LTC4269CDKD-2-TRPBF

FeaTures n n n n LTC4269-2 IEEE 802.3at High Power PD and Synchronous Forward Controller with AUX Support DescripTion The LTC®4269-2 is an integrated Powered Device (PD) interface and power supply controller featuring 2-event classification signaling, flexible auxiliary power options, and a power supply controller suitable for synchronously rectified forward supplies. These features make the LTC4269-2 ideally suited for an IEEE802.3at PD application. The PD controller features a 100V MOSFET that isolates the power supply during detection and classification, and provides 100mA inrush current limit. Also included are power good outputs, an undervoltage/overvoltage lockout and thermal protection. The current mode forward controller allows for synchronous rectification, resulting in an extremely high efficiency, green product. Soft-start for controlled output voltage start-up and fault recovery is included. Programmable frequency over 100kHz to 500kHz allows flexibility in efficiency vs size and low EMI. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Hot Swap, ThinSOT are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. n n n n n n n 25.5W IEEE 802.3at Compliant (Type-2) PD PoE+ 2-Event Classification IEEE 802.3at High Power Available Indicator Integrated State-of-the-Art Synchronous Forward Controller – Isolated Power Supply Efficiency >94% Flexible Auxiliary Power Interface Superior EMI Performance Robust 100V 0.7Ω (Typ) Integrated Hot Swap™ MOSFET Integrated Signature Resistor, Programmable Class Current, UVLO, OVLO and Thermal Protection Short-Circuit Protection with Auto-Restart Programmable Switching Frequency from 100kHz to 500kHz Thermally Enhanced 7mm × 4mm DFN Package applicaTions n n n IP Phones with Large Color Screens Dual Radio 802.11n Access Points PTZ Security Cameras Typical applicaTion PoE-Based Self-Driven Synchronous Forward Power Supply 1mH • 10µH 6.8µH • 2.2µF 33k VCC 0.1µF 10µF 133 SD_VSEC PWRGD VIN VPORTP SOUT 10k 4.7nF + • 5.1 + 5V 5A 220µF 10µF 237k 10.0k + 54V FROM DATA PAIR ~+ ~– ~+ ~– 0.1µF 30.9 OUT 54V FROM SPARE PAIR OC ISENSE VCC 1.5k 33k 22k 1.2k 22.1k 0.1µF 0.22µF TLV431 50m 4.7nF 2k 10nF 11.3k RCLASS SHDN VPORTN LTC4269-2 COMP FB VREF T2P VNEG PGND GND BLANK DELAY ROSC SS_MAXDC 82k 332k 158k 158k 3.65k 42692 TA01 TO MICROCONTROLLER 42692fb  LTC4269-2 absoluTe MaxiMuM raTings (Notes 1, 2) pin conFiguraTion TOP VIEW SHDN 1 T2P 2 RCLASS 3 NC 4 VPORTN 5 VPORTN 6 NC 7 NC 8 COMP 9 FB 10 ROSC 11 SYNC 12 SS_MAXDC 13 VREF 14 SD_VSEC 15 GND 16 32 VPORTP 31 NC 30 NC 29 PWRGD 28 PWRGD 27 VNEG 26 VNEG 25 NC 24 SOUT 23 VIN 22 OUT 21 PGND 20 DELAY 19 OC 18 ISENSE 17 BLANK Pins with Respect to VPORTN VPORTP Voltage ........................................ –0.3V to 100V VNEG Voltage ......................................... –0.3V to VPORTP VNEG Pull-Up Current ..................................................1A SHDN ....................................................... –0.3V to 100V RCLASS, Voltage ........................................... –0.3V to 7V RCLASS Source Current ..........................................50mA PWRGD Voltage (Note 3) Low Impedance Source .... VNEG – 0.3V to VNEG + 11V Sink Current .........................................................5mA PWRGD, T2P Voltage ............................... –0.3V to 100V PWRGD, T2P Sink Current .....................................10mA Pins with Respect to GND VIN (Note 4)................................................ –0.3V to 25V SYNC, SS_MAXDC, SD_VSEC, ISENSE, OC .................................................... –0.3V to 6V COMP, BLANK, DELAY .............................. –0.3V to 3.5V FB ................................................................ –0.3V to 3V ROSC Current ........................................................ –50µA VREF Source Current ..............................................10mA Operating Ambient Temperature Range LTC4269C-2 ............................................. 0°C to 70°C LTC4269I-2 ..........................................–40°C to 85°C 33 DKD PACKAGE 32-LEAD (7mm 4mm) PLASTIC DFN TJMAX = 125°C, θJA = 34°C/W, θJC = 2°C/W EXPOSED PAD (PIN 33) MUST BE SOLDERED TO HEAT SINKING PLANE THAT IS CONNECTED TO GND orDer inForMaTion LEAD FREE FINISH LTC4269CDKD-2#PBF LTC4269IDKD-2#PBF LEAD BASED FINISH LTC4269CDKD-2 LTC4269IDKD-2 TAPE AND REEL LTC4269CDKD-2#TRPBF LTC4269IDKD-2#TRPBF TAPE AND REEL LTC4269CDKD-2#TR LTC4269IDKD-2#TR PART MARKING* 42692 42692 PART MARKING* 42692 42692 PACKAGE DESCRIPTION 32-Lead (7mm × 4mm) Plastic DFN 32-Lead (7mm × 4mm) Plastic DFN PACKAGE DESCRIPTION 32-Lead (7mm × 4mm) Plastic DFN 32-Lead (7mm × 4mm) Plastic DFN TEMPERATURE RANGE 0°C to 70°C –40°C to 85°C TEMPERATURE RANGE 0°C to 70°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 42692fb  LTC4269-2 elecTrical characTerisTics PARAMETER Interface Controller (Note 5) Operating Input Voltage Signature Range Classification Range On Voltage Undervoltage Lockout Overvoltage Lockout ON/UVLO Hysteresis Window Signature/Class Hysteresis Window Reset Threshold Supply Current Supply Current at 57V Class O Current Signature Signature Resistance Invalid Signature Resistance, SHDN Invoked Invalid Signature Resistance During Mark Event Classification Class Accuracy Classification Stability Time Normal Operation Inrush Current Power FET On-Resistance Power FET Leakage Current at VNEG Digital Interface SHDN Input High Level Voltage SHDN Input Low Level Voltage SHDN Input Resistance PWRGD, T2P Output Low Voltage PWRGD, T2P Leakage Current PWRGDP Output Low Voltage PWRGDP Clamp Voltage PWRGDP Leakage Current PWM Controller (Note 11) Operational Input Voltage VIN Quiescent Current VIN Start-Up Current VIN Shutdown Current SD_VSEC Threshold IVREF = 0µA IVREF = 0µA, ISENSE = OC = Open FB = 0V, SS_MAXDC = 0V (Notes 12, 13) SD_VSEC = 0V (Notes 12, 13) 10V < SD < 25V l l l l l The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. CONDITIONS At VPORTP (Note 6) l l l l l l MIN TYP MAX 60 9.8 21.0 37.2 UNITS V V V V V V V V 1.5 12.5 30.0 4.1 1.4 2.57 71.0 State Machine Reset for 2-Event Classification Measured at VPORTP Pin VPORTP = 17.5V, No RCLASS Resistor 1.5V ≤ VPORTP ≤ 9.8V (Note 7) 1.5V ≤ VPORTP ≤ 9.8V, VSHDN = 3V (Note 7) (Notes 7, 8) 10mA < ICLASS < 40mA, 12.5V < VPORTP < 21V (Notes 9, 10) VPORTP Pin Step to 17.5V, RCLASS = 30.9Ω, ICLASS within 3.5% of Ideal Value (Notes 9, 10) VPORTP = 54V, VNEG = 3V Tested at 600mA into VNEG, VPORTP = 54V VPORTP = SHDN = VNEG = 57V l 5.40 1.35 0.40 V mA mA kΩ kΩ kΩ % ms l l l l l 23.25 26.00 11 11 ±3.5 1 l l l l l 60 100 0.7 180 1.0 1 mA Ω µA V 3 0.45 100 0.15 1 0.4 12.0 16.5 1 V kΩ V µA V V µA VPORTP = 9.8V, SHDN = 9.65V Tested at 1mA, VPORTP = 57V, For T2P, Must Complete 2-Event Classification to See Active Low Pin Voltage Pulled 57V, VPORTP = VPORTN = 0V Tested at 0.5mA, VPORTP = 52V, VNEG = 4V, Output Voltage is with Respect to VNEG Tested at 2mA, VNEG = 0V, Voltage is with Respect to VNEG VPWRGD = 11V, VNEG = 0V, Voltage is with Respect to VNEG l l l l l l VIN(OFF) 5.2 460 240 1.261 1.32 25 6.5 700 350 1.379 V mA µA µA V 42692fb  LTC4269-2 elecTrical characTerisTics PARAMETER SD_VSEC(ON) Current SD_VSEC(OFF) Current VIN(ON) VIN(OFF) VIN(HYST) VREF Output Voltage Line Regulation Load Regulation Oscillator Frequency, fOSC fOSC(MIN) fOSC(MAX) SYNC Input Resistance SYNC Switching Threshold SYNC Frequency/fOSC fOSC Line Regulation VROSC Error Amplifier FB Reference Voltage FB Input Bias Current Open-Loop Voltage Gain Unity-Gain Bandwidth COMP Source Current COMP Sink Current COMP Current (Disabled) COMP High Level VOH COMP Active Threshold COMP Low Level VOL Current Sense ISENSE Maximum Threshold ISENSE Input Current (Duty Cycle = 0%) ISENSE Input Current (Duty Cycle = 80%) OC Threshold OC Input Current Default Blanking Time Adjustable Blanking Time VBLANK SOUT Driver SOUT Clamp Voltage SOUT Low Level IGATE = 0µA, COMP = 2.5V, FB = 1V IGATE = 25mA 10.54 12 0.5 13.5 0.75 V V 42692fb The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. CONDITIONS SD_VSEC = SD_VSEC Threshold +100mV SD_VSEC = SD_VSEC Threshold – 100mV l l l MIN 8.3 TYP 0 10 14.25 8.75 MAX 11.7 15.75 9.25 7.0 2.575 10 10 240 120 560 2.2 1.5 0.33 UNITS µA µA V V V V mV mV kHz kHz kHz kΩ V %/V V 3.75 2.425 5.5 2.5 1 1 IVREF = 0 IVREF = 0, 10V < VIN < 25V 0mA < IVREF < 2.5mA ROSC = 178k, FB = 1V, SS_MAXDC = 1.84V ROSC = 365k, FB = 1V ROSC = 64.9k, COMP = 2.5V, SD_VSEC = 2.64V FB = 1V FB = 1V (Note 14) ROSC = 178k; 10V < VIN < 25V, SS_MAXDC = 1.84V ROSC Pin Voltage 10V < VIN < 25V, VOL + 0.2V < COMP < VOH – 0.2 FB = FB Reference Voltage VOL + 0.2V < COMP < VOH – 0.2 (Note 15) FB = 1V, COMP = 1.6V COMP = 1.6V FB = VREF , COMP = 1.6V FB = 1V, ICOMP = –250µA FB = 1V, SOUT Duty Cycle > 0% ICOMP = 250µA COMP = 2.5V, FB =1V COMP = 2.5V, FB = 1V (Note 12) COMP = 2.5V, FB = 1V (Note 12) COMP = 2.5V, FB = 1V (OC = 100mV) COMP = 2.5V, FB = 1V, RBLANK = 40k (Note 16) COMP = 2.5V, FB = 1V, RBLANK = 120k l l 165 80 440 200 100 500 18 1.5 1.25 0.05 1 l 1.201 65 –4 4 18 2.7 0.7 1.226 -75 85 3 –9 10 23 3.2 0.8 0.15 1.250 -200 V nA dB MHz mA mA 28 µA V V 0.4 243 V mV µA µA 197 220 –8 –35 98 107 –50 180 540 1 116 –100 mV nA ns ns V  LTC4269-2 elecTrical characTerisTics PARAMETER SOUT High Level SOUT Active Pull-Off in Shutdown SOUT to OUT (Rise) DELAY (tDELAY) VDELAY OUT Driver OUT Rise Time OUT Fall Time OUT Clamp Voltage OUT Low Level OUT High Level OUT Active Pull-Off in Shutdown OUT Max Duty Cycle OUT Max Duty Cycle Clamp FB = 1V, CL = 1nF (Notes 15, 16) FB = 1V, CL = 1nF (Notes 15, 16) IGATE = 0µA, COMP = 2.5V, FB = 1V IGATE = 20mA IGATE = 200mA IGATE = –20mA, VIN = 12V COMP = 2.5V, FB = 1V IGATE = –200mA, VIN = 12V COMP = 2.5V, FB = 1V VIN = 5V, SD_VSEC = 0V, OUT = 1V COMP = 2.5V, FB = 1V, RDELAY = 10k (fOSC = 200kHz), VIN = 10V, SD_VSEC = 1.4V, SS_MAXDC = VREF COMP = 2.5V, FB = 1V, RDELAY = 10k (fOSC = 200kHz), VIN = 10V SD_VSEC = 1.32V, SS_MAXDC = 1.84V SD_VSEC = 2.64V, SS_MAXDC = 