Electrical Specifications Subject to Change LTC4258 Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection U FEATURES ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®4258 is a quad –48V Hot SwapTM controller designed for use in IEEE 802.3af compliant Power Sourcing Equipment (PSE). It consists of four independent ports, each with output current limit, short-circuit protection, complete Powered Device (PD) detection and classification capability, and programmable PD disconnect using AC or DC sensing. Used with power MOSFETs and passives as in Figure 1, the LTC4258 can implement a complete IEEE 802.3af-compliant PSE. Controls Four Independent – 48V Powered Ethernet Ports Each Port Includes: – IEEE 802®.3af Compliant PD Detection and Classification – Output Current Limit with Foldback – Short-Circuit Protection with Fast Gate Pull-Down – PD Disconnect Using DC Sensing – Power Good Indication Operates Autonomously or Controlled by I2CTM Serial Interface 4-Bit Programmable Digital Address Allows Control of Up to 64 Ports Programmable INT Pin Eliminates Software Polling Current and Duty Cycle Limits Protect External FETs The LTC4258 can operate autonomously or be controlled by an I2C serial interface. Up to 16 LTC4258s may coexist on the same data bus, allowing up to 64 powered Ethernet ports to be controlled with only two digital lines. Fault conditions are optionally signaled with the INT pin to eliminate software polling. U APPLICATIO S ■ ■ ■ External power MOSFETs, current sense resistors and diodes allow easy scaling of current and power dissipation levels and provide protection against voltage and current spikes and ESD events. IEEE 802.3af Compliant Endpoint and Midspan Power Sources IP Phone Systems DTE Power Distribution The LTC4258 is available in a 36-pin SSOP package. Linear Technology also provides solutions for 802.3af PD applications with the LTC4257 and LTC4257-1. , LTC and LT are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation. 802 is a registered trademark of Instutute of Electrical and Electronics Engineers, Inc. I2C is a trademark of Philips Electronics N.V. U TYPICAL APPLICATIO 0.1µF 100V X7R 3.3V 0.1µF INT SCL SDAIN SDAOUT AD0 AD1 AD2 AD3 SHDN1 SHDN2 SHDN3 SHDN4 VDD AUTO BYP RESET DETECT1 DETECT2 DETECT3 LTC4258 DETECT4 CMPD3003 ×4 0.1µF 100V ×4 SMAJ58A ×4 DGND AGND VEE SENSE1 GATE1 OUT1 SENSE2 GATE2 OUT2 SENSE3 GATE3 OUT3 SENSE4 GATE4 OUT4 10k 10k RS1 10k 10k OUT1 –48V Q1 0.1µF RS2 OUT2 Q2 RS3 OUT3 Q3 RS1 TO RS4: 0.5Ω Q1 TO Q4: IRFM120A RS4 Q4 OUT4 4258 F01 Figure 1. Complete 4-Port Powered Ethernet Power Source 4258p 1 LTC4258 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltages VDD to DGND .......................................... – 0.3V to 5V VEE to AGND ......................................... 0.3V to – 70V DGND to AGND (Note 2) .................................... ±1V Digital Pins SCL, SDAIN, SDAOUT, INT, AUTO, RESET SHDNn, ADn ................. DGND – 0.3V to DGND + 5V Analog Pins GATEn (Note 3) ................... VEE – 0.3V to VEE + 12V DETECTn ...................... DGND –20V to DGND +0.3V SENSEn ................................. VEE – 0.3V to VEE + 1V OUTn .................................... VEE – 70V to VEE + 70V OSCIN .......................... DGND – 0.3V to DGND + 5V BYP Current ................................................. ±0.1mA Operating Ambient Temperature Range ...... 