LM5071MTX-50

LM5071 Power Over Ethernet PD Controller with Auxiliary Power Interface November 2005 LM5071 Power Over Ethernet PD Controller with Auxiliary Power Interface General Description The LM5071 power interface port and pulse width modulation (PWM) controller provides a complete integrated solution for Powered Devices (PD) that connect into Power over Ethernet (PoE) systems. The LM5071 is specifically designed for the PD that must accept power from auxiliary sources such as ac adapters. The auxiliary power interface of the LM5071 activates the PWM controller when the ac adapter is connected to power the PD when PoE network power is unavailable. The LM5071 integrates an 80V, 400mA line connection switch and associated control for a fully IEEE 802.3af compliant interface with a full featured current mode pulse width modulator dc-dc converter. All power sequencing requirements between the controller interface and switch mode power supply (SMPS) are integrated into the IC. n Detection Resistor Disconnect Function n Programmable Classification Current n Programmable Under-voltage Lockout with Programmable Hysteresis n Thermal Shutdown Protection n Auxiliary Power Enable Pin n Current Mode Pulse Width Modulator n Supports both Isolated and Non-Isolated Applications n Error Amplifier and Reference for Non-Isolated Applications n Programmable Oscillator Frequency n Programmable Soft-start n 80% Maximum Duty Cycle Limiter, Slope Compensation (-80 device) n 50% Maximum Duty Cycle Limiter, No Slope Compensation (-50 device) Features n Compatible with 12V ac adapters n Fully Compliant 802.3af Power Interface Port n 80V, 1Ω, 400 mA Internal MOSFET Packages n TSSOP-16 Block Diagram 20168401 © 2005 National Semiconductor Corporation DS201684 www.national.com LM5071 Block Diagram (Continued) 20168402 FIGURE 1. Simplified Block Diagram Connection Diagram 20168403 16 Lead TSSOP Ordering Information Order Number LM5071MT-50 LM5071MTX-50 LM5071MT-80 LM5071MTX-80 Description 50% Duty Cycle Limit 50% Duty Cycle Limit 80% Duty Cycle Limit 80% Duty Cycle Limit NSC Package Type / Drawing TSSOP-16/MTC-16 TSSOP-16/MTC-16 TSSOP-16/MTC-16 TSSOP-16/MTC-16 Supplied As 92 units per rail 2500 units on tape and reel 92 units per rail 2500 units on tape and reel www.national.com 2 LM5071 Pin Descriptions Pin 1 2 Name VIN RSIG Description System high potential input. Signature resistor pin. Application Information The diode “OR” of several lines entering the PD, it is the more positive input potential. Connect a resistor from VIN to this pin for signature detection. The resistor is in parallel with the UVLO resistors and should be valued accordingly. Connect the classification programming resistor from this pin to VEE. A resistor divider between the AUX voltage input to VEE programs the startup levels with a 2.5V threshold. A high value ( > 300kΩ) internal pull down resistor is present to pull the pin low if it is left open. In practice, the divider voltage should be set well above 2.5V by the programming resistors. An external resistor divider from VIN to UVLORTN programs the shutdown levels with a 2.00V threshold at the UVLO pin. Hysteresis is set by a switched internal 10uA current source that forces additional current into the resistor divider. Connect the bottom resistor of the resistor divider between the UVLO pin and this pin. Diode “OR’d” to the RJ45 connector and PSE’s –48V supply, it is the more negative input potential. The drain of the internal current limiting power MOSFET which connects VEE to the return path of the dc-dc converter. DC-DC converter gate driver output with 800mA peak sink current capability. When the auxiliary transformer winding (if used) raises the voltage on this pin above the regulation set point, the internal series pass regulator will shutdown, reducing the controller power dissipation. Inverting input of the internal error amplifier. The non-inverting input is internally connected to a 1.25V reference. COMP pull-up is provided by an internal 5K resistor which may be used to bias an opto-coupler transistor. Current sense input for current mode control and over-current protection. Current limiting is accomplished using a dedicated current sense comparator. If the CS pin voltage exceeds 0.5V the OUT pin switches low for cycle-by-cycle current limiting. CS is held low for 50ns after OUT switches high to blank leading edge current spikes. An external resistor connected from RT to ARTN sets the oscillator frequency. This pin will also accept narrow ac-coupled synchronization pulses from an external clock. An external capacitor and an internal 10uA current source set the soft-start ramp rate. RTN for sensitive analog circuitry including the SMPS current limit amplifier. 