19-2115; Rev 0; 7/01 Current-Mode PWM Controllers with Integrated Startup Circuit The MAX5019/MAX5020 integrate all the building blocks necessary for implementing DC-DC fixed-frequency power supplies. Either primary- or secondaryside regulation may be used to implement isolated or nonisolated power supplies. These devices are currentmode controllers with an integrated high-voltage startup circuit suitable for telecom/industrial voltage range power supplies. Current-mode control with leadingedge blanking simplifies control-loop design and internal ramp compensation circuitry stabilizes the current loop when operating at duty cycles above 50% (MAX5019). The MAX5019 allows 85% operating duty cycle and can be used to implement flyback converters whereas the MAX5020 limits the operating duty cycle to less than 50% and can be used in single-ended forward converters. A high-voltage startup circuit allows these devices to draw power directly from the 18V to 110V input supply during startup. The switching frequency is internally trimmed to 275kHz ±10%, thus reducing magnetics and filter component costs. The MAX5019/MAX5020 are available in 8-pin SO packages. Warning: The MAX5019/MAX5020 operate with high voltages. Exercise caution. Applications Features ♦ Wide Input Range: (18V to 110V) or (13V to 36V) ♦ Isolated (without optocoupler) or Nonisolated Power Supply ♦ Current-Mode Control ♦ Leading-Edge Blanking ♦ Internally Trimmed 275kHz ±10% Oscillator ♦ Low External Component Count ♦ Soft-Start ♦ High-Voltage Startup Circuit ♦ Pulse-by-Pulse Current Limiting ♦ Thermal Shutdown ♦ SO-8 Package Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX5019CSA* 0°C to +70°C 8-SO MAX5019ESA* -40°C to +85°C 8-SO MAX5020CSA* 0°C to +70°C 8-SO MAX5020ESA* -40°C to +85°C 8-SO *See Selector Guide at end of data sheet. Telecom Power Supplies Industrial Power Supplies Networking Power Supplies Isolated Power Supplies Typical Operating Circuit Pin Configuration VIN TOP VIEW VOUT V+ VDD MAX5020 V+ 1 VDD 2 FB 3 MAX5019/ MAX5020 8 VCC 7 NDRV 6 GND 5 CS VCC CS SS_SHDN SS_SHDN 4 NDRV GND FB 8-SO ________________________________________________________________ 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 MAX5019/MAX5020 General Description MAX5019/MAX5020 Current-Mode PWM Controllers with Integrated Startup Circuit ABSOLUTE MAXIMUM RATINGS V+ to GND ……………………………………...……-0.3V to +120V VDD to GND.………………………………….……….-0.3V to +40V VCC to GND.………………….……………………-0.3V to +12.5V FB, NDRV, SS_SHDN, CS to GND .……-0.3V to VCC + 0.3V VDD and VCC Current …………………...…………………..20mA NDRV Current Continuous...………………………………….25mA NDRV Current for Less than 1µs..………….…………….……±1A Continuous Power Dissipation (TA = +70°C) 8-Pin SO (derate 5.88mW/°C above +70°C) .………....471mW Operating Temperature Range…………..……...-40°C to +85°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 (VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV = open circuit, VFB = 3V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS SUPPLY CURRENT IV+(NS) VDD = 0, V+ = 110V, driver not switching V+ = 110V, VDD = 0, FB = GND, driver switching V+ = 110V, VDD = 13V, FB = GND 0.8 1.6 1.6 3.0 VDD = 36V, driver not switching 0.9 1.6 VDD = 36V, driver switching, FB = GND 2.1 3.0 V+ Shutdown Current VSS_SHDN = 0, V+ = 110V 180 290 µA VDD Shutdown Current VSS_SHDN = 0 4 20 µA V+ Supply Current IV+(S) V+ Supply Current After Startup VDD Supply Current