LM5027

LM5027 Voltage Mode Active Clamp Controller August 18, 2009 LM5027 Voltage Mode Active Clamp Controller General Description The LM5027 pulse-width modulation (PWM) controller contains all of the features necessary to implement power converters utilizing the Active Clamp / Reset technique. With the active clamp technique, higher efficiencies and greater power densities can be realized compared to conventional catch winding or RDC clamp / reset techniques. Three control outputs are provided: the main power switch control (OUTA), the active clamp switch control (OUTB), and secondary side synchronous rectifier control (OUTSR). The timing between the control outputs is adjustable with external resistors that program internal precision timers. This controller is designed for high-speed operation including an oscillator frequency range up to 1 MHz and total PWM propagation delays less than 50 ns. The LM5027 includes a high-voltage startup regulator with a maximum input voltage rating of 105V. Additional features include Line Under Voltage Lockout (UVLO), separate softstart of main and synchronous rectifier outputs, a timer for hiccup mode current limiting, a precision reference, and thermal shutdown. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Voltage-Mode control Line feed-forward PWM ramp Internal 105V rated start-up bias regulator Programmable line Under-Voltage Lockout (UVLO) with adjustable hysteresis Versatile dual mode over-current protection Programmable volt-second limiter Programmable soft-start Programmable synchronous rectifier soft-start and stop Precise 500mV over-current comparator Current sense leading edge blanking Programmable oscillator with 1 MHz maximum frequency and synchronization capability Precision 5V reference Programmable time delays between outputs Packages ■ eTSSOP-20 Typical Application Circuit 30099001 Simplified Active Clamp Converter © 2009 National Semiconductor Corporation 300990 www.national.com LM5027 Connection Diagram 30099002 20-Lead eTSSOP Ordering Information Order Number LM5027MH LM5027MHX Package Type eTSSOP-20 eTSSOP-20 NSC Package Drawing MXA20A MXA20A Package Marking LM5027MH LM5027MH Supplied As 73 units per Rail 2500 Units per Tape and Reel Pin Descriptions Pin 1 Name VIN Description Input voltage source Application Information Input to the Start-up Regulator. Operating input range is 13V to 90V. The Absolute Maximum Rating is 105V. For power sources outside of this range, the LM5027 can be biased directly at VCC by an external regulator. An external RC circuit from VIN sets the PWM ramp slope. This pin is discharged at the conclusion of every cycle by an internal FET. An internal comparator terminates the PWM pulse if the RAMP pin exceeds 2.5V thus limiting the maximum volt-second product to the transformer primary. An external resistor sets the overlap delay for the active clamp output. The RTIME3 resistor connected between TIME3 and AGND sets the OUTA turn-off (falling edge) to OUTB turn-on (falling edge) pulse delay. See Fig. 9. An external resistor sets the overlap delay for the OUTSR output. The RTIME2 resistor connected between TIME2 and AGND sets the OUTA turn-off (falling edge) to OUTSR turn-on (rising edge) pulse delay. See Fig. 9. An external resistor sets the overlap delay for the active clamp output. The RTIME1 resistor connected between TIME1 and AGND sets the OUTB and OUTSR turn-off to OUTA turn-on pulse delay. See Fig. 9. Connect directly to Power Ground. 2 RAMP Feed-forward modulation ramp 3 TIME3 Overlap delay 3 4 TIME2 Overlap delay 2 5 TIME1 Overlap delay 1 6 7 AGND RT Analog ground Oscillator frequency control and sync Normally biased at 2V by an internal amplifier. An external resistor clock input connected between RT and AGND sets the internal oscillator frequency. The internal oscillator can be synchronized to an external clock with a frequency higher than the free running frequency set by the RT resistor. www.national.com 2 LM5027 Pin 8 Name COMP Description Input to the pulse width modulator Application Information An external opto-coupler connected to the COMP pin sources current into an internal NPN current mirror. The PWM duty cycle is at its maximum value with zero input current, while 1mA reduces the duty cycle to zero. The current mirror improves the frequency response by reducing the ac voltage across the opto-coupler detector transistor. Output of a 5V reference. Maximum output current is 10 mA. Locally decouple with a 0.1 µF capacitor. Control output of the active clamp PFET gate. Capable of 1A peak source and sink current. Control output of the main PWM NFET gate. Capable of 2A peak source and sink current. Control output of the secondary side synchronous rectifier FET gates. Capable of 3A peak source and sink current. Connect directly to Analog Ground Output of the internal high voltage start-up regulator. Regulated at 9.5V during start-up and 7.5V during run mode. If the auxiliary winding raises the voltage on this pin above the regulation set point, the internal startup regulator will shutdown, thus reducing the IC power dissipation. Current sense input for cycle-by-cycle current limiting. If the CS pin exceeds 500mV the output pulse will be terminated, entering cycle-bycycle current limit. An internal switch holds CS low for 100 ns after OUTA switches high to blank leading edge transients. An internal 22 µA current source charges an external capacitor to set the soft-start