3 4 RCLASS AUX Classification resistor pin. Auxiliary input power startup pin. 5 UVLO Line under-voltage lockout. 6 7 8 9 10 UVLORTN Return for the external UVLO resistors. VEE RTN OUT VCC System low potential input. System return for the PWM converter. Output of the PWM controller. Output of the internal high voltage series pass regulator. Regulated output voltage is nominally 7.8V. Feedback signal. The output of the error amplifier and input to the Pulse Width Modulator. Current sense input. 11 12 13 FB COMP CS 14 RT / SYNC Oscillator timing resistor pin and synchronization input. SS ARTN Soft-start input. Analog PWM supply return. 15 16 3 www.national.com LM5071 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 ,RTN to VEE RSIG to VIN AUX to VEE UVLO RCLASS to VEE ARTN to RTN VCC, OUT to ARTN All other inputs to ARTN -0.3V to 80V -12V to 0V -0.3V to 57V -0.3V to 13V -0.3V to 7V -0.3V to 0.3V -0.3V to 16V -0.3V to 7V ESD Rating Human Body Model Lead Temperature (Note 2) Wave (4 seconds) Infrared (10 seconds) Vapor Phase (75 seconds) 2000V 260˚C 240˚C 219˚C Operating Ratings VIN voltage External voltage applied to VCC 1.8V to 60V 8.1V to 15V Electrical Characteristics (Note 3) Specifications in standard type face are for TJ= +25˚C and those in boldface type apply over the full operating junction temperature range. Unless otherwise specified: VIN = 48V, VCC = 10V, RT = 30.3kΩ. Symbol Powered Interface IOS VCLSS(ON) VCLSS(OFF) ICLASS IDC Offset Current Signature Resistor Disable / Classification Current Turn On Classification Current Turn Off Classification Voltage Supply Current During Classification Supply Current During Normal Operation UVLO Pin Reference Voltage UVLO Hysteresis Current Softstart Release Softstart Release Hysteresis RDS(ON) ILEAK PowerFET Resistance SMPS Bias Current AUX Pin Threshold AUX Pin Threshold Hysteresis ZAUX IINRUSH ILIM ILIM Startup Regulator VinMin VccReg Operational VIN Input Voltage VCC Regulation VCC Current Limit AUX = 5V Open ckt (Note 4) 9.5 7.5 15 7.8 20 8.1 V V mA AUX Pin Input Impedance Inrush Current Limit DC Current Limit DC Current Limit VIN < 10.0V VIN with respect to VEE VIN with respect to VEE With respect to VEE VIN =17V OUT floating VIN > 27V VIN > UVLO RTN falling with respect to VEE RTN rising with respect to VEE I = 350mA, VIN = 48V VEE = 0V, VIN = RTN = 57V AUX pin rising with respect to VEE AUX pin falling with respect to VEE AUX = 0.5V VEE = 0V, RTN = 3.0V VEE = 0V, RTN = 3.0V, Temp = 0˚C to 85˚C VEE = 0V, RTN = 3.0V, Temp = -40˚C to 125˚C 70 350 325 2.4 0.4 2.5 0.5 350 100 390 390 130 420 420 1.95 8.0 1.2 0.8 10.0 23.5 1.43 11.5 25.0 1.5 0.5 1 2.00 10 1.45 1.1 1 10 12.5 26.5 1.57 1.5 1.9 2.05 11.5 1.7 1.3 2.2 100 2.65 0.6 uA V V V mA mA V uA V V Ω uA V V kΩ mA mA mA Parameter Conditions Min Typ Max Units www.national.com 4 LM5071 Electrical Characteristics (Note 3) Symbol VCC Supply VCC UVLO (Rising) Parameter (Continued) Specifications in standard type face are for TJ= +25˚C and those in boldface type apply over the full operating junction temperature range. Unless otherwise specified: VIN = 48V, VCC = 10V, RT = 30.3kΩ. Conditions Min Typ Max Units VccReg VccReg – – 100mV 300mV 5.9 Cload = 0 6.25 1.5 4 75 FB = COMP FB=1.5V COMP=1V CS step from 0 to 0.6V, time to onset of OUT transition (90%) 0.44 1.219 1.212 5 20 20 0.5 55 25 7 175 505 10 200 580 3.1 COMP set to 2V CS stepped 0 to 0.4V, time to onset of OUT transition low COMP=0V 80 50 0.33 4.5 COMP= 0V Delta increase at PWM Comparator to CS Iout = 50mA, VCC - VOUT Iout = 100mA Cload = 1nF Cload = 1nF 0.6 5.4 1.1 105 6.3 1.5 V mA mV 25 55 13 225 665 3.8 0.56 1.281 1.288 6.6 3 V mA MHz dB V mA ns V ns Ω uA KHz KHz V ns VCC UVLO (Falling) Supply Current (Icc) Error Amplifier GBW Gain Bandwidth DC Gain Input Voltage COMP Sink Capability Current Limit ILIM Delay to Output Cycle by