IVDD(NS) IVDD(S) mA µA 14 mA PREREGULATOR/STARTUP V+ Input Voltage 18 110 V VDD Supply Voltage 13 36 V INTERNAL REGULATORS (VCC) VCC Output Voltage VCC Undervoltage Lockout VCC_UVLO Powered from V+, ICC = 7.5mA, VDD = 0 7.5 9.8 12.0 V Powered from VDD, ICC = 7.5mA 9.0 10.0 11.0 V VCC falling 6.6 V Peak Source Current VCC = 11V (externally forced) 570 mA Peak Sink Current VCC = 11V (externally forced) 1000 mA OUTPUT DRIVER NRDV High-Side Driver Resistance ROH VCC = 11V, externally forced, NDRV sourcing 50mA 4 12 Ω NDRV Low-Side Driver Resistance ROL VCC = 11V, externally forced, NDRV sinking 50mA 1.6 4 Ω ERROR AMPLIFIER FB Input Resistance RIN FB Input Bias Current IFB Error Amplifier Gain (Inverting) 50 VFB = VSS_SHDN AVCL Closed-Loop 3dB Bandwidth FB Input Voltage Range 2 kΩ ±1 µA -20 V/V 200 kHz 2 _______________________________________________________________________________________ 3 V Current-Mode PWM Controllers with Integrated Startup Circuit (VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV = open circuit, VFB = 3V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS SLOPE COMPENSATION Slope Compensation (MAX5019 only) 26 VSCOMP mV/µs THERMAL SHUTDOWN Thermal Shutdown Temperature 150 °C Thermal Hysteresis 25 °C CURRENT LIMIT CS Threshold Voltage VILIM FB = GND 419 465 510 mV 1 µA CS Input Bias Current 0 ≤ VCS ≤ 2V, FB = GND Current Limit Comparator Propagation Delay 50mV overdrive on CS, FB = GND 180 ns CS Blanking Time FB = GND, only PWM comparator is blanked 70 ns -1 OSCILLATOR Clock Frequency Range FB = GND Max Duty Cycle 247 275 302 MAX5019, FB = GND 75 85 MAX5020, FB = GND 44 50 VSS_SHDN = 0 2.0 kHz % SOFT-START SS Source Current ISSO SS Sink Current 4.5 µA 6.5 1.0 Steady State Reference Voltage at SS_SHDN VSS_SHDN Shutdown Threshold mA No external load 2.331 2.420 2.500 VSS_SHDN falling 0.25 0.37 0.41 VSS_SHDN rising 0.53 0.59 0.65 V V Typical Operating Characteristics (V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.) VSS_SHDN vs. TEMPERATURE (AT THE END OF SOFT-START) NDRV FREQUENCY (kHz) 1.001 1.000 FB = GND 276 275 274 0.999 -20 0 20 40 TEMPERATURE (°C) 60 80 80.9 80.8 FB = GND 80.7 80.6 80.5 273 -40 MAX5019 toc03 277 81.0 MAXIMUM DUTY CYCLE (%) VFB = 4V 1.002 MAX5019 toc02 278 MAX5019 toc01 1.003 VSS_SHDN (V) (NORMALIZED TO VREF = 2.4V) MAX5019 MAXIMUM DUTY CYCLE vs. TEMPERATURE NDRV FREQUENCY vs. TEMPERATURE 80.4 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (°C) _______________________________________________________________________________________ 3 MAX5019/MAX5020 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.) 47.2 47.0 1.59 1.58 1.57 1.56 0 20 60 V+ INPUT CURRENT vs. TEMPERATURE (AFTER STARTUP) V+ SHUTDOWN CURRENT vs. TEMPERATURE 40 60 80 MAX5019 toc07 13.80 13.75 V+ = 110V, VDD = 13V, FB = GND 13.70 13.65 13.60 0 20 40 60 182.0 V+ = 110V, FB = SS_SHDN = GND 181.5 181.0 180.5 180.0 -20 0 20 40 60 TEMPERATURE (°C) TEMPERATURE (°C) NDRV RESISTANCE vs. TEMPERATURE CURRENT-LIMIT DELAY vs. TEMPERATURE HIGH-SIDE DRIVER 3.5 3.0 2.5 2.0 LOW-SIDE DRIVER 80 0.486 0.485 0.484 206 0 20 40 60 80 2.410 2.408 FB = GND, 100mV OVERDRIVE ON CS 204 -20 VSS_SHDN vs. VDD 202 200 198 196 2.406 2.404 194 2.402 192 1.5 60 FB = GND -40 VSS_SHDN (V) 4.0 40 TEMPERATURE (°C) 208 CURRENT-LIMIT DELAY (ns) 4.5 20 0.487 80 210 MAX5019 toc10 5.0 0 0.483 -40 80 -20 0.488 MAX5019 toc11 -20 4.42 CS THRESHOLD VOLTAGE vs. TEMPERATURE 179.0 -40 4.43 -40 179.5 13.50 