1.84V ISS_MAXDC = 150µA, OC = 1V Measured on SS_MAXDC FB + 1V, DC > 0% SS_MAXDC = 1V, SD_VSEC = 1.4V, OC = 1V 9.9 9.75 20 83 90 11.5 50 30 13 0.45 1.25 14.5 0.75 1.8 ns ns V V V V V mA % The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. CONDITIONS IGATE = –25mA, VIN = 12V COMP = 2.5V, FB = 1V VIN = 5V, SD_VSEC = 0V, SOUT = 1V COMP = 2.5V, FB = 1V (Note 16) RDELAY = 120k MIN 10 1 40 120 0.9 TYP MAX UNITS V mA ns ns V 63.5 25 72 33 0.2 0.45 0.8 800 80.5 41 % % V V V µA Soft-Start SS_MAXDC Low Level: VOL SS_MAXDC Soft-Start Reset Threshold SS_MAXDC Active Threshold SS_MAXDC Input Current (Soft-Start Pull-Down: IDIS) Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: Pins with 100V absolute maximum guaranteed for T ≥ 0°C, otherwise 90V. Note 3: PWRGD voltage clamps at 14V with respect to VNEG. Note 4: In applications where the VIN pin is supplied via an external RC network from a system VIN > 25V, an external Zener with clamp voltage VIN ON(MAX) < VZ < 25V should be connected from the VIN pin to GND. Note 5: All voltages are with respect to VPORTN pin unless otherwise noted. Note 6: Input voltage specifications are defined with respect to LTC4269-2 pins and meet IEEE 802.3af/at specifications when the input diode bridge is included. Note 7: Signature resistance is measured via the ∆V/∆I method with the minimum ∆V of 1V. The LTC4269-2 signature resistance accounts for the additional series resistance in the input diode bridge. Note 8: An invalid signature after the 1st classification event is mandated by IEEE 802.3at standard. See the Applications Information section. Note 9: Class accuracy is respect to the ideal current defined as 1.237/RCLASS and does not include variations in RCLASS resistance. Note 10: This parameter is assured by design and wafer level testing. Note 11: Voltages are with respect to GND unless otherwise specified. Tested with COMP open, VFB = 1.4V, RROSC = 178k, VSYNC = 0V, VSS(MAXDC) set to VREF (but electrically isolated), CVREF = 0.1µF VSD_VSEC = 2V, RBLANK , = 121k, RDELAY = 121k, VISENSE = 0V, VOC = 0V, COUT = 1nF VIN = 15V, , SOUT open, unless otherwise specified. Note 12: Guaranteed by correlation to static test. Note 13: VIN start-up current is measured at VIN = VIN(ON) – 0.25V and scaled by × 1.18 (to correlate to worst-case VIN start-up current at VIN(ON). Note 14: Maximum recommended SYNC frequency = 500kHz. Note 15: Guaranteed but not tested. Note 16: Timing for R = 40k derived from measurement with R = 240k. 42692fb  LTC4269-2 Typical perForMance characTerisTics Input Current vs Input Voltage 25k Detection Range 0.5 TA = 25°C 50 Input Current vs Input Voltage TA = 25°C CLASS 4 VPORTP CURRENT (mA) 10.5 11.0 Input Current vs Input Voltage CLASS 1 OPERATION 0.4 VPORTP CURRENT (mA) VPORTP CURRENT (mA) 40 0.3 30 CLASS 3 CLASS 2 CLASS 1 CLASS 0 0 50 20 30 40 10 VPORTP VOLTAGE RISING (V) 60 85°C –40°C 0.2 20 10.0 0.1 0 10 0 0 2 6 VPORTP VOLTAGE (V) 4 8 10 42692 G01 9.5 12 14 20 18 16 VPORTP VOLTAGE (V) 22 42692 G03 42692 G02 Signature Resistance vs Input Voltage 28 RESISTANCE = V = V2 – V1 I I2 – I1 27 DIODES: HD01 TA = 25°C IEEE UPPER LIMIT LTC4269-2 + 2 DIODES 25 24 LTC4269-2 ONLY 23 IEEE LOWER LIMIT CLASS CURRENT 10mA/DIV VPORTP INPUT VOLTAGE 10V/DIV Class Operation vs Time 1.2 TA = 25°C 1.0 RESISTANCE (Ω) On-Resistance vs Temperature SIGNATURE RESISTANCE (k ) 26 0.8 0.6 0.4 22 V1: 1 V2: 2 3 4 7 5 8 6 VPORTP VOLTAGE (V) 9 10 42692 G04 TIME (10µs/DIV) 42692 G05 0.2 –50 0 25 50 75 –25 JUNCTION TEMPERATURE (°C) 100 42692 G06 PWRGD, T2P Output Low Voltage vs Current 0.8 TA = 25°C 1.0 Active High PWRGD Output Low Voltage vs Current TA = 25°C VPORTP – VNEG = 4V 1.25 FB Voltage vs Temperature VPWRGD – VPORTN (V) VT2P – VPORTN (V) 0.6 PWRGD (V) 0.8 FB VOLTAGE (V) 1.24 0.6 1.23 0.4 0.4 1.22 0.2 0.2 1.21 0 0 2 6 4 CURRENT (mA) 8 10 42692 G07 0 0 0.5 1 1.5 CURRENT (mA) 2 42692 G08 1.20 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 42692 G09 42692fb  LTC4269-2 Typical perForMance characTerisTics Switching Frequency vs Temperature 245 VIN SHUTDOWN CURRENT (µA) 230 215 200 185 170 155 –50 500 VIN Shutdown Current vs Temperature VIN = 15V 450 SD_VSEC = 0V 400 350 300 250 200 150 VIN STARTUP CURRENT (µA) 600 550 500 450 400 350 300 250 –25 50 25 0 75 TEMPERATURE (°C) 100 125 VIN Start-Up Current vs Temperature SD_VSEC = 1.4V SWITCHING FREQUENCY (kHz) –25 50 25 0 75 TEMPERATURE (°C) 100 125 100 –50 200 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 42692 G10 42692 G11 42692 G12 6.5 6.0 5.5 IQ (mA) 5.0 4.5 4.0 IQ (VIN) vs Temperature OC = OPEN SD_VSEC TURN ON THRESHOLD (V) SD_VSEC Turn On Threshold vs Temperature 1.42 15 SD_VSEC Pin Current vs Temperature PIN CURRENT BEFORE PART TURN ON 10 1.37 1.32 SD_VSEC PIN CURRENT (µA) 5 1.27 3.5 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 1.22 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 –50 0mA PIN CURRENT AFTER PART TURN ON –25 50 25 0 75 TEMPERATURE (°C) 100 125 42692 G13 42692 G14 42692 G15 VIN Turn On/Off Voltage vs Temperature 18 16 14 12 10 VIN TURN OFF VOLTAGE 8 6 –50 COMP (V) VIN (V) 1.6 1.4 VIN TURN ON VOLTAGE 1.2 1.0 0.8 0.6 0.4 0.2 –25 50 25 0 75 TEMPERATURE (°C) 100 125 COMP Active Threshold vs Temperature RISENSE = 0k COMP SOURCE CURRENT (mA) 12.5 COMP Source Current vs Temperature FB = 1V COMP = 1.6V 10.0 7.5 CURRENT OUT OF PIN 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 5.0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 42692 G16 42692 G17 42692 G18 42692fb  LTC4269-2 Typical perForMance characTerisTics COMP Sink Current vs Temperature 12.5 FB = 1.4V COMP = 1.6V COMP PIN CURRENT (µA) 50 (Disabled) COMP Pin Current vs Temperature FB = VREF COMP = 1.6V ISENSE MAX THRESHOLD (mV) 240 200 160 120 80 40 0 ISENSE Maximum Threshold vs COMP TA = 25°C RISENSE = 0k COMP SINK CURRENT (mA) 40 10.0 30 OC THRESHOLD 20 7.5 10 5.0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 –50 –25 50 0 75 25 TEMPERATURE (°C) 100 125 0 0.5 1.0 2.0 1.5 COMP (V) 2.5 3.0 42692 G19 42692 G20 42692 G21 ISENSE Maximum Threshold vs Temperature 240 COMP = 2.5V RISENSE = 0k ISENSE PIN CURRENT (µA) 40 ISENSE Pin Current (Out of Pin) vs Duty Cycle TA = 25°C ISENSE MAX THRESHOLD (mV) 225 ISENSE Maximum Threshold vs Duty Cycle (Programming Slope Compensation) RSLOPE = 0 ISENSE MAX THRESHOLD (mV) 230 30 215 205 RSLOPE = 470 195 220 20 210 10 185 TA = 25°C COMP = 2.5V 0 RSLOPE = 1k 200 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 42692 G23 175 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 42692 G24 42692 G22 OC (Overcurrent) Threshold vs Temperature 120 PRECISION OVERCURRENT THRESHOLD INDEPENDENT OF DUTY CYCLE BLANK DURATION (ns) 800 Blank Duration vs Temperature 1000 BLANK Duration vs RBLANK TA = 25°C OC THRESHOLD (mV) 110 600 RBLANK = 120k BLANK (ns) RBLANK = 40k 800 600 100 400 400 90 200 200 80 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 0 20 40 60 80 100 120 140 160 RBLANK (k) 42692 G27 42692 G25 42692 G26 42692fb  LTC4269-2 Typical perForMance characTerisTics tDELAY: SOUT Rise to OUT Rise vs Temperature 200 240 tDELAY: SOUT Rise to OUT Rise vs RDELAY TA = 25°C OUT Rise/Fall Time vs OUT Load Capacitance 125 TA = 25°C OUT RISE/FALL TIME (ns) 150 RDELAY = 120k tDELAY (ns) 100 tDELAY (ns) 160 100 tr tf 50 75 80 50 RDELAY = 40k 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 0 40 80 160 RDELAY (k) 120 200 240 42692 G29 25 0 0 2000 1000 3000 4000 OUT LOAD CAPACITANCE (pF) 5000 42692 G30 42692 G28 100 OUT : Max Duty Cycle vs fOSC OUT MAX DUTY CYCLE CLAMP (%) 90 80 70 60 50 40 30 20 10 OUT : Max Duty Cycle CLAMP vs SD_VSEC OUT MAX DUTY CYCLE CLAMP (%) OUT : Max Duty Cycle CLAMP vs SS_MAXDC 90 TA = 25°C fOSC = 200kHz 80 R DELAY = 10k 70 60 50 40 30 20 1.60 SD_VSEC = 2.64V SD_VSEC = 1.98V SD_VSEC = 1.32V OUT DUTY CYCLE (%) 90 80 TA = 25°C SS_MAXDC = 2.5V SD_VSEC = 1.4V 200 300 fOSC (kHz) 400 500 42692 G31 70 100 0 1.32 TA = 25°C SS_MAXDC = 1.84V fOSC = 200kHz RDELAY = 10k 1.65 1.98 SD_VSEC (V) 2.31 2.64 42692 G32 1.84 SS_MAXDC (V) 2.08 42692 G33 SS_MAXDC Setting vs fOSC (for OUT DC = 72%) 2.32 TA = 25°C SD_VSEC = 1.32V 2.20 RDELAY = 10k SS_MAXDC (mV) 2.08 1.96 1.84 1.72 1.60 100 1.2 1.0 0.8 0.6 0.4 0.2 SS_MAXDC Reset and Active Thresholds vs Temperature ACTIVE THRESHOLD SS_MAXDC (V) RESET THRESHOLD 200 300 fOSC (kHz) 400 500 42692 G34 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 42692 G35 42692fb  LTC4269-2 pin FuncTions SHDN (Pin 1): Shutdown Input. Use this pin for auxiliary power application. Drive SHDN high to disable LTC4269-2 operation and corrupt the signature resistance. If unused, tie SHDN to VPORTN. T2P (Pin 2): Type-2 PSE Indicator, Open-Drain. Low impedance indicates the presence of a Type-2 PSE. RCLASS (Pin 3): Class Select Input. Connect a resistor between RCLASS and VPORTN to set the classification load current. VPORTN (Pins 5, 6): Power Input. Tie to the PD input through the diode bridge. Pins 5 and 6 must be electrically tied together at the package. NC (Pins 4, 7, 8, 25, 30, 31): No Connect. COMP (Pin 9): Output Pin of the Error Amplifier. The error amplifier is an op amp, allowing various compensation networks to be connected between the COMP pin and FB pin for optimum transient response in a nonisolated supply. The voltage on this pin corresponds to the peak current of the external FET. Full operating voltage range is between 0.8V and 2.5V corresponding to 0mV to 220mV at the ISENSE pin. For applications using the 100mV OC pin for overcurrent detection, typical operating range for the COMP pin is 0.8V to 1.6V. For isolated applications where COMP is controlled by an opto-coupler, the COMP pin output drive can be disabled with FB = VREF , reducing the COMP pin current to (COMP – 0.7)/40k. FB (Pin 10): In a nonisolated supply, FB monitors the output voltage via an external resistor divider and is compared with an internal 1.23V reference by the error amplifier. FB connected to VREF disables error amplifier output. ROSC (Pin11): A resistor to GND programs