Cycle Current Limit Threshold Voltage Leading Edge Blanking Time CS Sink Impedance (clocked) Softstart Softstart Current Source Oscillator(Note 5) Frequency1 (RT = 30.3K) Frequency2 (RT = 10.5K) Sync threshold PWM Comparator Delay to Output Min Duty Cycle Max Duty Cycle (-80 Device) Max Duty Cycle (-50 Device) COMP to PWM Comparator Gain COMP Open Circuit Voltage COMP Short Circuit Current Slope Compensation Slope Comp Amplitude (LM5071-80 Device Only) Output Section Output High Saturation Output Low Saturation Rise time Fall time 0 % % % 0.25 0.25 15 15 0.75 0.75 V V ns ns 5 www.national.com LM5071 Electrical Characteristics (Note 3) Symbol Thermal Shutdown Tsd Thermal Shutdown Hysteresis Thermal Resistance θJA Junction to Ambient Thermal Shutdown Temp. Parameter (Continued) Specifications in standard type face are for TJ= +25˚C and those in boldface type apply over the full operating junction temperature range. Unless otherwise specified: VIN = 48V, VCC = 10V, RT = 30.3kΩ. Conditions Min Typ 165 25 Max Units ˚C ˚C MT Package 125 ˚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. The absolute maximum rating of VIN, RTN to VEE is derated to (-0.3V to 76V) at -40˚C. Note 2: For detailed information on soldering the plastic TSSOP package, refer to the Packaging Databook available from National Semiconductor. Note 3: Min and Max limits are 100% production tested at 25 ˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). Note 4: Device thermal limitations may limit usable range. Note 5: Specification applies to the oscillator frequency. The operational frequency of the LM5071-50 devices is divided by two. www.national.com 6 LM5071 Typical Performance Characteristics Default Current Limit vs Temperature Oscillator Frequency vs RT Resistance 20168413 20168409 UVLO Hysteresis Current vs Temperature Softstart Current vs Temperature 20168408 20168415 Error Amp Input Voltage vs temperature Oscillator Frequency vs Temperature RT = 15.2 kΩ 20168417 20168414 7 www.national.com LM5071 Typical Performance Characteristics VCC vs ICC (Continued) Input Current vs Input Voltage 20168407 20168412 UVLO Threshold vs Temperature 20168406 www.national.com 8 LM5071 Specialized Block Diagrams 20168404 FIGURE 2. Top Level Block Diagram 20168405 FIGURE 3. PWM Controller Block Diagram 9 www.national.com LM5071 Detailed Operating Description The LM5071 power interface port and pulse width modulation (PWM) controller provides a complete integrated solution for Powered Devices (PD) that connect into Power over Ethernet (PoE) systems. Major features of the PD interface portion of the IC include detection, classification, thermal limit, programmable undervoltage lockout, and current limit monitoring. The device also includes a high-voltage start-up bias regulator that operates over a wide input range up to 60V. The switch mode power supply (SMPS) control portion of the IC includes power good sensing, VCC regulator undervoltage lockout, cycle-by-cycle current limit, error amplifier, slope compensation, softstart, and oscillator sync capability. This high speed BiCMOS IC has total propagation delays less than 100ns and a 1MHz capable oscillator programmed by a single external resistor. The LM5071 PWM controller provides current-mode control for dc-dc converter topologies requiring a single drive output, such as Flyback and Forward topologies. The LM5071 PWM enables all of the advantages of current-mode control including line feed-forward, cycle-bycycle current limit and simplified loop compensation. The oscillator ramp is internally buffered and added to the PWM comparator input ramp to provide slope compensation necessary for current mode control at duty cycles greater than 50% (-80 suffix only). age, the internal power MOSFET is enabled to deliver a constant current to charge the input capacitor of the dc-dc converter. When the MOSFET Vds voltage falls below 1.5V, the internal Power Good signal enables the SMPS controller. The LM5071 is specified to operate with an input voltage as high as 60.0V. The SMPS controller and internal MOSFET are disabled when VIN falls to the lower UVLO threshold. Detection Signature To detect a potential powered device candidate, the PSE (Power Sourcing Equipment) will apply a voltage from 2.8V to 10V across the input terminals