0°C to 70°C Junction Temperature (Note 5) ............................ 150°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER TOP VIEW RESET 1 36 NC BYP 2 35 AUTO INT 3 34 OUT1 SCL 4 33 GATE1 SDAOUT 5 32 SENSE1 SDAIN 6 31 OUT2 AD3 7 30 GATE2 AD2 8 29 SENSE2 AD1 9 28 VEE AD0 10 27 OUT3 DETECT1 11 26 GATE3 DETECT2 12 25 SENSE3 DETECT3 13 24 OUT4 DETECT4 14 23 GATE4 DGND 15 VDD 16 LTC4258CGW 22 SENSE4 21 AGND SHDN1 17 20 SHDN4 SHDN2 18 19 SHDN3 GW PACKAGE 36-LEAD PLASTIC SSOP TJMAX = 150°C, θJA = 80°C/W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AGND = DGND = 0V, VDD = 3.3V, VEE = –48V unless otherwise noted (Note 5). SYMBOL PARAMETER Power Supplies VDD VDD Supply Voltage VEE VEE Supply Voltage IDD VDD Supply Current IEE VEE Supply Current VDDMIN VEEMINON VEEMINOFF Detection IDET VDD UVLO Voltage VEE UVLO Voltage (Turning On) VEE UVLO Voltage (Turning Off) Detection Current VDET Detection Voltage Compliance RDETMIN Minimum Valid Signature Resistance RDETMAX Maximum Valid Signature Resistance Classification VCLASS Classification Voltage ICLASS Classification Current Compliance CONDITIONS ● To Maintain IEEE Compliant Output (Note 6) ● MIN TYP MAX UNITS 3 –48 3.3 4 –57 5 –5 85 V V mA mA mA V V V 300 190 –23 19 33 µA µA V kΩ kΩ –21 75 V mA 2.5 –2 ● Normal Operation During Classification Into Short (VDETECTn = 0V) ● ● 2.7 –31 –28 VEE – AGND VEE – AGND First Point, VDETECTn = –10V Second Point, VDETECTn = –3.5V Open Circuit, Measured at DETECTn Pin ● ● ● ● ● 0mA < ICLASS < 31mA Into Short (VDETECT = 0V) 235 145 ● ● 15.2 26.7 –16.4 55 –20 17 29 4258p 2 LTC4258 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AGND = DGND = 0V, VDD = 3.3V, VEE = –48V unless otherwise noted (Note 5). SYMBOL ITCLASS PARAMETER Classification Threshold Current Gate Driver IGON GATE Pin Current IGOFF GATE Pin Current IGPD GATE Pin Short-Circuit Pull-Down ∆VGATE External Gate Voltage (VGATEn – VEE) Output Voltage Sense VPG Power Good Threshold Voltage IVOUT Out Pin Bias Current Current Sense VCUT Overcurrent Detection Sense Voltage VLIM Current Limit Sense Voltage VMIN DC Disconnect Sense Voltage VSC Short-Circuit Sense Voltage ISENSE SENSE Pin Bias Current Digital Interface VOLD Digital Output Low Voltage VILD Digital Input Low Voltage VIHD Digital Input High Voltage RPU Pull-Up Resistor to VDD RPD Pull-Down Resistor to DGND AC Characteristics tDETDLY Detection Delay tDET tCLSDLY Detection Duration Classification Delay tCLASS tPON Classification Duration Power On Delay, Auto Mode CONDITIONS Class 0-1 Class 1-2 Class 2-3 Class 3-4 Class 4-Overcurrent MIN 5.5 13 21 31 45 TYP 6.5 14.5 23 33 48 MAX 7.5 16 25 35 51 UNITS mA mA mA mA mA ● –20 30 –70 95 µA µA mA V 3 –6 –18 V µA µA µA 187.5 212.5 199 224 224 3.75 275 –50 4.97 mV mV mV mV mV mV µA ● ● ● ● ● Gate On, VGATEn = VEE Gate Off, VGATEn = VEE + 5V VGATEn = VEE + 5V IGATE = – 1µA (Note 3) ● ● 10 –50 50 100 13 VOUTn – VEE 0V > VOUT > –10V –10V > VOUT > –30V VOUT = –48V ● 1 2 ● ● 15 –20 VSENSEn – VEE, VOUT = VEE (Note 7) VSENSEn – VEE, VOUT = VEE VSENSEn – VEE, VOUT = AGND – 30V VSENSEn – VEE, VOUT = AGND – 10V VSENSEn – VEE 166 201 201 30.2 2.52 VSENSEn = VEE ISDAOUT = 3mA, IINT = 3mA ISDAOUT = 5mA, IINT = 5mA SCL, SDAIN, RESET, SHDNn, AUTO SCL, SDAIN, RESET, SHDNn, AUTO AD0 to AD3, RESET, SHDNn AUTO ● ● From Detect Command or Application of PD to Port to Detect Complete Time to Measure PD Signature Resistance (Figure 2) From Successful Detect in Auto or Semiauto Mode to