the operating frequency of the IC between 100kHz and 500kHz. Nominal voltage on the ROSC pin is 1.0V. SYNC (Pin 12): Used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 10% and 90% duty cycle. If unused, the pin should be connected to GND. SS_MAXDC (Pin 13): The external resistor divider from VREF sets the maximum duty cycle clamp (SS_MAXDC = 1.84V, SD_VSEC = 1.32V gives 72% duty cycle). Capacitor on SS_MAXDC pin in combination with external resistor divider sets soft-start timing. VREF (Pin 14): The output of an internal 2.5V reference which supplies internal control circuitry. Capable of sourcing up to 2.5mA drive for external use. Bypass to GND with a 0.1µF ceramic capacitor. SD_VSEC (Pin 15): The SD_VSEC pin, when pulled below its accurate 1.32V threshold, is used to turn off the IC and reduce current drain from VIN. The SD_VSEC pin is connected to system input voltage through a resistor divider to define undervoltage lockout (UVLO) for the power supply and to provide a volt-second clamp on the OUT pin. An 11µA pin current hysteresis allows external programming of UVLO hysteresis. GND (Pin 16): Analog Ground. Tie to VNEG. BLANK (Pin 17): A resistor to GND adjusts the extended blanking period of the overcurrent and current sense amplifier outputs during FET turn-on—to prevent false current limit trip. Increasing the resistor value increases the blanking period. ISENSE (Pin 18): The Current Sense Input for the Control Loop. Connect this pin to the sense resistor in the source of the external power MOSFET. A resistor in series with the ISENSE pin programs slope compensation. 42692fb 0 LTC4269-2 pin FuncTions OC (Pin 19): OC is an accurate 107mV threshold, independent of duty cycle, for overcurrent detection and trigger of soft-start. Connect this pin directly to the sense resistor in the source of the external power MOSFET. DELAY (Pin 20): A resistor to GND adjusts the delay period between SOUT rising edge and OUT rising edge. Used to maximize efficiency in forward converter applications by adjusting the timing. Increasing the resistor value increases the delay period. PGND (Pin 21): Power Ground. Carries the gate driver’s return current. Tie to VNEG. OUT (Pin 22): Drives the gate of an N-channel MOSFET between 0V and VIN with a maximum limit of 13V on OUT pin set by an internal clamp. Active pull-off exists in shutdown (see electrical specification). VIN (Pin 23): Input Supply for the Power Supply Controller. It must be closely decoupled to GND. An internal undervoltage lockout threshold exists for VIN at approximately 14.25V on and 8.75V off. SOUT (Pin 24): Switched Output in Phase with OUT Pin. Provides sync signal for control of secondary-side FETs in forward converter applications requiring highly efficient synchronous rectification. SOUT is actively clamped to 12V. Active pull-off exists in shutdown (see electrical specification). Can also be used to drive the active clamp FET of an active clamp forward supply. VNEG (Pins 26, 27): Power Output. Connects the PoE return line to the power supply through the internal Hot Swap power MOSFET. Pins 26 and 27 must be electrically tied together at the package. PWRGD (Pin 28): Active High Power Good Output, Open Collector. Signals that the internal Hot Swap MOSFET is on. High Impedance indicates power is good. PWRGD is referenced to VNEG and is low impedance during inrush and in the event of thermal overload. PWRGD is clamped 14V above VNEG. PWRGD (Pin 29): Active Low Power Good Output, Open Drain. Signals that the internal Hot Swap MOSFET is on. Low Impedance indicates power is good. PWRGD is referenced to VPORTN and is high impedance during inrush and in the event of thermal overload. PWRGD has no internal clamps. VPORTP (Pin 32): Input Voltage Positive Rail. This pin is connected to the PD’s positive rail. Exposed Pad (Pin 33): Tie to GND and PCB heat sink. 42692fb  LTC4269-2 block DiagraMs SHDN 1 REF T2P 2 + – CLASSIFICATION CURRENT LOAD 32 VPORTP EN 25k 16k RCLASS 3 CONTROL CIRCUITS 29 PWRGD 28 PWRGD 27 VNEG VPORTN 5 VPORTN 6 BOLD LINE INDICATES HIGH CURRENT PATH 26 VNEG 42692 BD1 VIN 23 VINON VINOFF START-UP INPUT CURRENT (ISTART) VREF 14 SS_MAXDC 13 0.45V + – – + SOFT-START CONTROL VREF >90% 2.5V SOURCE 2.5mA R Q S – + 1.23V ADAPTIVE MAXIMUM DUTY CYCLE CLAMP 50mA 24 SOUT 12V (TYPICAL 200kHz) OSC (100 TO 500)kHz RAMP (LINEAR) SLOPE COMP 8µA 0% DC 35µA 80% DC S R Q SD_VSEC 15 ROSC 11 1.32V SYNC 12 (VOLTAGE) ERROR AMPLIFIER 1.23V BLANK SENSE CURRENT EXPOSED PAD 33 10 FB 9 COMP 16 GND 0mV TO 220mV 20 17 107mV DELAY BLANK  + – + – + ON DELAY DRIVER 1A 22 OUT 21 PGND 13V OVER CURRENT 19 OC 18 ISENSE 42692 BD2 IHYST 10µA SD_VSEC = 1.32V 0µA SD_VSEC > 1.32V – – + + – 42692fb LTC4269-2 applicaTions inForMaTion OVERVIEW Power over Ethernet (PoE) continues to gain popularity as more products are taking advantage of having DC power and high speed data available from a single RJ45 connector. As PoE continues to grow in the marketplace, Powered Device (PD) equipment vendors are running into the 12.95W power limit established by the IEEE 802.3af standard. The IEE802.3at standard establishes a higher power allocation for Power over Ethernet while maintaining backwards compatibility with the existing IEEE 802.3af systems. Power Sourcing Equipment (PSE) and Powered Devices are distinguished as Type 1 complying with the IEEE 802.3af power levels, or Type 2 complying with the IEEE 802.3at power levels. The maximum available power of a Type 2 PD is 25.5W. The IEEE 802.3at standard also establishes a new method of acquiring power classification from a PD and communicating the presence of a Type 2 PSE. A Type 2 PSE has the option of acquiring PD power classification by performing 2-event classification (Layer 1) or by communicating with the PD over the data line (Layer 2). In turn, a Type 2 PD must be able to recognize both layers of communications and identify a Type 2 PSE. The LTC4269-2 is specifically designed to support a PD that must operate under the IEEE 802.3at standard. In particular, the LTC4269-2 provides the T2P indicator bit which recognizes 2-event classification. This indicator bit may be used to alert the LTC4269-2 output load that a Type 2 PSE is present. With an internal signature resistor, classification circuitry, inrush control, and thermal shutdown, the LTC4269-2 is a complete PD interface solution capable of supporting in the next generation PD applications. In addition to the PD front end, the LTC4269-2 also incorporates a high efficiency synchronous forward controller that minimizes component sizes while maximizing output power. MODES OF OPERATION The LTC4269-2 has several modes of operation depending on the input voltage applied between the VPORTP and VPORTN pins. Figure 1 presents an illustration of voltage VPORTP (V) 50 40 30 20 10 CLASSIFICATION DETECTION V2 DETECTION V1 dV = INRUSH dt C1 UVLO ON UVLO = RLOAD C1 TIME TIME VPORTP – PWRGD (V) –10 –20 –30 –40 –50 POWER BAD PWRGD TRACKS VPORTP POWER BAD PWRGD TRACKS VPORTP PWRGD TRACKS VPORTN ON UVLO TIME 50 VPORTP – VNEG (V) 40 30 20 10 POWER GOOD PWRGD – VNEG (V) 20 10 POWER BAD POWER GOOD POWER BAD IN DETECTION RANGE TIME INRUSH LOAD, ILOAD PD CURRENT CLASSIFICATION ICLASS DETECTION I2 DETECTION I1 I1 = V1 – 2 DIODE DROPS V2 – 2 DIODE DROPS I2 = 25kΩ 25kΩ TIME ICLASS DEPENDENT ON RCLASS SELECTION INRUSH = 100mA ILOAD = VPORTP RLOAD LTC4269-2 IIN PSE RCLASS VPORTN RCLASS VPORTP PWRGD PWRGD VNEG 42692 F01 RLOAD C1 Figure 1. Output Voltage, PWRGD, PWRGD and PD Current as a Function of Input Voltage 42692fb  LTC4269-2 applicaTions inForMaTion and current waveforms the LTC4269-2 may encounter with the various modes of operation summarized in Table 1. Table 1. LTC4269-2 Modes of Operation as a Function of Input Voltage VPORTP – VPORTN (V) LTC4269-2 MODES OF OPERATION 0V to 1.4V 1.5V to 9.8V (5.4V to 9.8V) 12.5V to On/UVLO On/UVLO to 60V >71V Inactive (Reset after 1st Classification Event) 25k Signature Resistor Detection Before 1st Classification Event (Mark, 11k Signature Corrupt After 1st Classification Event) Classification Load Current Active Inrush and Power Applied to PD Load Overvoltage Lockout, Classification and Hot Swap are Disabled. voltage ranges. Note that the Electrical Specifications are referenced with respect to the LTC4269-2 package pins. DETECTION During detection, the PSE looks for a 25k signature resistor which identifies the device as a PD. The PSE will apply two voltages in the range of 2.8V to 10V and measures the corresponding currents. Figure 1 shows the detection voltages V1 and V2 and the corresponding PD current. The PSE calculates the signature resistance using the ∆V/∆I measurement technique. The LTC4269-2 presents its precision, temperature-compensated 25k resistor between the VPORTP and VPORTN pins, alerting the PSE that a PD is present and requests power to be applied. The LTC4269-2 signature resistor also compensates for the additional series resistance introduced by the input diode bridge. Thus a PD built with the LTC4269-2 conforms to the IEEE 802.3af/at detection specifications. SIGNATURE CORRUPT OPTION In some designs that include an auxiliary power option, it is necessary to prevent a PD from being detected by a PSE. The LTC4269-2 signature resistance can be corrupted with the SHDN pin (Figure 3). Taking the SHDN pin high will reduce the signature resistor below 11k which is an invalid signature per the IEEE 802.3af/at specifications, and alerts the PSE not to apply power. Invoking the SHDN pin On/UVLO includes hysteresis. Rising input threshold: 37.2V max. Falling input threshold: 30.0V min. These modes satisfy the requirements defined in the IEEE 802.3af/at specification. INPUT DIODE BRIDGE In the IEEE 802.3af/at standard, the modes of operation reference the input voltage at the PD’s RJ45 connector. Since the PD must handle power received in either polarity from either the data or the spare pair, input diode bridges BR1 and BR2 are connected between the RJ45 connector and the LTC4269-2 (Figure 2). The input diode bridge introduces a voltage drop that affects the range for each mode of operation. The LTC4269-2 compensates for these voltage drops so that a PD built with the LTC4269-2 meets the IEEE 802.3af/at-established RJ45 1 2 3 POWERED DEVICE (PD) INPUT 6 TX+ TX– RX+ RX– TO PHY T1 BR1 VPORTP SPARE+ BR2 0.1µF 100V SPARE– D3 LTC4269-2 4 5 7 8 VPORTN 42692 F02 Figure 2. PD Front End Using Diode Bridge on Main and Spare Inputs 42692fb  LTC4269-2 applicaTions inForMaTion LTC4269-2 TO PSE 14k SHDN VPORTN 42692 F03 VPORTP 25k SIGNATURE RESISTOR Layer 2 communications takes place directly between the PSE and the PD, the LTC4269-2 concerns itself only with recognizing 2-event classification. In 2-event classification, a Type 2 PSE probes for power classification twice. Figure 4 presents an example of a 2-event classification. The 1st classification event occurs 50 40 VPORTP (V) 30 20 10 1ST CLASS 2ND CLASS SIGNATURE DISABLE Figure 3. 