4.44 TEMPERATURE (°C) 182.5 13.55 4.45 80 MAX5019 toc08 20 CS THRESHOLD VOLTAGE (V) -20 TEMPERATURE (°C) 0 V+ = 110V 4.46 4.40 -40 40 TEMPERATURE (°C) -20 V+ SHUTDOWN CURRENT (µA) -40 4.47 4.41 1.55 46.8 V+ INPUT CURRENT (µA) 1.60 VDD = FB = SS_SHDN = GND 4.48 MAX5019 toc09 47.4 FB = VDD = GND 1.61 4.49 MAX5019 toc12 47.6 1.62 4.50 SOFT-START SOURCE CURRENT (µA) 1.63 V+ SUPPLY CURRENT (mA) FB = GND MAX5019 toc05 47.8 MAXIMUM DUTY CYCLE (%) 1.64 MAX5019 toc04 48.0 190 1.0 2.400 188 -40 4 SOFT-START SOURCE CURRENT vs. TEMPERATURE V+ SUPPLY CURRENT vs. TEMPERATURE MAX5019 toc06 MAX5020 MAXIMUM DUTY CYCLE vs. TEMPERATURE NDRV RESISTANCE (Ω) MAX5019/MAX5020 Current-Mode PWM Controllers with Integrated Startup Circuit -20 0 20 40 TEMPERATURE (°C) 60 80 -40 -20 0 20 40 60 80 0 5 10 15 TEMPERATURE (°C) _______________________________________________________________________________________ 20 VDD (V) 25 30 35 40 Current-Mode PWM Controllers with Integrated Startup Circuit MAX5020 MAXIMUM DUTY CYCLE vs. VDD 269.5 269.0 268.5 FB = GND 268.0 47.8 VFB = 4V, CS = GND 47.7 47.6 47.5 DEVICE POWERED FROM VDD 47.4 47.3 20 25 30 35 9.5 0 40 5 10 15 20 25 30 35 0 5 10 15 VDD (V) VDD (V) 25 30 35 40 V+ SUPPLY CURRENT vs. V+ VOLTAGE (AFTER STARTUP) 1.59 1.58 VFB = VDD = GND 1.56 1.55 1.54 1.53 16 14 V+ LEAKAGE CURRENT (µA) MAX5019 toc16 1.60 1.57 20 VDD (V) V+ SUPPLY CURRENT vs. V+ VOLTAGE V+ SUPPLY CURRENT (mA) 40 MAX5019 toc17 15 DEVICE POWERED FROM V+ 12 VDD = 13V, FB = GND 10 8 6 4 2 1.52 0 1.51 0 20 40 60 80 0 10 20 30 40 50 60 70 80 90 100 110 100 V+ VOLTAGE (V) V+ VOLTAGE (V) VCC VOLTAGE vs. VCC CURRENT VCC VOLTAGE vs. VCC CURRENT V+ = 110V, VFB = 4V 10.2 VDD = 36V 10.0 MAX5019 toc18 10.4 9.8 VDD = 13V 9.6 VDD = GND, VFB = 4V 9.9 V+ = 110V V+ = 90V V+ = 72V V+ = 48V 9.8 VCC VOLTAGE (V) 10.0 MAX5019 toc19 10 VCC VOLTAGE (V) 5 FB = GND 9.8 9.6 47.0 0 9.9 9.7 DEVICE POWERED FROM V+ 47.1 267.0 DEVICE POWERED FROM VDD 10.0 47.2 267.5 10.1 VCC (V) 270.0 VCC vs. VDD 10.2 MAX5019 toc14 47.9 MAXIMUM DUTY CYCLE (%) 270.5 NDRV FREQUENCY (kHz) 48.0 MAX5019 toc13 271.0 MAX5019 toc15 NDRV FREQUENCY vs. VDD 9.7 9.6 9.5 V+ = 36V 9.4 V+ = 24V 9.3 9.4 9.2 9.2 9.1 9.0 9.0 0 5.0 10.0 15.0 VCC CURRENT (mA) 20.0 0 5.0 10.0 15.0 20.0 VCC CURRENT (mA) _______________________________________________________________________________________ 5 MAX5019/MAX5020 Typical Operating Characteristics (continued) (V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.) Current-Mode PWM Controllers with Integrated Startup Circuit MAX5019/MAX5020 Pin Description PIN NAME FUNCTION V+ High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects internally to a high-voltage linear regulator that generates VCC during startup. 2 VDD VDD is the Input of the Linear Regulator that Generates VCC. For supply voltages less than 36V, VDD and V+ can both be connected to the supply. For supply voltages greater than 36V, VDD receives its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass to GND with a 4.7µF capacitor. 3 FB Input of the Fixed-Gain Inverting Amplifier. Connect a voltage-divider from the regulated output to this pin. The noninverting input of the amplifier is referenced to 2.4V. 4 SS_SHDN Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 0.45ms/nF. This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The device goes into shutdown when SS_SHDN is pulled below 0.25V. 5 CS 6 GND Ground 7 NDRV Gate Drive. Drives a high-voltage external N-channel power MOSFET. 