rate. If cycle-by-cycle current limit is reached during any cycle, a 22 µA current is sourced into the RES capacitor. If the RES capacitor voltage charges to 1.0V, a hiccup sequence is initiated. The SS and SSSR capacitors are discharged and the control outputs are disabled. The voltage on the RES capacitor is ramped between 4V and 2V eight times. After the eighth cycle, the SS capacitor is released and the normal start-up sequence begins. An external capacitor and an internal 25 µA current source sets the soft-start and soft-stop ramps for the synchronous rectifier output (OUTSR). The OTP comparator can be used for over-temperature shutdown protection with an external NTC thermistor voltage divider setting the shutdown temperature. The OTP comparator threshold is 1.25V. Hysteresis is set by an internal current source that sources 20 µA into the external resistor divider when the OTP pin voltage is above the threshold. An external voltage divider from the power source sets the shutdown and standby comparator levels. When UVLO reaches the 0.4V threshold, the VCC and REF regulators are enabled. When UVLO reaches the 2.0V threshold, the SS pin is released and the device enters the active mode. Hysteresis is set by an internal current source that pulls 20 µA from the external resistor divider when the UVLO pin is below the 2.0V threshold. No electrical contact to the LM5027 integrated circuit. Connect to system ground plane for reduced thermal resistance. 9 10 11 12 13 14 REF OUTB OUTA OUTSR PGND VCC Reference Output Output driver Output driver Output driver Power ground Start-up regulator output 15 CS Current sense input 16 17 SS RES Soft-start Input Restart timer 18 SSSR Soft-start for synchronous rectifier output. Over-Temperature Protection 19 OTP 20 UVLO Line under-voltage lockout EP Exposed pad, underside of package 3 www.national.com LM5027 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND VCC to GND UVLO to GND All other inputs to GND COMP Input Current COMP, REF (Note 2) -0.3V to 105V -0.3V to 16V -0.3 to 8V -0.3 to 7V 10 mA ESD Rating (Note 3) Human Body Model Storage Temperature Range Junction Temperature 2kV -55°C to 150°C 150°C (Note 1) 13 to 90V 8 to 15V -40°C to +125°C Operating Ratings VIN VCC Operating Junction Temperature Electrical Characteristics Symbol VIN SUPPLY Ibias VIN Operating Current VIN Shutdown Current VCC REGULATOR VCC Regulation VCC Current Limit VCC Regulator load regulation VCC Under-voltage Lockout Voltage VccReg VCC Regulation VCC Under-voltage Lockout Voltage VCC Supply Current (Icc) REFERENCE SUPPLY Reference Voltage Reference Voltage Regulation Reference Current Limit REF Under-voltage Threshold UVLO/OTP THRESHOLDS UVLO Threshold UVLO Hysteresis Current UVLO Shutdown Threshold UVLO Standby Enable Threshold OTP Shutdown Threshold OTP Hysteresis Current SOFT-START SS Charging Current Source SS Rising Threshold for SSSR charge current enable Parameter Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature range of –40°C to +125°C. Unless otherwise specified, the following conditions apply: VIN = 48V, VCC = 10V, RT= 27.4K , No Load on OUTA, OUTB and OUTSR unless otherwise stated. Conditions COMP and VCC Open, UVLO and OTP = 3V UVLO = 0V, Vin = 100 V No Load (SS0.4V, VCC >9.5V 3.8 1.9 UVLO ULVLO voltage falling UVLO voltage rising OTP rising OTP SS = 0V 1.21 15 17 3.8 18 14 1.5 Sinking 100 µA UVLO = 0 16 2 20 0.3 0.4 1.25 20 22 4 25 20 2.2 120 100 1.29 24 26 4.2 30 26 3.0 2.1 25 4.85 5.0 10 17.5 VccReg –180mV 7.3 5.7 9.1 55 310 9.5 65 75 VccReg – 100mV 7.5 6.0 7.7 6.3 5.5 Min Typ Max 6 650 9.9 Units mA µA V mA mV V V V mA 5.15 20 V mV mA V V µA V V V µA µA V µA µA V mV mV SSSR Charging Current Source SSSR = 0V, SS>4V SSSR Discharge Current Source in Soft Stop SSSR Falling Threshold for SS Soft Stop SS output low voltage SSSR output low voltage www.national.com 4 LM5027 Symbol OSCILLATOR Parameter Frequency1 Frequency2 Sync Threshold Sync Pulse Width Sync Frequency Range RT = 27.4 kΩ RT = 11 kΩ Conditions Min 180 420 15 160 Typ 200 490 2.85 Max 220 550 150 Units kHz kHz V ns kHz PWM COMPARATORS Delay to Output COMP to PWM Offset Duty Cycle Maximum CURRENT LIMIT RESTART (RES Pin) RES Threshold Charge Source Current Level 1 VRES < 1.0V Charge Source Current Level 2 4.0V < VRES > 1.0V Discharge Current Source Ratio of RES Threshold to SS Low CURRENT LIMIT Cycle by cycle sense voltage threshold CS prop Current limit propagation delay RAMP = 0 CS step from 0 to 0.6V time to onset of OUTA transition (90%) Cgate = 0pen RDS(ON) Delta RAMP measured from onset of OUTA to Ramp peak. Comp = 5V 2.3 450 500 30 550 mV ns VRES ramping down VRES > 1V, Hiccup counter 4 4 1.1 22 5.0 5 125 25 6.5 7 V µA µA µA OUTA, OUT_A = Tdelay_min 90 50 1.0 ns V % VOLTAGE FEED-FORWARD (RAMP Pin) RAMP Discharge Device VOLT-SECOND CLAMP Ramp Clamp Level OUTA GATE DRIVER VOL VOH OUTA Low-state Output Voltage IOUTA = 100 mA OUTA High-state Output Voltage OUTA Rise Time OUTA Fall Time IOHL IOLL VOL VOH Peak OUTA Source Current Peak OUTA Sink Current IOUTA = -100 mA, VOHL = VCC -VLO C-load = 1000 pF C-load = 1000 pF VOUTA = 0V (VCC = 10V) VOUTA = VCC= 10V 0.35 0.15 0.21 10 13 2 2 0.2 0.5 0.31 15 13 1 1 0.4 0.5 V V ns ns A A V V ns ns A A 2.5 2.6 V 5 Ω OUTB GATE DRIVER OUTB Low-state Output Voltage IOUTB = 100 mA OUTB High-state Output Voltage OUTB Rise Time OUTB High Side Fall Time Peak OUTB Source Current Peak OUTB Source Current IOUTB = -100 mA, VOHL = VCC -VLO