25k Signature Resistor with Disable also ceases operation for classification and turns off the internal Hot Swap FET. If this feature is not used, connect SHDN to VPORTN. CLASSIFICATION Classification provides a method for more efficient power allocation by allowing the PSE to identify a PD power classification. Class 0 is included in the IEEE specification for PDs that don’t support classification. Class 1-3 partitions PDs into three distinct power ranges. Class 4 includes the new power range under IEEE 802.3at (see Table 2). During classification probing, the PSE presents a fixed voltage between 15.5V and 20.5V to the PD (Figure 1). The LTC4269-2 asserts a load current representing the PD power classification. The classification load current is programmed with a resistor RCLASS that is chosen from Table 2. Table 2. Summary of Power Classifications and LTC4269-2 RCLASS Resistor Selection MAXIMUM NOMINAL POWER LEVELS CLASSIFICATION AT INPUT OF PD LOAD CURRENT (W) (mA) 0.44 to 12.95 ON* ON OVLO OFF *INCLUDES ON-UVLO HYSTERESIS ON THRESHOLD 36.1V UVLO THRESHOLD 30.7V OVLO THRESHOLD 71.0V CURRENT-LIMITED TURN ON VPORTN 6 BOLD LINE INDICATES HIGH CURRENT PATH 26 VNEG 42692 F06 INRUSH COMPLETE ON < VPORTP < OVLO AND NOT IN THERMAL SHUTDOWN Figure 5. LTC4269-2 Undervoltage and Overvoltage Lockout POWER NOT GOOD POWER GOOD continues to power the PD load as long as the port voltage does not fall below the UVLO threshold. When the LTC4269-2 port voltage falls below the UVLO threshold, the PD is disconnected, and classification mode resumes. C1 discharges through the LTC4269-2 circuitry. COMPLEMENTARY POWER GOOD When LTC4269-2 fully charges the load capacitor (C1), power good is declared and the LTC4269-2 load can safely begin operation. The LTC4269-2 provides complementary power good signals that remain active during normal operation and are deasserted when the port voltage falls below the PoE UVLO threshold, when the voltage exceeds the overvoltage lockout (OVLO) threshold, or in the event of a thermal shutdown. See Figure 6. The PWRGD pin features an open-collector output referenced to VNEG which can interface directly with the SD_VSEC pin. When power good is declared and active, the PWRGD pin is high impedance with respect to VNEG. An internal 14V clamp limits the PWRGD pin voltage. Connecting the PWRGD pin to the SD_VSEC pin prevents the DC/DC converter from commencing operation before the PDI interface completely charges the reservoir capacitor, C1. The active low PWRGD pin connects to an internal, opendrain MOSFET referenced to VPORTN and can interface directly to the shutdown pin of a DC/DC converter product. VPORTP < UVLO VPORTP > OVLO OR THERMAL SHUTDOWN Figure 6. LTC4269-2 Power Good Functional and State Diagram When power good is declared and active, the PWRGD pin is low impedance with respect to VPORTN. PWRGD PIN WHEN SHDN IS INVOKED In PD applications where an auxiliary power supply invokes the SHDN feature, the PWRGD pin becomes high impedance. This prevents the PWRGD pin that is connected to the “RUN” pin of the DC/DC converter from interfering with the DC/DC converter operations when powered by an auxiliary power supply. OVERVOLTAGE LOCKOUT The LTC4269-2 includes an Overvoltage Lockout (OVLO) feature (Figure 5) which protects the LTC4269-2 and its load from an overvoltage event. If the input voltage exceeds the OVLO threshold, the LTC4269-2 discontinues PD operation. Normal operations resume when the input voltage falls below the OVLO threshold and when C1 is charged up. 42692fb  LTC4269-2 applicaTions inForMaTion THERMAL PROTECTION The IEEE 802.3af/at specification requires a PD to withstand any applied voltage from 0V to 57V indefinitely. However, there are several possible scenarios where a PD may encounter excessive heating. During classification, excessive heating may occur if the PSE exceeds the 75ms probing time limit. At turn-on, when the load capacitor begins to charge, the instantaneous power dissipated by the PD interface can be large before it reaches the line voltage. And if the PD experiences a fast input positive voltage step in its operational mode (for example, from 37V to 57V), the instantaneous power dissipated by the PD Interface can be large. The LTC4269-2 includes a thermal protection feature which protects the LTC4269-2 from excessive heating. If the LTC4269-2 junction temperature exceeds the overtemperature threshold, the LTC4269-2 discontinues PD operations. Normal operation resumes when the junction temperature falls below the overtemperature threshold and when C1 is charged up and power good becomes inactive. EXTERNAL INTERFACE AND COMPONENT SELECTION Transformer Nodes on an Ethernet network commonly interface to the outside world via an isolation transformer. For PDs, the isolation transformer must also include a center tap on the RJ45 connector side (see Figure 7). RJ45 1 2 3 6 TX+ TX– RX+ RX– SPARE+ 14 T1 1 12 13 10 11 9 3 2 5 4 6 VPORTP LTC4269-2 C1 TO PHY The increased current levels in a Type 2 PD over a Type 1 increase the current imbalance in the magnetics which can interfere with data transmission. In addition, proper termination is also required around the transformer to provide correct impedance matching and to avoid radiated and conducted emissions. Transformer vendors such as Bel Fuse, Coilcraft, Halo, Pulse and Tyco (Table 4) can assist in selecting an appropriate isolation transformer and proper termination methods. Table 4. Power over Ethernet Transformer Vendors VENDOR Bel Fuse Inc. CONTACT INFORMATION 206 Van Vorst Street Jersey City, NJ 07302 Tel: 201-432-0463 www.belfuse.com 1102 Silver Lake Road Gary, IL 60013 Tel: 847-639-6400 www.coilcraft.com 1861 Landings Drive Mountain View, CA 94043 Tel: 650-903-3800 www.haloelectronics.com 16799 Schoenborn Street North Hills, CA 91343 Tel: 818-892-0761 www.pca.com 12220 World Trade Drive San Diego, CA 92128 Tel: 858-674-8100 www.pulseeng.com 308 Constitution Drive Menlo Park, CA 94025-1164 Tel: 800-227-7040 www.circuitprotection.com Coilcraft Inc. Halo Electronics PCA Electronics Pulse Engineering Tyco Electronics BR1 HD01 Input Diode Bridge Figure 2 shows how two diode bridges are typically connected in a PD application. One bridge is dedicated to the data pair while the other bridge is dedicated to the spare pair. The LTC4269-2 supports the use of either silicon or Schottky input diode bridges. However, there are trade-offs in the choice of diode bridges. An input diode bridge must be rated above the maximum current the PD application will encounter at the temperature the PD will operate. Diode bridge vendors typically call out the operating current at room temperature, but derate the maximum current with increasing temperature. Consult the diode bridge vendors for the operating current de-rating curve. 42692fb COILCRAFT ETH1-230LD BR2 HD01 4 5 7 8 SPARE– C14 D3 0.1µF SMAJ58A 100V TVS VPORTN VNEG 42692 F07 Figure 7. PD Front End with Isolation Transformer, Diode Bridges, Capacitors and a Transient Voltage Suppressor (TVS)  LTC4269-2 applicaTions inForMaTion A silicon diode bridge can consume over 4% of the available power in some PD applications. Using Schottky diodes can help reduce the power loss with a lower forward voltage. A Schottky bridge may not be suitable for some high temperature PD applications. The leakage current has a temperature and voltage dependency that can reduce the perceived signature resistance. In addition, the IEEE 802.3af/at specification mandates the leakage back-feeding through the unused bridge cannot generate more than 2.8V across a 100k resistor when a PD is powered with 57V. Sharing Input Diode Bridges At higher temperatures, a PD design may be forced to consider larger bridges in a bigger package because the maximum operating current for the input diode bridge is drastically derated. The larger package may not be acceptable in some space-limited environments. One solution to consider is to reconnect the diode bridges so that only one of the four diodes conducts current in each package. This configuration extends the maximum operating current while maintaining a smaller package profile. Figure 7 shows how the reconnect the two diode bridges. Consult the diode bridge vendors for the de-rating curve when only one of four diodes is in operation. Input Capacitor The IEEE 802.3af/at standard includes an impedance requirement in order to implement the AC disconnect function. A 0.1µF capacitor (C14 in Figure 7) is used to meet this AC impedance requirement. Place this capacitor as close to the LTC4269-2 as possible. Transient Voltage Suppressor The LTC4269-2 specifies an absolute maximum voltage of 100V and is designed to tolerate brief overvoltage events. However, the pins that interface to the outside world can routinely see excessive peak voltages. To protect the LTC4269-2, install a transient voltage suppressor (D3) between the input diode bridge and the LTC4269-2 as close to