8 VCC 1 Current Sense Input. Turns power switch off if VCS rises above 465mV for cycle-by-cycle current limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM comparator through a leading-edge blanking circuit. Regulated IC Supply. Provides power for the entire IC. VCC is regulated from VDD during normal operation and from V+ during startup. Bypass VCC with a 10µF tantalum capacitor in parallel with 0.1µF ceramic capacitor to GND. Detailed Description Use the MAX5019/MAX5020 PWM current-mode controllers to design flyback- or forward-mode power supplies. Current-mode operation simplifies control-loop design while enhancing loop stability. An internal highvoltage startup regulator allows the device to connect directly to the input supply without an external startup resistor. Current from the internal regulator starts the controller. Once the tertiary winding voltage is established the internal regulator is switched off and bias current for running the IC is derived from the tertiary winding. The internal oscillator is set to 275kHz and trimmed to ±10%. This permits the use of small magnetic components to minimize board space. Both the MAX5019 and MAX5020 can be used in power supplies providing multiple output voltages. A functional diagram of the IC is shown in Figure 1. Typical applications circuits for forward and flyback topologies are shown in Figure 2 and Figure 3, respectively. For isolated flyback power supplies use the circuit of Figure 4. Current-Mode Control The MAX5019/MAX5020 offer current-mode control operation with added features such as leading-edge blanking with dual internal path that only blanks the 6 sensed current signal applied to the input of the PWM comparator. The current limit comparator monitors the CS pin at all times and provides cycle-by-cycle current limit without being blanked. The leading-edge blanking of the CS signal prevents the PWM comparator from prematurely terminating the on cycle. The CS signal contains a leading-edge spike that is the result of the MOSFET gate charge current, capacitive and diode reverse recovery current of the power circuit. Since this leading-edge spike is normally lower than the current limit comparator threshold, current limiting is not blanked and cycle-by-cycle current limiting is provided under all conditions. Use the MAX5019 in discontinuous flyback applications where wide line voltage and load current variation is expected. Use the MAX5020 for single transistor forward converters where the maximum duty cycle must be limited to less than 50%. Under certain conditions it may be advantageous to use a forward converter with greater than 50% duty cycle. For those cases use the MAX5019. The large duty cycle results in much lower operating primary RMS currents through the MOSFET switch and in most cases a smaller output filter inductor. The major disad- _______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit MAX5019/MAX5020 VDD VDD-OK V+ IN IN HIGHVOLTAGE REGULATOR GND EN BIAS WINDING REGULATOR OUT EN OUT 0.7V VCC MAX5019 ONLY UVLO SLOPE COMPENSATION 26mV/µs 6.6V 275kHz OSCILLATOR VCC R NDRV Q 26mV/µs 80%/50% DUTY CYCLE CLAMP 1MΩ FB S ∑ 50kΩ ILIM PWM 125mV CS ERROR AMP 5kΩ VCC SS_SHDN 70ns BLANKING 4µA 3R 2.4V BUF R 0.25V Figure 1. Functional Diagram _______________________________________________________________________________________ 7 MAX5019/MAX5020 Current-Mode