C-load = 1000 pF C-load = 1000 pF VOUTB = 0V (VCC = 10V) VOUTB = VCC= 10V 5 www.national.com LM5027 Symbol OUTSR GATE DRIVER VOL VOH Parameter OUTSR Low-state Output Voltage OUTSR High-state Output Voltage OUTSR Rise Time OUTSR High Side Fall Time Conditions IOUTSR = 100 mA IOUTSR = -100 mA, VOHL = VCC -VLO C-load = 1000 pF C-load = 1000 pF VOUTSR = 0V (VCC = 10V) VOUTSR = VCC= 10V RTIME1=10 kΩ – 100 kΩ RTIME1 = 33.2 kΩ RTIME2 = 10 kΩ – 100 kΩ RTIME2 = 28.7 kΩ RTIME3 = 10 kΩ – 100 kΩ RTIME3 = 29.4 kΩ Min Typ 0.1 Max 0.2 Units V V ns ns A A 0.25 0.11 12 10 3 3 IOHH IOLH T1 T1 T2 T2 T3 T3 THERMAL tsd RJA RJC Peak OUTSR Source Current Peak OUTSR Source Current Delay Leading Range Delay Leading Accuracy Delay Trailing Range Delay Trailing Accuracy Delay Leading Range Delay Leading Accuracy Thermal Shutdown Temp. Thermal Shutdown Hysteresis Junction to Ambient Junction to Exposed Pad OUTPUT TIMING CONTROL 30 75 30 75 30 75 150 100 165 25 40 4 100 100 300 125 300 125 300 125 ns ns ns ns ns ns °C °C °C/W °C/W Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: It is not recommended that external power sources be connected to these pins. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. www.national.com 6 LM5027 Typical Performance Characteristics VCC and VREF vs. VIN VCC vs. ICC 30099024 30099025 VREF vs. IREF Oscillator Frequency vs RT Resistor 30099026 30099027 Oscillator Frequency vs Temperature Time1 Delay vs RTIME1 (kΩ) 30099028 30099029 7 www.national.com LM5027 Time2 Delay vs RTIME2 (kΩ) Time3 Delay vs RTIME3 (kΩ) 30099030 30099031 Time1 Delay vs Temperature RTIME1 = 33.2 kΩ Time2 Delay vs Temperature RTIME2 = 28.7 kΩ 30099032 30099033 Time3 Delay vs Temperature RTIME3 = 29.4 kΩ SS Pin Current vs Temperature 30099035 30099034 www.national.com 8 LM5027 SSSR Pin Charging Current vs Temperature RES Pin Charging Current Level 1 vs Temperature 30099036 30099037 9 www.national.com LM5027 Block Diagram 30099003 FIGURE 1. Simplified Block Diagram www.national.com 10 LM5027 Detailed Operating Description The LM5027 PWM controller contains all the features necessary to implement power converters utilizing the Active Clamp Reset technique with synchronous rectification. The device is configured to control a P-Channel clamp switch. With the active clamp technique higher efficiencies and greater power densities can be realized compared to conventional catch winding or RDC clamp / reset techniques. The LM5027 provides three gate driver outputs: one to drive the primary side MOSFET (OUTA), one for the active clamp P-Channel MOSFET (OUTB), and one output to drive the synchronous rectifier through an isolation interface (OUTSR). This controller is designed for high-speed operation including an oscillator frequency range up to 1 MHz and total PWM and current sense propagation delay less than 50 ns. The LM5027 includes a high-voltage start-up regulator that operates over a wide input range of 13V to 90V. Additional features include: Line UnderVoltage lockout (UVLO), soft-start/soft-stop, oscillator with synchronization capability, cycle-by-cycle current limit, hiccup mode fault protection with adjustable delay, precision reference, and thermal shutdown. old initiates a soft-stop sequence described later in this document. There is 100mV of hysteresis provided in the 0.4V shutdown comparator. When the VCC and REF outputs exceed their respective under-voltage thresholds and the UVLO pin voltage is greater than 2.0V and the OTP pin voltage is greater than 1.25V, the outputs are enabled and normal operation begins. An external set-point voltage divider from the VIN to GND can be used to set the minimum operating voltage of the converter. The divider must be designed such that the voltage at the UVLO pin will be greater than 2.0V when Vin is in the desired operating range. If the under-voltage threshold is not met, all three outputs are disabled. UVLO hysteresis is accomplished with an internal 20 µA current sink that is switched on or off into the impedance of the set-point divider. When the UVLO pin voltage exceeds 2.0V threshold, the current sink is deactivated to quickly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 2.0V threshold, the current sink is turned on causing the voltage at the UVLO pin to quickly fall . Reference The REF pin is the output of a 5V linear regulator that can be used to bias an opto-coupler transistor and external housekeeping circuits. The regulator output is internally current limited to 10 mA. High Voltage Start-Up Regulator The LM5027 contains an internal high voltage start-up regulator that allows the input pin (VIN) to be connected directly to the line voltage. The regulator output is internally current limited to 55mA. When the UVLO pin potential is greater than 0.4V, the VCC regulator is enabled to charge an external capacitor connected to the VCC pin. The VCC regulator provides power to the voltage reference (REF) and the gate drivers (OUTA, OUTB, and OUTSR). The controller outputs are enabled when the voltage on the VCC pin reaches the regulation point of 9.5V, the internal voltage reference (REF) reaches its regulation point of 5V, the UVLO pin voltage is greater than 2V, and the OTP pin voltage is greater than 1.25V. The outputs will remain enabled unless one of the following conditions occurs, VCC falls below 6.0V, UVLO is below 2.0 V, or the OTP pin falls below 1.25V. The value selected for the VCC capacitor depends on the total system