Class Complete From Classify Command in Manual (Figure 2) From Valid Detect to Port On in Auto Mode (Figure 2) From Port On Command to GATE Pin Current = IGON (Note 8) ● ● 0.4 0.7 0.8 V V V V kΩ kΩ 170 590 ms 170 10.1 230 52 ms ms 10.1 10.1 420 13 90 1 ms ms ms ms ● ● 2.4 50 50 ● ● ● ● ● 4258p 3 LTC4258 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AGND = DGND = 0V, VDD = 3.3V, VEE = –48V unless otherwise noted (Note 5). SYMBOL tSTART PARAMETER Maximum Current Limit Duration During Port Start-Up tICUT Maximum Current Limit Duration After Port Start-Up DCCLMAX tDIS Maximum Current Limit Duty Cycle Disconnect Delay tVMIN DC Disconnect Minimum Pulse Width Sensitivity CONDITIONS tSTART1 = 0, tSTART0 = 0 (Figure 3) tSTART1 = 0, tSTART0 = 1 tSTART1 = 1, tSTART0 = 0 tSTART1 = 1, tSTART0 = 1 tICUT1 = 0, tICUT0 = 0 (Figure 3) tICUT1 = 0, tICUT0 = 1 tICUT1 = 1, tICUT0 = 0 tICUT1 = 1, tICUT0 = 1 Reg16h = 00h tDIS1 = 0, tDIS0 = 0 (Figure 4) tDIS1 = 0, tDIS0 = 1 tDIS1 = 1, tDIS0 = 0 tDIS1 = 1, tDIS0 = 1 VSENSEn – VEE > 5mV, VOUTn = –48V (Figure 4) (Note 8) Clock Frequency Bus Free Time Start Hold Time SCL Low Time SCL High Time Data Hold Time Data Set-Up Time Start Set-Up Time Stop Set-Up Time SCL, SDAIN Rise Time SCL, SDAIN Fall Time Fault Present to INT Pin Low Stop Condition to INT Pin Low ARA to INT Pin High Time (Note 8) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) Figure 5 (Notes 8, 9) (Notes 8, 9, 10) (Notes 8, 9, 10) (Notes 8, 9) I2C Timing fSCLK t1 t2 t3 t4 t5 t6 t7 t8 tr tf tFLTINT tSTOPINT tARAINT Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: DGND and AGND should be tied together in normal operation. Note 3: An internal clamp limits the GATE pins to a minimum of 12V above VEE. Driving this pin beyond the clamp may damage the part. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 5: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground (AGND and DGND) unless otherwise specified. ● ● ● ● ● ● ● ● ● ● ● ● ● MIN 50 25 100 200 50 25 100 200 5.8 300 75 150 600 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 1.3 600 1.3 600 150 200 600 600 20 20 20 60 20 TYP 60 30 120 240 60 30 120 240 6.3 360 90 180 720 0.02 MAX 70 35 140 280 70 35 140 280 6.7 400 100 200 800 1 UNITS ms ms ms ms ms ms ms ms % ms ms ms ms ms 400 kHz µs ns µs ns ns ns ns ns ns ns ns ns ns 300 150 150 200 300 Note 6: The LTC4258 is designed to maintain a port voltage of –46.6V to –57V and the VEE supply voltage range accounts for the drop across the diode, MOSFET and sense resistor. Note 7: The LTC4258 implements overload current detection per IEEE 802.3af. The minimum overload current (ICUT) is dependent on port voltage; ICUT_MIN = 15.4W/VPORT_MIN. An IEEE compliant system using the LTC4258 should maintain port voltage above –46.6V. Note 8: Guaranteed by design, not subject to test. Note 9: Values measured at VILD and VIHD. Note 10: If fault occurs during an I2C transaction, the INT pin will not be pulled down until a stop condition is present on the I2C bus. 4258p 4 LTC4258 UW TEST TI I G PD INSERTED VPORTn 0V tDET VCLASS PORT TURN ON (AUTO MODE) VT VEE VGATEn INT tCLSDLY 4258 F02 tCLASS tDETDLY tPON Figure 2. Detect, Class and Turn-On Timing in Auto or Semiauto Modes VLIM VCUT VSENSEn TO VEE 0V tSTART, tICUT INT 4258 F03 Figure 3. Current Limit Timing VSENSEn TO VEE VMIN INT tVMIN tDIS 4258 F04 Figure 4. DC Disconnect Timing t3 tr t4 tf SCL t2 t5 t6 t7 t8 SDA t1 4258 F05 Figure 