PWM Controllers with Integrated Startup Circuit 1N4148 6 VIN (36V TO 72V) NT N 14 R CMHD2003 V+ VDD CDD 4.7µF VCC CCC 10µF MAX5020 NDRV CIN 3✕ 0.47µF NP 14 SBL204OCT NS 5 L1 4.7µH VOUT 5V/10A COUT 3✕ 560µF 20Ω 0.1µF 1nF M1 IRF640N R1 2kΩ CS 100Ω SS_SHDN CSS 0.1µF RSENSE 100mΩ GND FB R2 2kΩ CFB (OPTIONAL) Figure 2. Forward Converter vantage to this is that the MOSFET voltage rating must be higher and that slope compensation must be provided to stabilize the inner current loop. The MAX5019 provides internal slope compensation. Internal Regulators The internal regulators of the MAX5019/MAX5020 enable initial startup without a lossy startup resistor and regulate the voltage at the output of a tertiary (bias) winding to provide power for the IC. At startup V+ is regulated down to VCC to provide bias for the device. The VDD regulator then regulates from the output of the tertiary winding to VCC. This architecture allows the tertiary winding to only have a small filter capacitor at its output thus eliminating the additional cost of a filter inductor. When designing the tertiary winding calculate the number of turns so the minimum reflected voltage is always higher than 12.7V. The maximum reflected voltage must be less than 36V. To reduce power dissipation the high-voltage regulator is disabled when the VDD voltage reaches 12.7V. This greatly reduces power dissipation and improves efficiency. If V CC falls below the undervoltage lockout threshold (VCC = 6.6V), the low-voltage regulator is dis- 8 abled, and soft-start is reinitiated. In undervoltage lockout the MOSFET driver output (NDRV) is held low. If the input voltage range is between 13V and 36V, V+ and VDD may be connected to the line voltage provided that the maximum power dissipation is not exceeded. This eliminates the need for a tertiary winding. Undervoltage Lockout (UVLO), Soft-Start, and Shutdown The soft-start feature of the MAX5019/MAX5020 allows the load voltage to ramp up in a controlled manner, thus eliminating output voltage overshoot. While the part is in UVLO, the capacitor connected to the SS_SHDN pin is discharged. Upon coming out of UVLO an internal current source starts charging the capacitor to initiate the soft-start cycle. Use the following equation to calculate total soft-start time: tstartup = 0.45 ms × Css nF where CSS is the soft-start capacitor as shown in Figure 2. Operation begins when VSS_SHDN ramps above 0.6V. When soft-start has completed, VSS_SHDN is regulated _______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit V+ VDD VOUT CCC COUT CIN NP CDD VCC MAX5019 MAX5020 MAX5019/MAX5020 NT VIN NS M1 NDRV CS 100Ω SS_SHDN RSENSE R1 CSS GND FB R2 Figure 3. Nonisolated Flyback Converter NT VOUT VIN V+ VDD COUT CIN NP CDD NS R1 FB R2 MAX5019 MAX5020 VCC NDRV M1 CS CCC 100Ω RSENSE SS_SHDN GND CSS Figure 4. Isolated Flyback Converter to 2.4V, the internal voltage reference. Pull VSS_SHDN below 0.25V to disable the controller. Undervoltage lockout shuts down the controller when VCC is less than 6.6V. The regulators for V+ and the reference remain on during shutdown. Current-Sense Comparator The current-sense (CS) comparator and its associated logic limit the peak current through the MOSFET. Current is sensed at CS as a voltage across a sense resistor between the source of the MOSFET and GND. To reduce switching noise, connect CS to the external MOSFET source through a 100Ω resistor or an RC