of the PD. The voltage can be of either polarity so a diode barrel network is required on both lines to ensure this capability. The PSE will take two measurements, separated by at least 1V and 2ms of time. The voltage ramp between measurement points will not exceed 0.1V/us. The delta voltage / delta current calculation is then performed; if the detected impedance is above 23.75kΩ and below 26.25kΩ, the PSE will consider a PD to be present. If the impedance is less than 15kΩ or greater than 33kΩ a PD will be considered not present and will not receive power. Impedances between these values may or may not indicate the presence of a valid PD. The LM5071 will enable the signature resistor at a controller input voltage of 1.5V to take into account the diode voltage drops. An external signature resistor should be placed between the VIN and RSIG pins. The signature resistor is in parallel with the external UVLO resistor divider, and its value should be calculated accordingly. Targeting 24.5kΩ increases margin in the signature design as the input bridge rectifier diodes contribute to the series resistance measured at the PD input terminals. The PSE will tolerate no more than 1.9V of offset voltage (caused by the external diodes) or more than 10uA of offset current (bias current). The input capacitance must be greater than 0.05uF and less than 0.12uF. To increase efficiency, the signature resistor is disabled by the LM5071 controller once the input voltage is above the detection range ( > 11V). Modes of Operation The LM5071 PD interface is designed to provide a fully compliant IEEE 802.3af system. As such, the modes of operation take into account the barrel rectifiers often utilized to correctly polarize the dc input from the Ethernet cable. Table 1 shows the LM5071 operating modes and associated input voltage range. TABLE 1. Operating Modes With Respect to Input Voltage Input Voltage VIN wrt VEE 1.8V to 10.0V 12.5V to 25.0V 25.0V to UVLO Rising Vth 60V to UVLO Falling Vth Mode of Operation Detection (Signature) Classification Awaiting Full Power Normal Powered Operation Classification To classify the PD, the PSE will present a voltage between 14.5V and 20.5V to the PD. The LM5071 enables classification mode at a nominal input voltage of 11.5V. An internal 1.5V linear regulator and an external resistor connected to the RCLASS pin provide classification programming current. Table 2 shows the external classification resistor required for a particular class. The classification current flows through the IC into the classification resistor. The suggested resistor values take into account the bias current flowing into the IC. A different desired RCLASS can be calculated by dividing 1.5V by the desired classification current. Per the IEEE 802.3af specification, classification is optional, and the PSE will default to class 0 if a valid classification current is not detected. If PD classification is not desired (i.e., Class 0), simply leave the RCLASS pin open. The classification time period may not last longer than 75ms as per IEEE 802.3af. The LM5071 will remain in classification mode until VIN is greater than 25V. An external signature resistor is connected to VEE when VIN exceeds 1.8V, initiating detection mode. During detection mode, quiescent current drawn by the LM5071 is less than 10uA. Between 10.0V and 12.5V, the device enters classification mode and the signature resistor is disabled. The nominal range for classification mode is 11.5V to 25.0V. The classification current is turned off once the classification range voltage is exceeded, to reduce power dissipation. Between 25.0V and UVLO release, the device is in a standby state, awaiting the input voltage to reach the operational range to complete the power up sequence. Once the VIN voltage increases above the upper UVLO threshold volt- www.national.com 10 LM5071 Classification (Continued) TABLE 2. Classification Levels and Required External Resistors Class 0 1 2 3 4 PMIN 0.44W 0.44W 3.84W 6.49W Reserved PMAX 12.95W 3.84W 6.49W 12.95W Reserved ICLASS ICLASS RCLASS (MIN) (MAX) 0mA 9mA 17mA 26mA 36mA 4mA 12mA 20mA 30mA 44mA Open 150Ω 82.5Ω 54.9Ω 38.3Ω of the UVLO set point divider. When the UVLO threshold is exceeded, the current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 2.00V