the LTC4269-2 as possible as shown in Figure 7. Classification Resistor (RCLASS) The RCLASS resistor sets the classification load current, corresponding to the PD power classification. Select the value of RCLASS from Table 2 and connect the resistor between the RCLASS and VPORTN pins as shown in Figure 4, or float the RCLASS pin if the classification load current is not required. The resistor tolerance must be 1% or better to avoid degrading the overall accuracy of the classification circuit. Load Capacitor The IEEE 802.3af/at specification requires that the PD maintains a minimum load capacitance of 5µF and does not specify a maximum load capacitor. However, if the load capacitor is too large, there may be a problem with inadvertent power shutdown by the PSE. This occurs when the PSE voltage drops quickly. The input diode bridge reverses bias, and the PD load momentarily powers off the load capacitor. If the PD does not draw power within the PSE’s 300ms disconnection delay, the PSE may remove power from the PD. Thus, it is necessary to evaluate the load current and capacitance to ensure that an inadvertent shutdown cannot occur. The load capacitor can store significant energy when fully charged. The PD design must ensure that this energy is not inadvertently dissipated in the LTC4269-2. For example, if the VPORTP pin shorts to VPORTN while the capacitor is charged, current will flow through the parasitic body diode of the internal MOSFET and may cause permanent damage to the LTC4269-2. T2P Interface When a 2-event classification sequence successfully completes, the LTC4269-2 recognizes this sequence, and provides an indicator bit, declaring the presence of a Type 2 PSE. The open-drain output provides the option to use this signal to communicate to the LTC4269-2 load, or to leave the pin unconnected. Figure 8 shows two interface options using the T2P pin and the opto-isolator. The T2P pin is active low and connects to an optoisolater to communicate across the 42692fb  LTC4269-2 applicaTions inForMaTion VPORTP TO PSE LTC4269-2 V+ RP TO PD’s MICROPROCESSOR These options come with various trade-offs and design considerations. Contact Linear Technology applications support for detailed information on implementing custom auxiliary power sources. IEEE 802.3AT SYSTEM POWER-UP REQUIREMENT Under the IEEE 802.3at standard, a PD must operate under 12.95W in accordance with IEEE 802.3at standard until it recognizes a Type 2 PSE. Initializing PD operation in 12.95W mode eliminates interoperability issue in case a Type 2 PD connects to a Type 1 PSE. Once the PD recognizes a Type 2 PSE, the IEEE 802.3at standard requires the PD to wait 80ms in 12.95W operation before 25.5W operation can commence. MAINTAIN POWER SIGNATURE In an IEEE 802.3af/at system, the PSE uses the maintain power signature (MPS) to determine if a PD continues to require power. The MPS requires the PD to periodically draw at least 10mA and also have an AC impedance less than 26.25k in parallel with 0.05µF If one of these conditions . is not met, the PSE may disconnect power to the PD. Isolation The 802.3 standard requires Ethernet ports to be electrically isolated from all other conductors that are user accessible. This includes the metal chassis, other connectors, and any auxiliary power connection. For PDs, there are two common methods to meet the isolation requirement. If there are any user-accessible connections to the PD, then an isolated DC/DC converter is necessary to meet the isolation requirements. If user connections can be avoided, then it is possible to meet the safety requirement by completely enclosing the PD in an insulated housing. Switcher Controller Operation The LTC4269-2 has a current mode synchronous PWM controller optimized for control of a forward converter topology. The LTC4269-2 is ideal for power systems where very high efficiency and reliability, low complexity and cost are required in a small space. Key features of the LTC4269-2 include an adaptive maximum duty cycle clamp. 42692fb –54V VPORTN T2P OPTION 1: SERIES CONFIGURATION FOR ACTIVE LOW/LOW IMPEDANCE OUTPUT V+ VPORTP TO PSE LTC4269-2 T2P TO PD’s MICROPROCESSOR 42692 F08 RP –54V VPORTN VNEG OPTION 2: SHUNT CONFIGURATION FOR ACTIVE HIGH/OPEN COLLECTOR OUTPUT Figure 8. T2P Interface Examples DC/DC converter isolation barrier. The pull-up resistor RP is sized according to the requirements of the opto-isolator operating current, the pull-down capability of the T2P pin, and the choice of V+. V+ for example can come from the PoE supply rail (which the LTC4269-2 VPORTP is tied to), or from the voltage source that supplies power to the DC/DC converter. Option 1 has the advantage of not drawing power unless T2P is declared active. Shutdown Interface To corrupt the signature resistance, the SHDN pin can be driven high with respect to VPORTN . If unused, connect SHDN directly to VPORTN. Exposed Pad The LTC4269-2 uses a thermally enhanced DFN12 package that includes an Exposed Pad. The Exposed Pad should be electrically connected to the GND pin’s PCB copper plane. This plane should be large enough to serve as the heat sink for the LTC4269-2. Auxiliary Power Source In some applications, it is desirable to power the PD from an auxiliary power source such as a wall adapter. Auxiliary power can be injected into the PD at several locations with priority chosen between PoE or auxiliary power sources. 0 LTC4269-2 applicaTions inForMaTion An additional output signal is included for synchronous rectifier control or active clamp control. A precision 107mV threshold senses overcurrent conditions and triggers softstart for low stress short-circuit protection and control. The key functions of the LTC4269-2 PWM controller are shown in the Block Diagrams. Part Start-Up In normal operation, the SD_VSEC pin must exceed 1.32V and the VIN pin must exceed 14.25V to allow the part to turn on. This combination of pin voltages allows the 2.5V VREF pin to become active, supplying the LTC4269-2 control circuitry and providing up to 2.5mA external drive. SD_VSEC threshold can be used for externally programming the power supply undervoltage lockout (UVLO) threshold on the input voltage to the forward converter. Hysteresis on the UVLO threshold can also be programmed since the SD_VSEC pin draws 11µA just before part turn-on and 0µA after part turn-on. With the LTC4269-2 turned on, the VIN pin can drop as low as 8.75V before part shutdown occurs. This VIN pin hysteresis (5.5V) combined with low 460µA start-up input current allows low power start-up using a resistor/capacitor network from power supply input voltage to supply the VIN pin (Figure 10). The VIN capacitor value is chosen to prevent VIN falling below its turn-off threshold before a bias winding in the converter takes over supply to the VIN pin. Output Drivers The LTC4269-2 has two outputs, SOUT and OUT. The OUT pin provides a ±1A peak MOSFET gate drive clamped to 13V. The SOUT pin has a ±50mA peak drive clamped to 12V and provides sync signal timing for synchronous rectification control or active clamp control. For SOUT and OUT turn-on, a PWM latch is set at the start of each main oscillator cycle. OUT turn-on is delayed from SOUT turn-on by a time, tDELAY (Figure 14). tDELAY is programmed using a resistor from the DELAY pin to GND and is used to set the timing control of the secondary synchronous rectifiers for optimum efficiency. SOUT and OUT turn off at the same time each cycle by one of three methods: (1) MOSFET peak current sense at ISENSE pin (2) Adaptive maximum duty cycle clamp reached during load/line transients (3) Maximum duty cycle reset of the PWM latch During any of the following conditions—low VIN, low SD_VSEC or overcurrent detection at the OC pin—a softstart event is latched and both SOUT and OUT turn off immediately (Figure 11). Leading Edge Blanking To prevent MOSFET switching noise causing premature turn-off of SOUT or OUT, programmable leading edge blanking exists. This means both the current sense comparator and overcurrent comparator outputs are ignored during MOSFET turn-on and for an extended period after the OUT leading edge (Figure 12). The extended blanking period is programmable by adjusting a resistor from the BLANK pin to GND. Adaptive Maximum Duty Cycle Clamp (Volt-Second Clamp) For forward converter applications, a maximum switch duty cycle clamp which adapts to transformer input voltage is necessary for reliable control of the MOSFET. This volt-second clamp provides a safeguard for transformer reset that prevents transformer saturation. Instantaneous load changes can cause the converter loop to demand maximum duty cycle. If the maximum duty cycle of the switch is too great, the transformer reset voltage can exceed the voltage rating of the primary-side MOSFETs with catastrophic damage. Many converters solve this problem by limiting the operational duty cycle of the MOSFET to 50% or less—or by using a fixed (non-adaptive) maximum duty cycle clamp with very large voltage rated MOSFETs. The LTC4269-2 provides a volt-second clamp to allow MOSFET duty cycles well above 50%. This gives greater power utilization for the MOSFETs, rectifiers and transformer resulting in less space for a given power output. In addition, the volt-second clamp can allow a reduced voltage rating on the MOSFET resulting in lower RDS(ON) for greater efficiency. The volt-second clamp defines a maximum duty cycle ‘guard rail’ which falls when power supply input voltage increases. 