5. I2C Interface Timing 4258p 5 LTC4258 W UW TI I G DIAGRA S SCL SDA 0 1 AD3 AD2 AD1 AD0 R/W ACK A7 0 START BY MASTER A6 A5 A4 A3 A2 A1 ACK BY SLAVE FRAME 1 SERIAL BUS ADDRESS BYTE A0 ACK D7 D6 D5 D4 D3 D2 ACK BY SLAVE D1 D0 ACK STOP BY MASTER ACK BY SLAVE FRAME 2 REGISTER ADDRESS BYTE FRAME 3 DATA BYTE 4258 F06 Figure 6. Writing to a Register SCL SDA 0 1 0 AD3 AD2 AD1 AD0 R/W ACK A7 START BY MASTER A6 A5 A4 A3 A2 FRAME 1 SERIAL BUS ADDRESS BYTE A0 ACK A1 ACK BY SLAVE 0 1 0 AD3 AD2 AD1 AD0 R/W ACK D7 REPEATED START BY MASTER ACK BY SLAVE D6 FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 REGISTER ADDRESS BYTE D5 D4 D3 ACK BY SLAVE D2 D1 D0 ACK NO ACK BY MASTER FRAME 2 DATA BYTE STOP BY MASTER 4258 F07 Figure 7. Reading from a Register SCL SDA 0 1 0 AD3 AD2 AD1 AD0 R/W ACK D7 START BY MASTER D6 D5 D4 D3 ACK BY SLAVE D2 D1 D0 ACK STOP BY MASTER NO ACK BY MASTER FRAME 1 SERIAL BUS ADDRESS BYTE FRAME 2 DATA BYTE 4258 F08 Figure 8. Reading the Interrupt Register (Short Form) SCL SDA 0 0 0 1 1 0 0 R/W ACK START BY MASTER FRAME 1 ALERT RESPONSE ADDRESS BYTE 0 1 ACK BY SLAVE 0 AD3 AD2 AD1 AD0 1 NO ACK BY MASTER FRAME 2 SERIAL BUS ADDRESS BYTE ACK STOP BY MASTER 4258 F09 Figure 9. Reading from Alert Response Address 4258p 6 LTC4258 U U U PI FU CTIO S RESET (Pin 1): Chip Reset, Active Low. When the RESET pin is low, the LTC4258 is held inactive with all ports off and all internal registers reset to their power-up states. When RESET is pulled high, the LTC4258 begins normal operation. RESET can be connected to an external capacitor or RC network to provide a power turn-on delay. Internal filtering of the RESET pin prevents glitches less than 1µs wide from resetting the LTC4258. Pull RESET high with ≤10k or tie to VDD. BYP (Pin 2): Bypass Output. The BYP pin is used to connect the internally generated – 20V supply to an external 0.1µF bypass capacitor. Use a 100V rated 0.1µF, X7R capacitor. Do not connect the BYP pin to any other external circuitry. INT (Pin 3): Interrupt Output, Open Drain. INT will pull low when any one of several events occur in the LTC4258. It will return to a high impedance state when bits 6 or 7 are set in the Reset PB register (1Ah). The INT signal can be used to generate an interrupt to the host processor, eliminating the need for continuous software polling. Individual INT events can be disabled using the Int Mask register (01h). See Register Functions and Applications Information for more information. The INT pin is only updated between I2C transactions. SCL (Pin 4): Serial Clock Input. High impedance clock input for the I2C serial interface bus. The SCL pin should be connected directly to the I2C SCL bus line. SDAOUT (Pin 5): Serial Data Output, Open Drain Data Output for the I2C Serial Interface Bus. The LTC4258 uses two pins to implement the bidirectional SDA function to simplify optoisolation of the I2C bus. To implement a standard bidirectional SDA pin, tie SDAOUT and SDAIN together. See Applications Information for more information. SDAIN (Pin 6): Serial Data Input. High impedance data input for the I2C serial interface bus. The LTC4258 uses two pins to implement the bidirectional SDA function to simplify optoisolation of the I2C bus. To implement a standard bidirectional SDA pin, tie SDAOUT and SDAIN together. See Applications Information for more information. AD3 (Pin 7): Address Bit 3. Tie the address pins high or low to set the I2C serial