threshold, the current source is turned off, causing the voltage at the UVLO pin to fall. The LM5071 UVLO thresholds cannot be programmed lower than 25V, the AUX pin should be used to force UVLO release below 25V. There are many additional uses for the UVLO pin. The UVLO function can also be used to implement a remote enable / disable function. Pulling the UVLO pin down below the UVLO threshold disables the interface and SMPS controller unless forced on via AUX pin operation. Undervoltage Lockout (UVLO) The IEEE 802.3af specification states that the PSE will supply power to the PD within 400ms after completion of detection. The LM5071 contains a programmable line Under Voltage Lock Out (UVLO) circuit. The first resistor should be connected between the VIN to UVLO pins; the bottom resistor in the divider should be connected between the UVLO and UVLORTN pins. The divider must be designed such that the voltage at the UVLO pin equals 2.0V when VIN reaches the desired minimum operating level. If the UVLO threshold is not met, the interface control and SMPS control will remain in standby. UVLO hysteresis is accomplished with an internal 10uA current source that is switched on and off into the impedance AUX Pin Operation The AUX pin can be used to force operation (UVLO release) of the interface and switching regulator at any input voltage above 9.5V. This is especially useful for auxiliary input (wall transformer) input voltages. The pin has a 2.5V threshold (0.5V hysteresis) and an input impedance of approximately 350kΩ. The input resistor provides a defined pull down impedance if the pin is left open by the user. An external pull down resistor should be used to provide additional noise immunity. The resultant pin voltage from the external resistor divider should be well above the 2.5V threshold to ensure proper auxiliary operation. See Figure 4 for an example of a simple yet robust auxiliary configuration. 20168422 FIGURE 4. Simplified Schematic Showing Auxiliary Implementation 11 www.national.com LM5071 Power Supply Operation Once the UVLO threshold has been satisfied, the interface controller of the LM5071 will charge up the SMPS input capacitor through the internal power MOSFET. This load capacitance provides input filtering for the power converter section and must be at least 5uF per the IEEE 802.3af specification. To accomplish the charging in a controlled manner, the power MOSFET is current limited to 100mA. The SMPS controller will not initiate operation until the load capacitor is completely charged. The power sequencing between the interface circuitry and the SMPS controller occurs automatically within the LM5071. Detection circuitry monitors the RTN pin to detect interface startup completion. When the RTN pin potential drops below 1.5V with respect to VEE, the VCC regulator of the SMPS controller is enabled. The softstart function is enabled once the VCC regulator achieves minimum operating voltage. The inrush current limit only applies to the initial charging phase. The interface power MOSFET current limit will revert to the default protection current limit of 390mA once the SMPS is powered up and the soft-start pin sequence begins. tance range for the VCC regulator output is 0.1uF to 10uF. When the voltage on the V CC pin reaches the regulation point of 7.8V, the controller output is enabled. The controller will remain enabled until VCC falls below 6.25V. In typical applications, a transformer auxiliary winding is diode connected to the VCC pin. This winding should raise the VCC voltage above 8.1V to shut off the internal startup regulator. Though not required, powering VCC from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller. The external VCC capacitor must be selected such that the capacitor maintains the VCC voltage greater than the VCC UVLO falling threshold (6.25V) during the initial start-up. During a fault condition when the converter auxiliary winding is inactive, external current draw on the VCC line should be limited such that the power dissipated in the start-up regulator does not exceed the maximum power dissipation capability of the LM5071 package. If the VCC auxiliary winding is used with a low voltage auxiliary supply (wall transformer), the VCC pin could back feed through the LM5071 to the VIN pin. A