MAX8540 8V SS_RESET 5V REFERENCE REF OVERVOLTAGE COMPARATOR 3.02V ENABLE OV UNDERVOLTAGE COMPARATOR CLK ENABLE DRIVE LOGIC DRV UV 1.25V EN SLOPE COMPENSATION CS SCOMP 1.2V SKTOFF SKTON SHORT-CIRCUIT TIMER PWM COMPARATOR OPTO CS 80% MAX ILIM GND SS MAX TON 5µA SS_RESET OSC DECODER RAMP MAXDTY CLK REF FREQ/SYNC Figure 2. MAX8540 Functional Diagram ______________________________________________________________________________________ 13 MAX8540/MAX8541 Functional Diagrams Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541 Functional Diagrams (continued) VCC UVLO COMPARATOR VCC VCC TEMP OK MAX8541 8V SS_RESET 5V REFERENCE REF OVERVOLTAGE COMPARATOR 3.02V ENABLE OV UNDERVOLTAGE COMPARATOR CLK ENABLE DRIVE LOGIC DRV UV 1.25V EN VOLTAGE RAMP CLK PRAMP 1.2V SKTOFF SKTON SHORT-CIRCUIT TIMER PWM COMPARATOR OPTO CS 80% MAX ILIM GND SS MAX TON 5µA SS_RESET OSC DECODER RAMP CLK REF FREQ/SYNC Figure 3. MAX8541 Functional Diagram 14 ______________________________________________________________________________________ MAXDTY Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies INPUT 36V TO 76V OUTPUT 2.5V AT 20A OUT IN FB MAX8515 VIN PGND GND OPTIONAL SYNCHRONIZATION OV VCC UV FREQ/SYNC SS U1 REF MAX8540 DRV CS MAXDTY ILIM SKTOFF SCOMP SKTON OFF OPTO EN GND ON ______________________________________________________________________________________ 15 MAX8540/MAX8541 Typical Operating Circuit 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. QSOP.EPS MAX8540/MAX8541 Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH 21-0055 F 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. 16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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