address to which the LTC4258 responds. This address will be (010A3A2A1A0)b. Pull AD3 high or low with ≤10k or tie to VDD or DGND. AD2 (Pin 8): Address Bit 2. See AD3. AD1 (Pin 9): Address Bit 1. See AD3. AD0 (Pin 10): Address Bit 0. See AD3. DETECT1 (Pin 11): Detect Sense, Port 1. The LTC4258 Powered Device (PD) detection, classification and AC disconnect hardware monitors port 1 with this pin. Connect DETECT1 to the output port via a low leakage diode (see Figure 1). DETECT2 (Pin 12): Detection Sense, Port 2. See DETECT1. DETECT3 (Pin 13): Detection Sense, Port 3. See DETECT1. DETECT4 (Pin 14): Detection Sense, Port 4. See DETECT1. DGND (Pin 15): Digital Ground. DGND should be connected to the return from the 3.3V supply. DGND and AGND should be tied together. VDD (Pin 16): Logic Power Supply. Connect to a 3.3V power supply relative to DGND. VDD must be bypassed to DGND near the LTC4258 with at least a 0.1µF capacitor. SHDN1 (Pin 17): Shutdown Port 1, Active Low. When pulled low, SHDN1 shuts down port 1, regardless of the state of the internal registers. Pulling SHDN1 low is equivalent to setting the Reset Port 1 bit in the Reset Pushbutton register (1Ah). Internal filtering of the SHDN1 pin prevents glitches less than 1µs wide from reseting the LTC4258. Pull SHDN1 high with ≤10k or tie to VDD. SHDN2 (Pin 18): Shutdown Port 2, Active Low. See SHDN1. SHDN3 (Pin 19): Shutdown Port 3, Active Low. See SHDN1. SHDN4 (Pin 20): Shutdown Port 4, Active Low. See SHDN1. 4258p 7 LTC4258 U U U PI FU CTIO S AGND (Pin 21): Analog Ground. AGND should be connected to the return from the – 48V supply. AGND and DGND should be tied together. SENSE4 (Pin 22): Port 4 Current Sense Input. SENSE4 monitors the external MOSFET current via a 0.5Ω sense resistor between SENSE4 and VEE. Whenever the voltage across the sense resistor exceeds the overcurrent detection threshold VCUT, the current limit fault timer counts up. If the voltage across the sense resistor reaches the current limit threshold VLIM (typically 25mV/50mA higher), the GATE4 pin voltage is lowered to maintain constant current in the external MOSFET. See Applications Information for further details. If the port is unused, the SENSE4 pin must be tied to VEE. GATE4 (Pin 23): Port 4 Gate Drive. GATE4 should be connected to the gate of the external MOSFET for port ␣ 4. When the MOSFET is turned on, a 50µA pull-up current source is connected to the pin. The gate voltage is clamped to 13V (typ) above VEE. During a current limit condition, the voltage at GATE4 will be reduced to maintain constant current through the external MOSFET. If the fault timer expires, GATE4 is pulled down with 50µA, turning the MOSFET off and recording a tICUT or tSTART event. If the port is unused, float the GATE4 pin or tie it to VEE. OUT4 (Pin 24): Port 4 Output Voltage Monitor. OUT4 should be connected to the output port through a 10k series resistor. A current limit foldback circuit limits the power dissipation in the external MOSFET by reducing the current limit threshold when the port voltage is within 18V of AGND. The port 4 Power Good bit is set when the voltage from OUT4 to VEE drops below 2V (typ). A 2.5MΩ resistor is connected internally from OUT4 to AGND. If the port is unused, the OUT4 pin can be tied to AGND or allowed to float. SENSE3 (Pin 25): Port 3 Current Sense Input. See SENSE4. GATE3 (Pin 26): Port 3 Gate Drive. See GATE4. OUT3 (Pin 27): Port 3 