42692fb  LTC4269-2 applicaTions inForMaTion An increase of voltage at the SD_VSEC pin causes the maximum duty cycle clamp to decrease. If SD_VSEC is resistively divided down from power supply input voltage, a volt-second clamp is realized. To adjust the initial maximum duty cycle clamp, the SS_MAXDC pin voltage is programmed by a resistor divider from the 2.5V VREF pin to GND. An increase of programmed voltage on SS_MAXDC pin provides an increase of switch maximum duty cycle clamp. Soft-Start The LTC4269-2 provides true PWM soft-start by using the SS_MAXDC pin to control soft-start timing. The proportional relationship between SS_MAXDC voltage and switch maximum duty cycle clamp allows the SS_MAXDC pin to slowly ramp output voltage by ramping the maximum switch duty cycle clamp—until switch duty cycle clamp seamlessly meets the natural duty cycle of the converter. A soft-start event is triggered whenever VIN is too low, SD_VSEC is too low (power supply UVLO), or a 107mV overcurrent threshold at OC pin is exceeded. Whenever a soft-start event is triggered, switching at SOUT and OUT is stopped immediately. The SS_MAXDC pin is discharged and only released for charging when it has fallen below its reset threshold of 0.45V and all faults have been removed. Increasing voltage on the SS_MAXDC pin above 0.8V will increase switch maximum duty cycle. A capacitor to GND on the SS_MAXDC pin in combination with a resistor divider from VREF , defines the soft-start timing. Current Mode Topology (ISENSE Pin) The LTC4269-2 current mode topology eases frequency compensation requirements because the output inductor does not contribute to phase delay in the regulator loop. This current mode technique means that the error amplifier (nonisolated applications) or the opto-coupler (isolated applications) commands current (rather than voltage) to be delivered to the output. This makes frequency compensation easier and provides faster loop response to output load transients. A resistor divider from the application’s output voltage generates a voltage at the inverting FB input of the LTC4269-2 error amplifier (or to the input of an external opto-coupler) and is compared to an accurate reference (1.23V for LTC4269-2). The error amplifier output (COMP) defines the input threshold (ISENSE) of the current sense comparator. COMP voltages between 0.8V (active threshold) and 2.5V define a maximum ISENSE threshold from 0mV to 220mV. By connecting ISENSE to a sense resistor in series with the source of an external power MOSFET, the MOSFET peak current trip point (turn off) can be controlled by COMP level and hence by the output voltage. An increase in output load current causing the output voltage to fall, will cause COMP to rise, increasing ISENSE threshold, increasing the current delivered to the output. For isolated applications, the error amplifier COMP output can be disabled to allow the opto-coupler to take control. Setting FB = VREF disables the error amplifier COMP output, reducing pin current to (COMP – 0.7)/40k. Slope Compensation The current mode architecture requires slope compensation to be added to the current sensing loop to prevent subharmonic oscillations which can occur for duty cycles above 50%. Unlike most current mode converters which have a slope compensation ramp that is fixed internally, placing a constraint on inductor value and operating frequency, the LTC4269-2 has externally adjustable slope compensation. Slope compensation can be programmed by inserting an external resistor (RSLOPE) in series with the ISENSE pin. The LTC4269-2 has a linear slope compensation ramp which sources current out of the ISENSE pin of approximately 8µA at 0% duty cycle to 35µA at 80% duty cycle. Overcurrent Detection and Soft-Start (OC Pin) An added feature to the LTC4269-2 is a precise 107mV sense threshold at the OC pin used to detect overcurrent conditions in the converter and set a soft-start latch. The OC pin is connected directly to the source of the primaryside MOSFET to monitor peak current in the MOSFET (Figure 13). The 107mV threshold is constant over the entire duty cycle range of the converter because it is unaffected by the slope compensation added to the ISENSE pin. 42692fb  LTC4269-2 applicaTions inForMaTion Synchronizing A SYNC pin allows the LTC4269-2 oscillator to be synchronized to an external clock. The SYNC pin can be driven from a logic-level output, requiring less than 0.8V for a logic-level low and greater than 2.2V for a logic-level high. Duty cycle should run between 10% and 90%. To avoid loss of slope compensation during synchronization, the free running oscillator frequency, fOSC, should be programmed to 80% of the external clock frequency (fSYNC). The RSLOPE resistor chosen for nonsynchronized operation should be increased by 1.25x (= fSYNC/fOSC). Shutdown and Programming the Power Supply Undervoltage Lockout The LTC4269-2 has an accurate 1.32V shutdown threshold at the SD_VSEC pin. This threshold can be used in conjunction with a resistor divider to define the power supply undervoltage lockout threshold (UVLO) of the power supply input voltage (VS) (Figure 9). A pin current hysteresis (11µA before part turn-on, 0µA after part turn-on) allows power supply UVLO hysteresis to be programmed. Calculation of the on/off thresholds for the power supply input voltage can be made as follows: VS(OFF) Threshold = 1.32[1 + (R1/R2)] VS(ON) Threshold = VS(OFF) + (11µA • R1) Connect the PWRGD pin to the resistive divider network at the SD_VSEC pin to prevent the DC/DC converter from starting before the PD interface completely charges the reservoir capacitor, C1 (Figure 9). The SD_VSEC pin must not be left open since there must be an external source current >11µA to lift the pin past its 1.32V threshold for part turn-on. Micropower Start-Up: Selection of Start-Up Resistor and Capacitor for VIN The LTC4269-2 uses turn-on voltage hysteresis at the VIN pin and low start-up current to allow micropower start-up (Figure 10). The LTC4269-2 monitors VIN pin voltage to allow the part to turn-on at 14.25V and the part to turnoff at 8.75V. Low start-up current (460µA) allows a large resistor to be connected between the power supply input supply and VIN. Once the part is turned on, input current Figure 10. Low Power Start-Up POWER SUPPLY INPUT VOLTAGE (VS) R1 SD_VSEC 11µA PWRGD 1.32V LTC4269-2 – + R2 42692 F09 Figure 9. Programming Power Supply Undervoltage Lockout (UVLO) POWER SUPPLY INPUT VOLTAGE (VS) FROM AUXILIARY WINDING RSTART D1* VIN LTC4269-2 – 1.32V CSTART + 42692 F10 *FOR VS > 25V, ZENER D1 RECOMMENDED (VIN ON(MAX) < VZ < 25V) increases to drive the IC (5.2mA) and the output drivers (IDRIVE). A large enough capacitor is chosen at the VIN pin to prevent VIN falling below its turn-off threshold before a bias winding in the converter takes over supply to VIN. This technique allows a simple resistor/capacitor for start-up which draws low power from the system supply to the converter. For system input voltages exceeding the absolute maximum rating of the LTC4269-2 VIN pin, an external Zener should be connected from the VIN pin to GND. This covers the condition where VIN charges past VIN(ON) but the part does not turn on because SD_VSEC < 1.32V. In this condition, VIN will continue to charge towards system VIN, possibly exceeding the rating for the VIN pin. The Zener voltage should obey VIN(ONMAX) < VZ < 25V. 42692fb  LTC4269-2 applicaTions inForMaTion Programming Oscillator Frequency The oscillator frequency (fOSC) of the LTC4269-2 is programmed using an external resistor, ROSC, connected between the ROSC pin and GND. Figure 11 shows typical fOSC vs ROSC resistor values. The LTC4269-2 free-running oscillator frequency is programmable in the range of 100kHz to 500kHz. Stray capacitance and potential noise pickup on the ROSC pin should be minimized by placing the ROSC resistor as close as possible to the ROSC pin and keeping the area of the ROSC node as small as possible. The ground side of the ROSC resistor should be returned directly to the (analog ground) GND pin. ROSC can be calculated by: ROSC = 9.125k [(4100k/fOSC) – 1] Programming Leading Edge Blank Time For PWM controllers driving external MOSFETs, noise can be generated at the source of the MOSFET during gate rise time and some time thereafter. This noise can potentially exceed the OC and ISENSE pin thresholds of the LTC4269-2 to cause premature turn-off of SOUT and OUT pins in addition to false trigger of soft-start. The LTC4269-2 provides a programmable leading edge blanking of the OC and ISENSE comparator outputs to avoid false current sensing during MOSFET switching. Blanking is provided in two phases (Figure 12): The first phase automatically blanks during gate rise time. Gate rise 500 450 400 FREQUENCY (kHz) 350 300 250 200 150 100 50 BLANKING 100 150 200 250 ROSC (k ) 300 350 400 0 Xns X + 45ns [X + 45(RBLANK/10k)]ns 42692 F12 times can vary depending on MOSFET type. For this reason the LTC4269-2 performs true ‘leading edge blanking’ by automatically blanking OC and ISENSE comparator outputs until OUT rises to within 0.5V of VIN or reaches its clamp level of 13V. The second phase of blanking starts after the leading edge of OUT has been completed. This phase is programmable by the user with a resistor connected from the BLANK pin to GND. Typical durations for this portion of the blanking period are from 45ns at RBLANK = 10k to 540ns at RBLANK = 120k. Blanking duration can be approximated as: Blanking (extended) = [45(RBLANK/10k)]ns (See graph in the Typical Performance Characteristics section). Programming Current Limit (OC Pin) The LTC4269-2 uses a precise 107mV sense threshold at the OC pin to detect overcurrent conditions in the converter and set a soft-start latch. It is independent of duty cycle because it is not affected by slope compensation programmed at the ISENSE pin. The OC pin monitors the peak current in the primary MOSFET by sensing the voltage across a sense resistor (RS) in the source of the MOSFET. The overcurrent limit for the converter can be programmed by: Overcurrent limit = (107mV/RS)(NP/NS) – (½)(IRIPPLE) (AUTOMATIC) LEADING EDGE BLANKING (PROGRAMMABLE) EXTENDED BLANKING CURRENT SENSE DELAY OUT RBLANK = 10k (MIN) 10k < RBLANK 240k 100ns 42692 F11 Figure 11. Oscillator Frequency, fOSC, vs ROSC Figure 12. Leading Edge Blank Timing 42692fb  LTC4269-2 applicaTions inForMaTion where: RS = sense resistor in source of primary MOSFET IRIPPLE = IP-P ripple current in the output inductor L1 NS = number of transformer secondary turns NP = number of transformer primary turns Programming Slope Compensation The LTC4269-2 uses a current mode architecture to provide fast response to load transients and to ease frequency compensation requirements. Current mode switching regulators which operate with duty cycles above 50% and have continuous inductor current must add slope compensation to their current sensing loop to prevent subharmonic oscillations. (For more information on slope compensation, see Application Note 19.) The LTC4269-2 has programmable slope compensation to allow a wide range of inductor values, to reduce susceptibility to PCB generated noise and to optimize loop bandwidth. The LTC4269-2 programs slope compensation by inserting a resistor, RSLOPE, in series with the ISENSE pin (Figure 13). The LTC4269-2 generates a current at the ISENSE pin which is linear from 0% duty cycle to the maximum duty cycle of the OUT pin. A simple calculation of ISENSE • RSLOPE gives an added ramp to the voltage at the ISENSE pin for programmable slope compensation. (See both graphs ISENSE Pin Current vs Duty Cycle and ISENSE Maximum Threshold vs Duty Cycle in the Typical Performance Characteristics section.) CURRENT SLOPE = 35µA • DC LTC4269-2 OUT OC ISENSE RSLOPE RS 42692 F13 Programming Synchronous Rectifier Timing: SOUT to OUT delay (‘tDELAY’) The LTC4269-2 has an additional output SOUT which provides a ±50mA peak drive clamped to 12V. In applications requiring synchronous rectification for high efficiency, the LTC4269-2 SOUT provides a sync signal for secondary side control of the synchronous rectifier MOSFETs (Figure 14). Timing delays through the converter can cause non-optimum control timing for the synchronous rectifier MOSFETs. The LTC4269-2 provides a programmable delay (tDELAY, Figure 14) between SOUT rising edge and OUT rising edge to optimize timing control for the synchronous rectifier MOSFETs to achieve maximum efficiency gains. A resistor RDELAY connected from the DELAY pin to GND sets the value of tDELAY. Typical values for tDELAY range from 10ns with RDELAY = 10k to 160ns with RDELAY = 160k (see graph in the Typical Performance Characteristics section). tDELAY LTC4269-2 DELAY 42692 F14 SOUT OUT RDELAY Figure 14. Programming SOUT and OUT Delay: tDELAY Programming Maximum Duty Cycle Clamp For forward converter applications, a maximum switch duty cycle clamp which adapts to transformer input voltage is necessary for reliable control of the MOSFETs. This volt-second clamp provides a safeguard for transformer reset that prevents transformer saturation. The LTC4269-2 SD_VSEC and SS_MAXDC pins provide a capacitor-less, programmable volt-second clamp solution using simple resistor ratios (Figure 15). An increase of voltage at the SD_VSEC pin causes the maximum duty cycle clamp to decrease. Deriving SD_VSEC from a resistor divider connected to system input voltage VSOURCE V(ISENSE) = VSOURCE + (ISENSE • RSLOPE) ISENSE = 8µA + 35DC µA DC = DUTY CYCLE FOR SYNC OPERATION ISENSE(SYNC) = 8µA + (k • 35DC)µA k = fOSC/fSYNC Figure 13. Programming Slope Compensation 42692fb  LTC4269-2 applicaTions inForMaTion POWER SUPPLY INPUT VOLTAGE R1 ADAPTIVE DUTY CYCLE CLAMP INPUT LTC4269-2 SD_VSEC R2 RT* SS_MAXDC VREF 42692 F15 (3) The maximum duty cycle clamp calculated in (2) should be programmed to be 10% greater than the maximum operational duty cycle calculated in (1). Simple adjustment of maximum duty cycle can be achieved by adjusting SS_MAXDC. Example calculation for (2): For RT = 35.7k, RB = 100k, VREF = 2.5V, RDELAY = 40k, fOSC = 200kHz and SD_VSEC = 1.32V, this gives SS_MAXDC(DC) = 1.84V, tDELAY = 40ns and k = 1 Maximum Duty Cycle Clamp = 1 • 0.522(1.84/1.32) – (40ns • 200kHz) = 0.728 – 0.008 = 0.72 (Duty Cycle Clamp = 72%) Note 1: To achieve the same maximum duty cycle clamp at 100kHz as calculated for 200kHz, the SS_MAXDC voltage should be reprogrammed by, SS_MAXDC(DC) (100kHz) = SS_MAXDC(DC) (200kHz) • k (200kHz)/k (100kHz) = 1.84 • 1.0/1.055 = 1.74V (k = 1.055 for 100kHz) Note 2 : To achieve the same maximum duty cycle clamp while synchronizing to an external clock at the SYNC pin, the SS_MAXDC voltage should be reprogrammed as, SS_MAXDC (DC) (fsync) = SS_MAXDC (DC) (200kHz) • [(fosc/fsync) + 0.09(fosc/200kHz)0.6] For SS_MAXDC (DC) (200kHz) = 1.84V for 72% duty cycle SS_MAXDC (DC) (fsync = 250kHz) for 72% duty cycle = 1.84 • [(200kHz/250kHz) + 0.09(1)0.6] = 1.638V Programming Soft-Start Timing The LTC4269-2 has built-in soft-start capability to provide low stress controlled start-up from a list of fault conditions that can occur in the application (see Figures 16 and 17). The LTC4269-2 provides true PWM soft-start by RB MAX DUTY CYCLE CLAMP ADJUST INPUT *MINIMUM ALLOWABLE RT IS 10k TO GUARANTEE SOFT-START PULL-OFF Figure 15. Programming Maximum Duty Cycle Clamp creates the volt-second clamp. The maximum duty cycle clamp can be adjusted by programming voltage on the SS_MAXDC pin using a resistor divider from VREF . An increase of voltage at the SS_MAXDC pin causes the maximum duty cycle clamp to increase. To program the volt-second clamp, the following steps should be taken: (1) The maximum operational duty cycle of the converter should be calculated for the given application. (2) An initial value for the maximum duty cycle clamp should be calculated using the equation below with a first pass guess for SS_MAXDC. Note: Since maximum operational duty cycle occurs at minimum system input voltage (UVLO), the voltage at the SD_VSEC pin = 1.32V. Max Duty Cycle Clamp (OUT Pin) = k • 0.522(SS_MAXDC(DC)/SD_VSEC) – (tDELAY • fOSC) where: SS_MAXDC(DC) = VREF(RB/(RT + RB) SD_VSEC = 1.32V at minimum system input voltage tDELAY = programmed delay between SOUT and OUT k = 1.11 – 5.5e–7 • (fOSC) 42692fb  LTC4269-2 applicaTions inForMaTion tDELAY: PROGRAMMABLE SYNCHRONOUS DELAY SOUT OUT SS_MAXDC FAULTS TRIGGERING SOFT-START VIN < 8.75V OR SD_VSEC < 1.32V (UVLO) OR OC > 107mV (OVERCURRENT) SOFT-START LATCH SET 0.8V (ACTIVE THRESHOLD) 0.45V (RESET THRESHOLD) 0.2V SOFT-START LATCH RESET: VIN > 14.25V (> 8.75V IF LATCH SET BY OC) AND SD_VSEC > 1.32V AND OC < 107mV AND SS_MAXDC < 0.45V 42692 F16 using the SS_MAXDC pin to control soft-start timing. The proportional relationship between SS_MAXDC voltage and switch maximum duty cycle clamp allows the SS_MAXDC pin to slowly ramp output voltage by ramping the maximum switch duty cycle clamp—until switch duty cycle clamp seamlessly meets the natural duty cycle of the converter. A capacitor CSS on the SS_MAXDC pin and the resistor divider from VREF used to program maximum switch duty cycle clamp, determine soft-start timing (Figure 18). A soft-start event is triggered for the following faults: (1) VIN < 8.75V, or (2) SD_VSEC < 1.32V (UVLO), or (3) OC > 107mV (overcurrent condition) When a soft-start event is triggered, switching at SOUT and OUT is stopped immediately. A soft-start latch is set and SS_MAXDC pin is discharged. The SS_MAXDC pin can only recharge when the soft-start latch has been reset. Note: A soft-start event caused by (1) or (2) above, also causes VREF to be disabled and to fall to GND. Soft-start latch reset requires all of the following: (A) VIN > 14.25V*, and (B) SD_VSEC > 1.32V, and (C) OC < 107mV, and Figure 16. Timing Diagram SS_MAXDC SOFT-START EVENT TRIGGERED 0.8V (ACTIVE THRESHOLD) 0.45V (RESET THRESHOLD) TIMING (A): SOFT START FAULT REMOVED BEFORE SS_MAXDC FALLS TO 0.45V SS_MAXDC 0.8V (ACTIVE THRESHOLD) 0.45V (RESET THRESHOLD) 0.2V (D) SS_MAXDC < 0.45V (SS_MAXDC reset threshold) *VIN > 8.75V is okay for latch reset if the latch was only set by overcurrent condition in (3) above. SS_MAXDC Discharge Timing It can be seen in Figure 17 that two types of discharge can occur for the SS_MAXDC pin. In timing (A) the fault that caused the soft-start event has been removed before SS_MAXDC falls to 0.45V. This means the soft-start latch will be reset when SS_MAXDC falls to 0.45V and SS_MAXDC will begin charging. In timing (B), the fault that caused the soft-start event is not removed until some time after SS_MAXDC has fallen past 0.45V. The SS_MAXDC pin continues to discharge to 0.2V and remains low until all faults are removed. 42692fb 42692 F17 TIMING (B): SOFT-START FAULT REMOVED AFTER SS_MAXDC FALLS PAST 0.45V Figure 17. Soft-Start Timing SS_MAXDC(DC) LTC4269-2 SS_MAXDC RT CSS RB VREF SS_MAXDC CHARGING MODEL SS_MAXDC(DC) = VREF [RB/(RT + RB)] RCHARGE = [RT • RB/(RT + RB)] RCHARGE CSS 42692 F18 LTC4269-2 SS_MAXDC Figure 18. Programming Soft-Start Timing  LTC4269-2 applicaTions inForMaTion The time for SS_MAXDC to fall to a given voltage can be approximated as: SS_MAXDC(tFALL)= (CSS/IDIS) • [SS_MAXDC(DC) – VSS(MIN)] where: IDIS = net discharge current on CSS CSS = capacitor value at SS_MAXDC pin SS_MAXDC(DC) = programmed DC voltage VSS(MIN) = minimum SS_MAXDC voltage before recharge IDIS ≅ 8e–4 + (VREF – VSS(MIN))[(1/2RB) – (1/RT)] For faults arising from (1) and (2): VREF = 100mV. For a fault arising from (3): VREF = 2.5V. SS_MAXDC(DC) = VREF[RB/(RT + RB)] VSS(MIN) = SS_MAXDC reset threshold = 0.45V (if fault removed before tFALL) Example For an overcurrent fault (OC > 100mV), VREF = 2.5V, RT = 35.7k, RB = 100k, CSS = 0.1µF and assume VSS(MIN) = 0.45V, IDIS ≅ 8e–4 + (2.5 – 0.45)[(½ • 100k) – (1/35.7k)] = 8e–4 + (2.05)(–0.23e–4) = 7.5e–4 SS_MAXDC(DC) = 1.84V SS_MAXDC(tFALL) = (1e–7/7.5e–4) • (1.84 – 0.45)=1.85e–4s If the OC fault is not removed before 185µs then SS_MAXDC will continue to fall past 0.45V towards a new VSS(MIN). The typical VOL for SS_MAXDC at 150µA is 0.2V. SS_MAXDC Charge Timing When all faults are removed and the SS_MAXDC pin has fallen to its reset threshold of 0.45V or lower, the SS_MAXDC pin will be released and allowed to charge. SS_MAXDC will rise until it settles at its programmed DC voltage—setting the maximum switch duty cycle