low- _______________________________________________________________________________________ 9 MAX5019/MAX5020 Current-Mode PWM Controllers with Integrated Startup Circuit pass filter (Figures 2, 3). Select the current-sense resistor, RSENSE according to the following equation: RSENSE = 0.465V / ILimPrimary where ILimPrimary is the maximum peak primary-side current. When VCS > 465mV, the power MOSFET switches off. The propagation delay from the time the switch current reaches the trip level to the driver turn-off time is 180ns. Internal Error Amplifier The MAX5019/MAX5020 include an internal error amplifier that can be used to regulate the output voltage in the case of a nonisolated power supply (see Figure 2). Calculate the output voltage using the following equation: R VOUT = 1+ 1 × VREF R2 where VREF = 2.4V. Choose R1//R2 << RIN, where RIN, ≅ 50kΩ is the input resistance of FB. The gain of the error amplifier is internally configured for -20 (see Figure 1). The error amplifier may also be used to regulate the output of the tertiary winding for implementing a primaryside regulated isolated power supply (see Figure 4). Calculate the output voltage using the following equation: VOUT = NS NT R1 1+ R × VREF 2 where NS is the number of secondary turns and NT is the number of tertiary winding turns. PWM Comparator and Slope Compensation An internal 275kHz oscillator determines the switching frequency of the controller. At the beginning of each cycle, NDRV switches the N-channel MOSFET on. NDRV switches the external MOSFET off after the maximum duty cycle has been reached, regardless of the feedback. The MAX5019 uses an internal ramp generator for slope compensation. The internal ramp signal is reset at the beginning of each cycle and slews at 26mV/µs. The PWM comparator uses the instantaneous current, the error voltage, the internal reference, and the slope compensation (MAX5019 only) to determine when to 10 switch the N-channel MOSFET off. In normal operation the N-channel MOSFET turns off when: IPRIMARY × RSENSE > VEA - VREF - VSCOMP where IPRIMARY is the current through the N-channel MOSFET, VREF is the 2.4V internal reference, VEA is the output voltage of the internal amplifier, and VSCOMP is a ramp function starting at 0 and slewing at 26mV/µs (MAX5019 only). When using the MAX5019 in a forward-converter configuration the following condition must be met to avoid control-loop subharmonic oscillations: NS k × RSENSE × VOUT × = 26mV / µs L NP where k = 0.75 to 1, and NS and NP are the number of turns on the secondary and primary side of the transformer, respectively. L is the output filter inductor. This makes the output inductor current downslope as referenced across RSENSE equal to the slope compensation. The controller responds to transients within one cycle when this condition is met. N-Channel MOSFET Gate Driver NDRV drives an N-channel MOSFET. NDRV sources and sinks large transient currents to charge and discharge the MOSFET gate. To support such switching transients, bypass VCC with a ceramic capacitor. The average current as a result of switching the MOSFET is the product of the total gate charge and the operating frequency. It is this current plus the DC quiescent current that determines the total operating current. Applications Information Design Example The following is a general procedure for designing a forward converter using the MAX5020. 1) Determine the requirements. 