diode from VCC to VIN should be used to clamp the VCC pin and prevent this internal back feed. The winding voltage will remain the same and extra power will be dissipated in the series resistor. Also, note that when using a very low voltage auxiliary supply, a diode from the AUX supply to the VCC pin should be used to ensure VCC startup. High Voltage Start-up Regulator The LM5071 contains an internal high voltage startup regulator that allows the input pin (VIN) to be connected directly to line voltages as high as 60V. The regulator output is internally current limited to 15mA. The recommended capaci- 20168423 FIGURE 5. Simplified Schematic Showing Low Voltage Auxiliary Supply www.national.com 12 LM5071 Error Amplifier An internal high gain error amplifier is provided within the LM5071. The amplifier’s non-inverting reference is set to a fixed reference voltage of 1.25V. The inverting input is connected to the FB pin. In non-isolated applications, the power converter output is connected to the FB pin via voltage scaling resistors. Loop compensation components are connected between the COMP and FB pins. For most isolated applications the error amplifier function is implemented on the secondary side of the converter and the internal error amplifier is not used. The internal error amplifier is configured as an open drain output and can be disabled by connecting the FB pin to ARTN. An internal 5K pull-up resistor between a 5V reference and COMP can be used as the pull-up for an optocoupler in isolated applications. LM5071-50: Current Limit / Current Sense The LM5071 provides a cycle-by-cycle over current protection function. Current limit is accomplished by an internal current sense comparator. If the voltage at the current sense comparator input CS exceeds 0.5V with respect to RTN/ ARTN, the output pulse will be immediately terminated. A small RC filter, located near the CS pin of the controller, is recommended to filter noise from the current sense signal. The CS input has an internal MOSFET which discharges the CS pin capacitance at the conclusion of every cycle. The discharge device remains on an additional 50ns after the beginning of the new cycle to attenuate the leading edge spike on the current sense signal. The LM5071 current sense and PWM comparators are 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 located very close to the device and connected directly to the pins of the controller (CS and ARTN). If a current sense transformer is used, both leads of the transformer secondary should be routed to the sense resistor and the current sense filter network. A sense resistor located in the source of the primary power MOSFET may be used for current sensing, but a low inductance resistor is required. When designing with a current sense resistor, all of the noise sensitive low power ground connections should be connected together local to the controller and a single connection should be made to the high current power return (sense resistor ground point). The LM5071 can also be synchronized to an external clock. The external clock must have a higher frequency than the free running oscillator frequency set by the RT resistor. The clock signal should be capacitively coupled into the RT pin with a 100pF capacitor. A peak voltage level greater than 3.7 volts at the RT pin is required for detection of the sync pulse. The sync pulse width should be set between 15 to 150ns by the external components. The RT resistor is always required, whether the oscillator is free running or externally synchronized. The voltage at the RT pin is internally regulated to a 2 volts. The RT resistor should be located very close to the device and connected directly to the pins of the controller (RT and ARTN). PWM Comparator / Slope Compensation The PWM comparator compares the current ramp signal with the loop error voltage derived from the error amplifier output. The error amplifier output voltage at the COMP pin is offset by 1.4V and then further attenuated by a 3:1 resistor divider. The PWM comparator polarity is such that 0 Volts on the COMP pin will result in zero duty cycle at the controller output. For duty cycles greater than 50 percent, current mode control circuits are subject to sub-harmonic oscillation. By adding an additional fixed slope