design and the start-up characteristics. The recommended capacitance range for the VCC regulator is 0.1 µF to 100 µF. In a typical application, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding must raise the VCC voltage above the VCC regulation set point to shut off the internal start-up regulator. The LM5027 lowers the VCC regulation set point from 9.5V to 7.5V after the output of the first OUTSR drive pulse. Powering VCC from an auxiliary winding improves efficiency while reducing the controller power dissipation. When the converter auxiliary winding is inactive, external current draw on the VCC line should be limited so the power dissipation in the start-up regulator does not exceed the maximum power dissipation of the LM5027 package. An external start-up regulator or other bias rail can be used instead of the internal start-up regulator by connecting the VCC and the VIN pins together and feeding the external bias into the two pins. Cycle-by-Cycle Current Limit The CS pin is to be driven by a signal representative of the transformer primary current. If the voltage on the CS pin exceeds 0.5V, the current sense comparator terminates the output driver pulse, with the duty cycle determined by the current sense comparator instead of the PWM comparator. A small R-C filter connected to the CS pin and located near the controller is recommended to suppress noise. An internal 5Ω MOSFET discharges the external current sense filter capacitor at the conclusion of every cycle. The discharge MOSFET remains on for an additional 30 ns after either OUTA driver switches high to blank leading edge transients in the current sensing circuit. Discharging the CS pin filter each cycle and blanking leading edge spikes reduces the filtering requirements and improves the current sense response time. The current sense comparator is very fast and may respond to short duration noise pulses. Layout considerations are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be placed very close to the device and connected directly to the CS and AGND pins. If a current sense transformer is used, both leads of the transformer secondary should be routed to the filter network, which should be located close to the IC. When designing with a current sense resistor, all of the noise sensitive low power ground connections should be connected together near the AGND pin, and a single connection should be made to the power ground (sense resistor ground point). Restart Time Delay (Hiccup Mode) The LM5027 provides a current limit restart timer to disable the outputs and force a delayed restart (hiccup mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current limit events required to trigger the restart is programmable by the external capacitor at the RES pin. During each PWM cycle, the LM5027 either sources or sinks current from the RES pin capacitor. If no current limit is detected during a cycle, a 5 µA current sink is enabled to pull the RES pin to ground. If a current limit is detected, the 5 µA current sink is disabled and a 22 µA current source causes Line Under-Voltage Detector The LM5027 contains a dual level line Under Voltage Lock Out (UVLO) circuit. When the UVLO pin voltage is greater than 0.4V but less than 2.0V, the controller is in a standby mode. In the standby mode the VCC and REF bias regulators are active while the controller outputs are disabled. This feature allows the UVLO pin to be used as a remote enable/ disable function. Pulling the UVLO pin below the 2.0V thresh11 www.national.com LM5027 the voltage at the RES pin to gradually increase. If the RES voltage reaches the 1.0V threshold, the following restart sequence occurs (also see Figure 2). • The SS and SSSR capacitors are fully discharged. • The RES 20 µA current source is turned-off and the 5 µA current source is turned-on. • The voltage on the RES pin is allowed to charge up to 4.0V. • When voltage on the RES pin reaches 4.0V the 5 µA current source is turned-off and a 5 µA current sink is turned-on, ramping the voltage on the RES capacitor down to 2.0V. • The RES capacitor voltage is ramped between 4.0V and 2.0V eight times. • When the counter reaches eight, the RES pin voltage is pulled low and the Soft-Start capacitor is released to begin a soft-start sequence. The SS capacitor voltage slowly increases. When the SS voltage reaches 1.0V, the PWM comparator will produce the first narrow output pulse at OUTA. • When the SS voltage reaches 4.0V the capacitor on the SSSR pin is released and is charged with a 25 µA current source, soft-starting the free-wheeling synchronous rectifier. • If the overload condition persists after restart, cycle-by-cycle current limiting will begin to increase the voltage on the RES capacitor again, repeating the hiccup mode sequence. • If the overload condition no longer exists after restart, the RES pin will be held at ground by the 5 µA current sink and normal operation resumes. 