2) Set the output voltage. 3) Calculate the transformer primary to secondary winding turns ratio. 4) Calculate the reset to primary winding turns ratio. 5) Calculate the tertiary to primary winding turns ratio. 6) Calculate the current-sense resistor value. 7) Calculate the output inductor value. 8) Select the output capacitor. The circuit in Figure 2 was designed as follows: ______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit R VOUT ≅ VREF 1+ 1 R2 R1 // R2 << 50kΩ VREF = VSS_SHDN ≅ 2.4V where VREF is the reference voltage of the shunt regulator, and R1 and R2 are the resistors shown in Figures 2 and 3. 3) The turns ratio of the transformer is calculated based on the minimum input voltage and the lower limit of the maximum duty cycle for the MAX5020 (44%). To enable the use of MOSFETs with drain-source breakdown voltages of less than 200V use the MAX5020 with the 50% maximum duty cycle. Calculate the turns ratio according to the following equation: NS VOUT + (VD1 × DMAX ) ≥ NP DMAX × VIN_MIN where: NS/NP = Turns ratio (NS is the number of secondary turns and NP is the number of primary turns). VOUT = Output voltage (5V). VD1 = Voltage drop across D1 (typically 0.5V for power Schottky diodes). DMAX = Minimum value of maximum operating duty cycle (44%). VIN_MIN = Minimum Input voltage (36V). In this example: NS 5V + (0.5V × 0.44) ≥ = 0.330 0.44 × 36V NP Choose N P based on core losses and DC resistance. Use the turns ratio to calculate NS, rounding up to the nearest integer. In this example NP = 14 and NS = 5. For a forward converter choose a transformer with a magnetizing inductance in the neighborhood of 200µH. Energy stored in the magnetizing inductance of a forward converter is not delivered to the load and must be returned back to the input; this is accomplished with the reset winding. The transformer primary to secondary leakage inductance should be less than 1µH. Note that all leakage energy will be dissipated across the MOSFET. Snubber circuits may be used to direct some or all of the leakage energy to be dissipated across a resistor. To calculate the minimum duty cycle (DMIN) use the following equation: VOUT DMIN = NS VIN_MAX × N - VD1 P where VIN_MAX is the maximum input voltage (72V). 4) The reset winding turns ratio (NR/NP) needs to be low enough to guarantee that the entire energy in the transformer is returned to V+ within the off cycle at the maximum duty cycle. Use the following equation to determine the reset winding turns ratio: NR ≤ NP × 1-DMAX ′ DMAX ′ where: NR/NP = Reset winding turns ratio. DMAX’ = Maximum value of Maximum Duty Cycle. NR ≤ 14 × 1- 0.5 = 14 0.5 Round NR to the nearest smallest integer. The turns ratio of the reset winding (N R /N P ) will determine the peak voltage across the N-channel MOSFET. Use the following equation to determine the maximum drain-source voltage across the N-channel MOSFET: N VDSMAX ≥ VIN_MAX × 1 + P NR VDSMAX = Maximum MOSFET drain-source voltage. VIN_MAX = Maximum input voltage. ______________________________________________________________________________________ 11 MAX5019/MAX5020 1) 36V ≤ VIN ≤ 72V, VOUT = 5V, IOUT = 10A, VRIPPLE ≤ 50mV 2) To set the output voltage calculate the values of resistors R1 and R2 according to the following equation: MAX5019/MAX5020 Current-Mode PWM Controllers with Integrated Startup Circuit 14 VDSMAX ≥ 72V × 1 + = 144V 14 Choose MOSFETs with appropriate avalanche power ratings. 