voltage ramp signal (slope compensation) to the current sense signal, this oscillation can be avoided. The LM5071-80 integrates this slope compensation by summing a current ramp generated by the oscillator with the current sense signal. Additional slope compensation may be added by increasing the source impedance of the current sense signal (with an external resistor between the CS pin and current sense resistor). Since the LM5071-50 is not capable of duty cycles greater than 50%, there is no slope compensation feature in this device. Oscillator, Shutdown and Sync Capability A single external resistor connected between the RT and ARTN pins sets the LM5071 oscillator frequency. Internal to the LM5071–50 device (50% duty cycle limited option) is an oscillator divide by two circuit. This divide by two circuit creates an exact 50% duty cycle clock which is used internally to create a precise 50% duty cycle limit function. Because of this divide by two, the internal oscillator actually operates at twice the frequency of the output (OUT). For the LM5071–80 device the oscillator frequency and the operational output frequency are the same. To set a desired output operational frequency (F), the RT resistor can be calculated from: LM5071-80: Softstart The softstart feature allows the power converter to gradually reach the initial steady state operating point, thereby reducing start-up stresses, output overshoot and current surges. At power on, after the VCC undervoltage lockout threshold is satisfied, an internal 10µA current source charges an external capacitor connected to the SS pin. The capacitor voltage will ramp up slowly and will limit the COMP pin voltage and the duty cycle of the output pulses. Gate Driver and Maximum Duty Cycle Limit The LM5071 provides an internal gate driver (OUT), which can source and sink a peak current of 800mA. The LM5071 is available in two duty cycle limit options. The maximum output duty cycle is typically 80% for the LM5071-80 option and precisely equal to 50% for the LM5071-50 option. The maximum duty cycle function for the LM5071-50 is accom13 www.national.com LM5071 Gate Driver and Maximum Duty Cycle Limit (Continued) plished with an internal toggle flip-flop which ensures an accurate duty cycle limit. The internal oscillator frequency of the LM5071-50 is therefore twice the operating frequency of the PWM controller (OUT pin). The 80% maximum duty cycle limit of the LM5071-80 is determined by the internal oscillator and varies more than the 50% limit of the LM5071-50. For the LM5071-80, the internal oscillator frequency and the operational frequency of the PWM controller are equal. perature is exceeded. This feature prevents catastrophic failures from accidental device overheating. When activated, typically at 165 degrees Celsius, the controller is forced into a low power standby state, disabling the output driver, bias regulator, main interface pass MOSFET, and classification regulator if enabled. After the temperature is reduced (typical hysteresis = 25˚C ) the VCC regulator will be enabled and a softstart sequence initiated. Thermal shutdown is not enabled during auxiliary power operation as the power MOSFET is not running any current and should not experience an over-temperature condition. If the drain of the MOSFET exceeds 2.5V with respect to VEE (internal Power Good de-assertion), PoE UVLO becomes de-asserted (insertion of PoE or other 48V supply), or the auxiliary power is removed, thermal limit will be re-enabled immediately. Thermal Protection Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction tem- www.national.com 14 LM5071 Application Circuit Diagrams LM5071 15 20168424 FIGURE 6. Single Isolated Output with Diode Rectification www.national.com LM5071 (Continued) www.national.com 20168425 LM5071 Application Circuit Diagrams 16 FIGURE 7. Dual Isolated Output with Diode Rectification LM5071 Application Circuit Diagrams (Continued) LM5071 17 20168426 FIGURE 8. Non-Isolated Output Buck with Diode Rectification www.national.com LM5071 Power Over Ethernet PD Controller with Auxiliary Power Interface Physical Dimensions inches (millimeters) unless otherwise noted Package Number MTC16 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. 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