30099004 FIGURE 2. Restart and Soft-Start Delay Timing www.national.com 12 LM5027 Soft-Start The soft-start circuit allows the regulator to gradually increase its output voltage until the steady state operating point is reached; thereby reducing start-up stresses and surge currents. When bias power is supplied to the LM5027, the SS pin capacitor is discharged by an internal MOSFET. When the voltages on the UVLO, OTP, VCC, and REF pins reach the operating thresholds, the soft-start capacitor is released and is charged with a 22 µA current source. When the SS pin voltage reaches 1V, output pulses commence with a slowly increasing duty cycle (refer to Figure 3, Figure 4 and Figure 5). The voltage on the SS pin eventually increases to 5V, while the voltage at the PWM comparator is limited to the level required for regulation as determined by the voltage feedback loop via the COMP pin. When the soft-start voltage reaches 4.0V, the capacitor on the SSSR pin is released and charged with a 25 µA current source (refer to Figure 3, Figure 4 and Figure 5) . When the SSSR pin voltage reaches approximately 2.5V (refer to Figure 6), the internal synchronous rectifier PWM circuit gradually increases the synchronous rectifier drive duty cycle (OUTSR) in proportion the rising SSSR pin voltage. Delaying the start of the SSSR gate drive pulses until after the main soft-start is completed allows the output voltage to reach regulation before the synchronous rectifiers begins operation. This delay prevents the synchronous rectifier from sinking current from the output in applications where the output voltage may be pre-biased. Soft-Stop If the UVLO pin voltage falls below the 2.0V standby threshold, but above the 0.4V shutdown threshold, the synchronous rectification soft-start capacitor is discharged with a 20 µA current source which gradually disables the synchronous rectifiers (refer to Figure 4). After the SSSR capacitor has been discharged to 2.0V the soft-start and synchronous rectification soft-start capacitors are quickly discharged to ground to terminate PWM pulses at OUTA ,OUTB, and OUTSR. The PWM pulses may cease before the SSSR voltage reduces the synchronous rectifier duty cycle if the VCC or REF voltage drops below the respective under-voltage thresholds during the soft-stop process. This soft-stop method of turning off the converter prevents oscillations in the synchronous rectifiers during a shutdown sequence. 30099005 FIGURE 3. Soft-Start Timing 13 www.national.com LM5027 30099006 FIGURE 4. Soft-Start/Soft-Stop Timing 30099007 FIGURE 5. Soft-Start and Drive Enable www.national.com 14 LM5027 30099008 FIGURE 6. Soft-Start Synchronous Rectifier Timing External Over-Temperature Protection An external set-point voltage divider between the REF, OTP, and AGND pins, as shown in Figure 7, is one method to implement over-temperature protection shutdown. Typically a NTC thermistor is installed as the lower device of the voltage divider. The divider must be designed such that the voltage at the OTP pin will be greater than 1.25V when there is not an over-temperature condition, and the OTP pin must drop below 1.25V during an over-temperature event. OTP hysteresis is accomplished with an internal 20 µA current source that is switched on or off into the impedance of the external setpoint divider. When the OTP pin voltage exceeds 1.25V threshold, the current source is activated to quickly raise the voltage at the OTP pin. When the OTP pin voltage falls below the 1.25V threshold, the current source is turned off causing the voltage at the OTP pin to quickly fall. When OTP falls below 1.25V the LM5027 will go through a soft-stop turn-off sequence. 30099009 FIGURE 7. External OTP Protection Fault/Events Summary Table Table 1 is a Truth Table which describes the faults and events that control the LM5027 drive outputs. For example the first event is with UVLO being pulled below 0.4V, a possible remote shutdown condition. When this occurs the SS, and SSSR capacitors are discharged, and the three drive outputs will stop switching (outputs low). The last fault is if the OTP pin is pulled low 6.2V UVLO OTP >1.25V >1.25V SS Fast discharge Fast discharge after SSSR4.0V Fast discharge after SSSR6.2V >2.0V >2.0V >2.0V >1.25V >1.25V - Fast discharge Slow discharge Low Switching Low Low Switching Low Low Low Low PWM Comparators The pulse width modulator (PWM) comparator compares the voltage ramp signal at the RAMP pin to the loop error signal. The loop error signal is received from the external feedback and isolation circuit in the form of a control current into the matched pair of NPN transistors which sink current through a 5 kΩ resistor connected to the 5V reference. The resulting control voltage is compared at the PWM input to a 1V level shifted ramp signal. An opto-coupler detector can be connected directly between the REF pin and the COMP pin. Since the COMP pin is a current mirror input, the potential difference across the opto-coupler detector is nearly constant. The bandwidth limiting phase delay which is normally introduced by the significant capacitance of the opto-coupler is thereby greatly reduced. Higher loop bandwidths can be realized since the bandwidth-limiting pole associated with the optocoupler is now at a much higher frequency. The PWM comparator polarity is configured such that with no current into the COMP pin, the controller produces maximum duty cycle at the main gate drive output (OUTA). Feed-Forward Ramp An external resistor (RFF) and capacitor (CFF) connected to VIN, AGND, and the RAMP pins is required to create the PWM ramp signal as shown in Figure 8. The slope of the signal at the RAMP pin will vary in proportion to the input line voltage. This varying slope provides line feed-forward information necessary to improve line transient response with voltage mode control. The RAMP signal is compared to the error signal by the pulse width modulator comparator to control the duty cycle of the