5) Choose the tertiary winding turns ratio (NT/NP) so that the minimum input voltage provides the minimum operating voltage at VDD (13V). Use the following equation to calculate the tertiary winding turns ratio: VDDMIN + 0.7 × NP ≤ NT ≤ VIN_MIN VDDMAX + 0.7 × NP VIN_MAX where: VDDMIN is the minimum VDD supply voltage (13V). VDDMAX is the maximum VDD supply voltage (36V). VIN_MIN is the minimum input supply voltage (36V). VIN_MAX is the maximum input supply voltage (72V in this design example). NP is the number of turns of the primary winding. NT is the number of turns of the tertiary winding. 13.7 36.7 × 14 ≤ NT ≤ × 14 36 72 5.33 ≤ NT ≤ 7.14 Choose NT = 6. 6) Choose RSENSE according to the following equation: RSENSE ≤ VILIM NS × 1.2 × IOUTMAX NP where: VILim is the current-sense comparator trip threshold voltage (0.465V). NS/NP is the secondary side turns ratio (5/14 in this example). IOUTMAX is the maximum DC output current (10A in this example). RSENSE ≤ 12 7) Choose the inductor value so that the peak ripple current (LIR) in the inductor is between 10% and 20% of the maximum output current. L≥ (VOUT + VD ) × (1- DMIN ) 2 × LIR × 275kHz × IOUTMAX where VD is the output Schottky diode forward voltage drop (0.5V). L≥ (5.5) × (1- 0.198) 0.4 × 275kHz × 10A = 4.01µH 8) The size and ESR of the output filter capacitor determine the output ripple. Choose a capacitor with a low ESR to yield the required ripple voltage. Use the following equations to calculate the peak-topeak output ripple: 2 2 VRIPPLE = VRIPPLE ,ESR + VRIPPLE,C where: VRIPPLE is the combined RMS output ripple due to V RIPPLE,ESR , the ESR ripple, and V RIPPLE,C , the capacitive ripple. Calculate the ESR ripple and capacitive ripple as follows: VRIPPLE,ESR = IRIPPLE x ESR VRIPPLE,C = IRIPPLE/(2 x π x 275kHz x COUT) Layout Recommendations All connections carrying pulsed currents must be very short, be as wide as possible, and have a ground plane as a return path. The inductance of these connections must be kept to a minimum due to the high di/dt of the currents in high-frequency switching power converters. Current loops must be analyzed in any layout proposed, and the internal area kept to a minimum to reduce radiated EMI. Ground planes must be kept as intact as possible. Chip Information TRANSISTOR COUNT: 589 PROCESS: BiCMOS 0.465V = 109mΩ 5 × 1.2 × 10 14 ______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit Power FETS Current-Sense Resistors Diodes Capacitors Magnetics International Rectifier www.irf.com Fairchild www.fairchildsemi.com Vishay-Siliconix www.vishay.com/brands/siliconix/main.html Dale-Vishay www.vishay.com/brands/dale/main.html IRC www.irctt.com/pages/index.cfm On Semi www.onsemi.com General Semiconductor www.gensemi.com Central Semiconductor www.centralsemi.com Sanyo www.sanyo.com Taiyo Yuden www.t-yuden.com AVX www.avxcorp.com Coiltronics www.cooperet.com Coilcraft www.coilcraft.com Pulse Engineering www.pulseeng.com MAX5019/MAX5020 Table 1. Component Manufacturers Selector Guide PART MAXIMUM DUTY CYCLE SLOPE COMPENSATION MAX5019CSA 85% Yes MAX5019ESA 85% Yes MAX5020CSA 50% No MAX5020ESA 50% No ______________________________________________________________________________________ 13 Current-Mode PWM Controllers with Integrated Startup Circuit SOICN.EPS MAX5019/MAX5020 Package Information 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 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.