clamp. The calculation of charging time for the SS_MAXDC pin between any two voltage levels can be approximated as an RC charging waveform using the model shown in Figure 16. The ability to predict SS_MAXDC rise time between any two voltages allows prediction of several key timing periods: (1) No Switching Period (time from SS_MAXDC(DC) to VSS(MIN) + time from VSS(MIN) to VSS(ACTIVE)) (2) Converter Output Rise Time (time from VSS(ACTIVE) to VSS(REG); VSS(REG) is the level of SS_MAXDC where maximum duty cycle clamp equals the natural duty cycle of the switch) (3) Time For Maximum Duty Cycle Clamp within X% of Target Value The time for SS_MAXDC to charge to a given voltage VSS is found by re-arranging: VSS(t) = SS_MAXDC(DC) (1 – e(–t/RC)) to give, t = RC • (–1) • ln(1 – VSS/SS_MAXDC(DC)) where, VSS = SS_MAXDC voltage at time t SS_MAXDC(DC) = programmed DC voltage setting maximum duty cycle clamp = VREF(RB/(RT + RB) R = RCHARGE (Figure 16) = RT • RB/(RT + RB) C = CSS (Figure 16) Example (1) No Switching Period The period of no switching for the converter, when a softstart event has occurred, depends on how far SS_MAXDC can fall before recharging occurs and how long a fault exists. It will be assumed that a fault triggering soft-start is removed before SS_MAXDC can reach its reset threshold (0.45V). No Switching Period = tDISCHARGE + tCHARGE tDISCHARGE = discharge time from SS_MAXDC(DC) to 0.45V tCHARGE = charge time from 0.45V to VSS(ACTIVE) tDISCHARGE was already calculated earlier as 185µs. 42692fb  LTC4269-2 applicaTions inForMaTion tCHARGE is calculated by assuming the following: VREF = 2.5V, RT = 35.7k, RB = 100k, CSS = 0.1µF and VSS(MIN) = 0.45V. tCHARGE = t(VSS = 0.8V) – t(VSS = 0.45V) Step 1: SS_MAXDC(DC) = 2.5[100k/(35.7k + 100k)] = 1.84V RCHARGE = (35.7k • 100k/135.7k) = 26.3k Step 2: t(VSS = 0.45V) is calculated from: t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS_MAXDC(DC)) = 2.63e4 • 1e–7 • (–1) • ln(1 – 0.45/1.84) = 2.63e–3 • (–1) • ln(0.755) = 7.3e–4 s Step 3: t(VSS = 0.8V) is calculated from: t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS–MAXDC(DC)) = 2.63e4 • 1e–7 • (–1) • ln(1 – 0.8/1.84) = 2.63e–3 • (–1) • ln(0.565) = 1.5e–3 s From Step 1 and Step 2 tCHARGE = (1.5 – 0.73)e–3 s = 7.7e–4 s The total time of no switching for the converter due to a soft-start event = tDISCHARGE + tCHARGE = 1.85e–4 + 7.7e–4 = 9.55e–4 s Example (2) Converter Output Rise Time The rise time for the converter output to reach regulation can be closely approximated as the time between the start of switching (SS_MAXDC = VSS(ACTIVE)) and the time where converter duty cycle is in regulation (DC(REG)) and no longer controlled by SS_MAXDC (SS_MAXDC = VSS(REG)). Converter output rise time can be expressed as: Output Rise Time = t(VSS(REG)) – t(VSS(ACTIVE)) Step 1: Determine converter duty cycle DC(REG) for output in regulation. The natural duty cycle DC(REG) of the converter depends on several factors. For this example it is assumed that DC(REG) = 60% for power supply input voltage near the power supply UVLO. This gives SD_VSEC = 1.32V. Also assume that the maximum duty cycle clamp programmed for this condition is 72% for SS_MAXDC(DC) = 1.84V, fOSC = 200kHz and RDELAY = 40k. Step 2: Calculate VSS(REG) To calculate the level of SS_MAXDC (VSS(REG)) that no longer clamps the natural duty cycle of the converter, the equation for maximum duty cycle clamp must be used (see previous section Programming Maximum Duty Cycle Clamp). The point where the maximum duty cycle clamp meets DC(REG) during soft-start is given by: DC(REG) = Max Duty Cycle Clamp 0.6 = k • 0.522(SS_MAXDC(DC)/SD_VSEC) – (tDELAY • fOSC) For SD_VSEC = 1.32V, fOSC = 200kHz and RDELAY = 40k This gives k = 1 and tDELAY = 40ns. Rearranging the above equation to solve for SS_MAXDC = VSS(REG) = [0.6 + (tDELAY • fOSC)(SD_VSEC)]/(k • 0.522) = [0.6 + (40ns • 200kHz)(1.32V)]/(1 • 0.522) = (0.608)(1.32)/0.522 = 1.537V Step 3: Calculate t(VSS(REG)) – t(VSS(ACTIVE)) Recall the time for SS_MAXDC to charge to a given voltage VSS is given by: t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS_MAXDC(DC)) (Figure 16 gives the model for SS_MAXDC charging) For RT = 35.7k, RB = 100k, RCHARGE = 26.3k For CSS = 0.1µF this gives t(VSS(ACTIVE)) , = t(VSS(0.8V)) = 2.63e4 • 1e–7 • (–1) • ln(1 – 0.8/1.84) = 2.63e–3 • (–1) • ln(0.565) = 1.5e–3 s t(VSS(REG)) = t(VSS(1.537V)) = 26.3k • 0.1µF • –1 • ln(1 – 1.66/1.84) = 2.63e–3 • (–1) • ln(0.146) = 5e–3 s The rise time for the converter output: = t(VSS(REG)) – t(VSS(ACTIVE)) = (5 – 1.5)e–3 s = 3.5e–3 s 42692fb  LTC4269-2 applicaTions inForMaTion Example (3) Time For Maximum Duty Cycle Clamp to Reach Within X% of Target Value A maximum duty cycle clamp of 72% was calculated previously in the section ‘Programming Maximum Duty Cycle Clamp’. The programmed value used for SS_MAXDC(DC) was 1.84V. The time for SS_MAXDC to charge from its minimum value VSS(MIN) to within X% of SS_MAXDC(DC) is given by: t(SS_MAXDC charge time within X% of target) = t[(1 – (X/100) • SS_MAXDC(DC)] – t(VSS(MIN)) For X = 2 and VSS(MIN) = 0.45V, t(0.98 • 1.84) – t(0.45) = t(1.803) – t(0.45) From previous calculations, t(0.45) = 7.3e–4s. Using previous values for RT , RB and CSS, t(1.803) = 2.63e–4 • 1e–7 • (–1) • ln(1 – 1.803/1.84) = 2.63e–3 • (–1) • ln(0.02) = 1.03e–2 s Hence the time for SS_MAXDC to charge from its minimum reset threshold of 0.45V to within 2% of its target value is given by: t(1.803) – t(0.45) = 1.03e–2 – 7.3e–4 = 9.57e–3s 42692fb 0 PoE-Based Self-Driven Synchronous Forward Power Supply 1mH DO1608C-105 BAS516 PA2431NL 18V PDZ18B BAS516 48V AUXILIARY POWER VPORTP 10µH DO1608C-103 • 6.8µH PG0702.682 5V 5A 5.1 • 4.7nF 250V B1100 107k 237k VCC 0.1µF IRF6217 BAS516 FDS2582 2.2nF 2kV VCC 1.5k 50m 22k 33k PS2801-1-L 2k 10nF OUT OC ISENSE LTC4269-2 COMP FB VREF SS_MAXDC 22.1k 0.1µF VPORTP 82k 158k 332k 158k 100pF 0.22µF 51k 5V 20k 10k 10µF 16V 133 SOUT FDS8880 5.1 FDS8880 5.1 1nF 1nF 10.0k BSS63LT1 36V PDZ36B 10µF 100V 2.2µF 100V 33k 8 PLCS + + • + 220µF 6.3V PSLV0J227M(12)A 54V FROM DATA PAIR 10k Typical applicaTion S1B BAS21 SD_VSEC PWRGD VIN 4.7nF 11.3k VPORTP SHDN SMAJ58A 24k VPORTN T2P VNEG PGND GND BLANK ROSC DELAY 30.9 RCLASS 0.1µF 100V BC857BF 54V FROM SPARE PAIR 1.2k TLV431A 3.65k 2692 TA02 T2P TO MICROCONTROLLER PS2801-1-L Efficiency vs Load Current 95 90 85 80 75 70 65 42V 50V 57V 0.5 1 1.5 2 2.5 3 LOAD (A) 42692 TA02b EFFICIENCY (%) 3.5 4 4.5 5 LTC4269-2  42692fb LTC4269-2 package DescripTion (Reference LTC DWG # 05-08-1734 Rev A) DKD Package 32-Lead Plastic DFN (7mm × 4mm) 0.70 4.50 0.05 3.10 0.05 2.65 0.05 0.05 6.43 0.05 PACKAGE OUTLINE 0.40 BSC 6.00 REF RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.20 0.05 7.00 0.10 R = 0.05 TYP 17 R = 0.115 TYP 32 0.40 0.10 6.43 0.10 4.00 0.10 2.65 0.10 PIN 1 NOTCH R = 0.30 TYP OR 0.35 45 CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 16 0.75 0.05 0.40 BSC 0.20 0.05 1 6.00 REF BOTTOM VIEW—EXPOSED PAD (DKD32) QFN 0707 REV A 0.200 REF 0.00 – 0.05 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE NOTE: 1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WXXX) IN JEDEC PACKAGE OUTLINE M0-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 42692fb  LTC4269-2 revision hisTory REV B DATE 04/10 DESCRIPTION Connected PWRGD Pin to SD_VSEC Pin in Typical Applications Circuits. Added Text Clarifying Connecting PWRGD Pin to SD_VSEC Pin in Shutdown and Programming the Power Supply Undervoltage Lockout Section of Applications Information. (Revision history begins at Rev B) PAGE NUMBER 1, 23, 31 23 42692fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.  LTC4269-2 relaTeD parTs PART NUMBER LT 1952 ® DESCRIPTION Single Switch Synchronous Forward Controller Current Mode Flyback DC/DC Controller in ThinSOT™ Adjustable Frequency Current Mode Flyback Controller Isolate No-Opto Synchronous Flyback Controller with Wide Input Supply Range IEEE 802.3af PD Interface Controller Quad IEEE 802.3af Power over Ethernet Controller Quad IEEE 802.3af Power over Ethernet Controller Single IEEE 802.3af Power over Ethernet Controller High Power Single PSE Controller IEEE 802.3at PD Interface Controller Quad IEEE 802.3at PSE Controller IEEE 802.3af PD Interface with Integrated Switching Regulator 35W High Power PD Interface with Integrated Switching Regulator COMMENTS Synchronous Controller, Programmable Volt-Sec Clamp, Low Start Current 200kHz Constant Frequency, Adjustable Slope Compensation, Optimized for High Input Voltage Applications Slope Comp Overcurrent Protect, Internal/External Clock Adjustable Switching Frequency, Programmable Undervoltage Lockout, Accurate Regulation Without Trim, Synchronous for High Efficiency. 100V 400mA Internal Switch, Programmable Classification Dual Current Limit DC Disconnect Only, IEEE-Compliant PD Detection and Classification, Autonomous Operation or I2C Control AC or DC Disconnect IEEE-Compliant PD Detection and Classification, Autonomous Operation or I2C Control AC or DC Disconnect IEEE-Compliant PD Detection and Classification, Autonomous Operation Internal Switch, Autonomous Operation, 30W 2-Event Classification Signaling, Programmable Classification, Auxiliary Support Type 1 and 2 Compliant, 180mW/Port at 720mA, Advanced Power Management, 4-Point PD Detection 100V 400mA Internal Switch, Programmable Classification, 200KHz or 300kHz Constant Frequency PWM, Interface and Switcher Optimized for IEEE-Compliant PD System. 750mA MOSFET, Programmable Classification, Synchronous No-Opto Flyback Controller, 50kHz to 250kHz LTC3803 LTC3805 LTC3825 LTC4257-1 LTC4258 LTC4259A-1 LTC4263 LTC4263-1 LTC4265 LTC4266 LTC4267-1/ LTC4267-3 LTC4268-1 LTC4269-1 IEEE 802.3at PD Interface Integrated Switching Regulator 2-Event Classification, Programmable Classification, Synchronous No-Opto Flyback Controller, 50kHz to 250kHz 42692fb  Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0409 REV B • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORATION 2009