outputs. With a constant error signal, the on-time (tON) varies inversely with the input voltage (VIN) to stabilize the volt-second product of the transformer primary. The power path gain of the conventional voltage-mode pulse with modulator (oscillator generated ramp) varies directly with input voltage. The use of a line generated ramp (input voltage feed-forward) nearly eliminates the gain variation. As a result, the feedback loop is only required to make very small corrections for large changes in input voltage. At the end of each clock period, an internal MOSFET with an RDS(ON) of 10Ω (typical) is enabled to reset the CFF capacitor voltage to ground. www.national.com 16 LM5027 30099010 FIGURE 8. Feed-Forward Voltage Mode Configuration Volt-Second Clamp An external resistor (RFF) and a capacitor (CFF) connected between the VIN, RAMP and AGND pins is required to create a saw-tooth modulation ramp signal as shown in Figure 8. The slope of the RAMP will vary in proportion to the input line voltage. Varying the PWM ramp slope inversely with the input voltage provides line feed-forward information necessary to improve line transient response with voltage mode control. With a constant error signal, the on time (tON) varies inversely with the input voltage (VIN) to stabilize the volt-second product of the transformer primary. The volt-second clamp compares the ramp signal (RAMP) to a fixed 2.5V reference. By proper selection of RFF and CFF, the maximum on-time of the main switch can be set to the desired duration. An example will illustrate the use of the voltsecond clamp comparator to achieve a 90% duty cycle limit at 200 kHz and 18V line input: A 90% duty cycle at 200 kHz requires a 4.5 µs on-time. At 18V input the volt-second product is 81µs (18V x 4.5 µs). To achieve this clamp level: RFF x CFF = VIN x tON / 2.5V 18V x 4.5 µs/ 2.5V = 32.4µ Select CFF = 470 pF RFF = 68.9 kΩ (closest standard value 68.1 kΩ) The recommended capacitor value range for CFF is 100 pF to 1000 pF. The CFF ramp capacitor is discharged at the conclusion of every cycle by an internal discharge switch controlled by either the PWM comparator, the CS comparator or by the volt-second clamp comparator, which ever occurs first. Oscillator and Sync Capability The LM5027 oscillator frequency is set by the external resistor connected between the RT pin and the ground (AGND). To set a desired oscillator frequency, the necessary RT resistor is calculated from: For example, if the desired oscillator frequency is 200 kHz, a 27.4 kΩ resistor would be the nearest standard one percent value. The RT resistor should be located as close as possible to the IC and connected directly to the pins (RT and AGND). The tolerance of the external resistor and the frequency tolerance indicated in the Electrical Characteristics must be taken into account when determining the worst case operating frequency. The LM5027 can be synchronized to an external clock by applying a narrow pulse to the RT pin. The external clock must be at least 10% higher than the free-running oscillator frequency set by the RT resistor. If the external clock frequency is less than the RT resistor programming frequency, the LM5027 will ignore the synchronizing pulses. The synchronization pulse should be coupled to the RT pin through a 100 pF capacitor with a pulse width of 15 ns to 150 ns. When a synchronizing pulse transitions from low-to-high (rising edge), the voltage at the RT pin must be driven to exceed 3.2V from its nominal 2.85V level. During the clock signal low time, the voltage at the RT pin will be clamped at 2V by an internal regulator. The output impedance of the RT regulator is approximately 100Ω. The RT resistor is always required, whether the oscillator is free running or externally synchronized. 17 www.national.com LM5027 Gate Drive Outputs The LM5027 contains three unique gate drivers. OUTA, the primary switch driver, is designed to drive the gate of an NChannel MOSFET and is capable of sourcing and sinking a peak current of 2A. The active clamp drive, OUTB, is designed to drive a P-Channel MOSFET and is capable of sourcing and sinking peak currents of 1A. The third driver, OUTSR, is designed to drive the gate of a synchronous rectification MOSFET through a gate drive transformer. OUTSR gate driver has a source and sink capability of 3A. Thermal Protection Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum rated junction temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power standby state with the output drivers (OUTA, OUTB, and OUTSR), the bias regulators (VCC and REF) disabled. The thermal protection feature is provided to prevent catastrophic failures from accidental device overheating. During a restart, after thermal shutdown, the soft-start capacitors (SS and SSSR) are fully discharged and the controller follows a normal start-up sequence after the junction temperature falls below the Thermal Shutdown hysteresis threshold (typically 145°C). Driver Delay Timing The three independent time delay adjustments allow a great deal of flexibility for the user to optimize the efficiency of the system. The active clamp output (OUTB) is in phase with the main output (OUTA), with the active clamp output overlapping the main output. The overlap time provides dead-time between the operation of the main switch and the P-Channel active clamp switch at both the rising and falling edges. The rising edge control is set by a resistor from the TIME1 pin to the AGND pin. The falling edge control is set with a resistor from the TIME3 pin to the AGND pin. The rising edge of the PWM comparator output coincides with the rising edge of OUTB and the falling edge of the OUTSR output without delay, as shown in Figure 9. The rising edge control for turning on the OUTSR output after the main output (OUTA) has turned off is set with a resistor from the TIME2 pin to the AGND pin. VIN The voltage applied to the VIN pin, normally the same as the system voltage applied to the power transformer’s primary (VPWR), can vary in the range of the 13 to 90V with transient capability of 105V. The current into VIN depends primarily on the output driver capacitive loads, the switching frequency, and any external loads on the VCC pin. If the power dissipation associated with the VIN current exceeds the package capability, an external voltage should be applied to the VCC pin (see Figure 10 ) to disable the internal start-up regulator. The VCC regulation set point voltage is initially internally regulated to 9.5V. After the first OUTSR pulse, the VCC set point voltage is reduced to 7.5V. If an external voltage is applied to the VCC pin the required range is 8V to 15V . The VIN to VCC series pass regulator includes a parasitic diode between VIN and VCC. This diode should not be forward biased in normal operation. The VCC voltage should never exceed the VIN voltage. It is recommended the circuit of Figure 10 be used to suppress transients which may occur at the input supply, in particular where VIN is operated close to the maximum operating rating of the LM5027. 30099012 FIGURE 10. Start-up Regulator Power Reduction 30099011 FIGURE 9. LM5027 Driver Output Timing www.national.com 18 LM5027 For Applications > 100V For applications where the system input voltage (VPWR) exceeds 100V, VIN can be powered from an external start-up regulator as shown in Figure 11. Connecting the VIN and VCC pins together allows the LM5027 to be operated with VIN be- low 13V. To turn-off the internal start-up regulator the VCC voltage must be raised above 9.9V. The voltage at the VCC pin must not exceed 15V. The voltage source at the right side of Figure 11 is typically derived from the power stage, and becomes active when the LM5027 outputs are active. 30099014 FIGURE 11. Start-up Regulator for VPWR > 100V UVLO The under-voltage lockout threshold (UVLO) is internally set to 2.0V at the UVLO pin. With two external resistors as shown in Figure 12, the LM5027 is enabled when VPWR causes the UVLO pin to exceed threshold voltage of 2V. When VPWR is below the threshold, the internal 20 µA current sink is enabled to reduce the voltage at the UVLO pin. When the UVLO pin voltage exceeds the 2V threshold, the 20 µA current sink is turned off causing the UVLO voltage to increase and providing hysteresis. The values of R1 and R2 can be determined from the following equation: R1 = VHYS/20 µA Where VHYS is the desired UVLO hysteresis at VPWR, and VPWR in the second equation is the turn-on voltage. For example, if the LM5027 is to be enabled when VPWR reaches 34V, and the hysteresis is 1.8V, then R1 is 90 kΩ and 5.6 kΩ. For this application R1 was selected to be 90.0 kΩ, R2 was selected to be 6.19 kΩ. The LM5027 can be remotely shutdown by taking the UVLO pin below 0.4V with an external open collector or open drain device, as shown in Figure 12. The outputs and the VCC regulator are disabled in shutdown mode. 19 www.national.com LM5027 30099016 FIGURE 12. UVLO Circuit with Shutdown Control Oscillator The oscillator frequency is generally selected in conjunction with the system magnetic components and any other aspects of the system which may be affected by the frequency. The RT resistor selection equation is specified in the Oscillator and Sync Capability section. If the required frequency tolerance is critical in particular application, the tolerance of the external resistor and the frequency tolerance specified in the Electrical Characteristics table must be considered when selecting the RT resistor. Voltage Feedback The COMP pin is designed to accept a current input typically from an opto-coupler. A typical configuration is shown in Figure 13, where the emitter of the opto-coupler transistor is connected to the COMP pin and the collector is connected to the REF pin of the LM5027. When the output voltage is below regulation, no current flows into the COMP pin and the LM5027 operates at maximum duty cycle. At the secondary side, VOUT is compared to a reference by the error amplifier which has an appropriate frequency compensation network. The amplifier output drives the opto-coupler, which in turn drives the LM5027 COMP pin current mirror. 30099017 FIGURE 13. Typical COMP Configuration www.national.com 20 LM5027 Current Sense The CS pin receives an input signal representative of the transformer primary current, either from a current sense transformer or from a resistor in series with the source of the primary switch, as shown in Figure 14 and Figure 15. In both cases the sensed current creates a ramping voltage across RSENSE, and the RF/CF filter suppresses noise and leading edge transients. The filtering components RSENSE, RF and CF should be physically as close to the LM5027 as possible. The current sense components must be scaled for 0.5V at the CS pin when an over-current condition exists. If the voltage on the CS pin reaches 0.5V, the present cycle will be immediately terminated. If the over-load event continues and the RES pin reaches 1V, the soft-start capacitor is discharged and the LM5027 will go through an auto re-start (Hiccup Mode). The Hiccup Mode time is set by the capacitor on the RES pin. 30099018 FIGURE 14. Transformer Current Sense 21 www.national.com LM5027 30099019 FIGURE 15. Resistor Current Sense Soft-Start The capacitor on the SS pin determines the time required for the output duty cycle to increase from zero to its final value for regulation. The minimum acceptable time is dependent on the output capacitance and the response of the feedback loop that controls the COMP pin. If the soft-start time is too quick, the output could significantly overshoot its intended voltage before the feedback loop has a chance to regulate the PWM controller. After power is applied and VCC has passed its upper UV threshold (9.5V), the voltage at the SS pin ramps up as the external capacitor is charged with an internal 20 μA current source. The voltage at the internal PWM comparator input node follows the voltage at the SS pin. When the voltage on the SS pin has reached 1.0V, PWM pulses appear at the drive output with a very low duty cycle. The voltage at the SS pin eventually increases to approximately 5.0V. The voltage at the input to the PWM comparator and the PWM duty cycle increase to the value required for regulation as determined by the voltage regulation loop. Hiccup Mode Current Limit Restart Hiccup mode operation is described in the Restart Time Delay (Hiccup Mode) section. In the case of continuous current limit detection at the CS pin, the time required to reach the 1.0V RES pin threshold is: For example, if CRES = 0.047 µF the time tCS in Figure 16 is approximately 2.14 ms. After the voltage on the RES pin reaches 1.0V, the 22 µA current source is turned-off and a 5 µA current source is turned-on. The Hiccup Mode time is: where ΔV is 2.0V. With a CRES = 0.047 µF, the Hiccup Mode time is 179 ms. After the Hiccup Mode off time is complete, the RES pin voltage is pulled low and soft-start capacitor is released allowing a soft-start sequence to commence. The soft-start time trestart is set by the internal 22 µA current source, and is equal to: If CSS = 0.1 µF, trestart is = 6.41 ms www.national.com 22 LM5027 The hiccup mode provides the periodic cool-down time for the power converter in the event of a sustained overload or short circuit. This off time results in lower average input current and lower power dissipation within the power components. 30099023 FIGURE 16. Hiccup Mode Timing Printed Circuit Board Layout The LM5027 Current Sense and PWM comparators are very fast and respond to short duration noise pulses. The components at the CS, COMP, SS, UVLO, TIME1, TIME2, and TIME3 pins should be physically close as possible to the IC, thereby minimizing noise pickup on the PC board trace inductance. Layout consideration is critical for the current sense filter. If a current sense transformer is used, both leads of the transformer secondary should be routed to the sense filter components and to the IC pins. The ground side of the transformer should be connected via a dedicated PC board trace to the AGND pin, rather than through the ground plane. If the current sense circuit employs a sense resistor in the drive transistor source, low inductance resistors should be used. In this case, all the noise sensitive, low-current ground trace should be connected in common near the IC, and then a single connection made to the power ground (sense resistor ground point). The gate drive outputs of the LM5027 should have short, direct paths to the power MOSFETs in order to minimize inductance in the PC board. The two ground pins (AGND, PGND) must be connected together with a short, direct connection, to avoid jitter due to relative ground bounce. Application Circuit Example The following schematic shows an example of the LM5027 controlling a 100W active clamp forward power converter. The input voltage range (VPWR) is 36V to 76V, and the output voltage is 3.3V. The output current capability is 30 Amps. Current sense transformer T3 provides information to the CS pin for current limit protection. The error amplifier and reference U3 and U5 provide voltage feedback via opto-coupler U2. Synchronous rectifiers Q3-Q6 minimize rectification losses in the secondary. An auxiliary winding on the power transformer T1 provides power to the LM5027 VCC pin when the output is in regulation. The input UVLO levels are 34V for increasing VPWR, and approximately 32V for decreasing VPWR. The circuit can be shutdown by forcing the ON/OFF input J2 below 0.4V. An external synchronizing frequency can be applied to the Oscillator input. The converter output current limit is limited at 32A. Special care was taken in this design such that the converter will turn on with the output pre-biased without sinking current from the output. More information is available in AN-1976. 23 www.national.com LM5027 www.national.com 24 30099039 LM5027 Physical Dimensions inches (millimeters) unless otherwise noted 20 Lead eTSSOP NS Package Number MXA20A 25 www.national.com LM5027 Voltage Mode Active Clamp Controller Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage Reference PowerWise® Solutions Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero PowerWise® Design University Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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