Output Voltage Monitor. See OUT4. VEE (Pin 28): – 48V Supply Input. Connect to a – 48V to – 57V supply, relative to AGND. SENSE2 (Pin 29): Port 2 Current Sense Input. See SENSE4. GATE2 (Pin 30): Port 2 Gate Drive. See GATE4. OUT2 (Pin 31): Port 2 Output Voltage Monitor. See OUT4. SENSE1 (Pin 32): Port 1 Current Sense Input. See SENSE4. GATE1 (Pin 33): Port 1 Gate Drive. See GATE 4. OUT1 (Pin 34): Port 1 Output Voltage Monitor. See OUT4. AUTO (Pin 35): Auto Mode Input. Auto mode is intended to allow the LTC4258 to detect and power up a PD even if there is no host controller present on the I2C bus. The voltage of the AUTO pin determines the state of the internal registers when the LTC4258 is reset or comes out of VDD UVLO (see the Register map in Table 1). The states of these register bits can subsequently be changed via the I2C interface if desired. The real-time state of the AUTO pin can be read at bit 0 in the Pin Status register (11h). Pull AUTO high or low with ≤10k or tie to VDD or DGND. NC (Pin 36): No Internal Connection. 4258p 8 RO CoR 0Ah Supply Event 0Bh Supply Event CoR RO RO RO RO RO 0Eh Port 3 Status 0Fh Port 4 Status 10h Power Status 11h Pin Status R/W R/W R/W R/W R/W 13h Disconnect Enable 14h Detect/Class Enable 15h Reserved 16h Timing Config 17h Misc Config WO 1Ah Reset PB Change 4 Pwr Good Change 3 Pwr Good Mask 6 Change 2 Pwr Good Mask 5 Change 1 Pwr Good Mask 4 Class Complete Change 4 Pwr Enable Mask 3 Detect Complete Change 3 Pwr Enable Mask 2 Disconnect BIT 2 Change 2 Pwr Enable Mask 1 Pwr Good Event BIT 1 Change 1 Pwr Enable Mask 0 Pwr Enable Event BIT 0 Global 4321 4321 Global Global 4321 4321 4321 Global 4321 4 3 2 1 4321 4321 4321 Clear All Interrupts Power Off 4 Restart Class 4 Interrupt Pin Enable Reserved Reserved Class Enable 4 Reserved Port 4 Mode 1 Reserved Power Good 4 Reserved Reserved Reserved Reserved Over Temp Reserved Disconnect 4 Reserved Power Off 2 Restart Class 2 Reserved tSTART1 Reserved Class Enable 2 Reserved Port 3 Mode 1 AD3 Pin Status Power Good 2 Class Status 1 Class Status 1 Class Status 1 Class Status 1 VDD UVLO Reserved Disconnect 2 Power Good 1 Class Status 0 Class Status 0 Class Status 0 Class Status 0 VEE UVLO Reserved Disconnect 1 Reset Port 4 Power On 4 Restart Detect 4 Reserved tICUT1 Reserved Detect Enable 4 DC Discon En 4 Port 2 Mode 1 AD1 Pin Status Power Enable 4 Reserved Reserved Reserved Reserved Reserved tSTART Fault 4 tICUT Fault 4 Reset Port 3 Power On 3 Restart Detect 3 Reserved tICUT0 Reserved Detect Enable 3 DC Discon En 3 Port 2 Mode 0 AD0 Pin Status Power Enable 3 Detect Status 2 Detect Status 2 Detect Status 2 Detect Status 2 Reserved tSTART Fault 3 tICUT Fault 3 Power Off, Detection and Class Off Will Not Advance Between States Detect and Class But Wait to Turn On Power Detect, Class and Power Automatically Reset All Power Off 1 Restart Class 1 Reserved tSTART0 Reserved Class Enable 1 Reserved Port 3 Mode 0 AD2 Pin Status MODE BIT ENCODING 00 Shutdown 01 Manual 10 Semiauto 11 Auto Clear Interrupt Pin Power Off 3 Restart Class 3 Reserved Reserved Reserved Class Enable 3 Reserved Port 4 Mode 0 Reserved Power Good 3 Class Status 2 Class Status 2 Class Status 2 Class Status 2 Reserved Reserved Disconnect 3 RESET STATE 0000,0000 0000,0000 0000,0000 1000,0000 0000,0000 0000,0000 0000,0000 0000,0000 0000,0000 00A3A2,A1A000 0000,0000 0000,0000 0000,0000 0000,0000 0000,0000 0011,0000** 0000,0000 0000,0000 0000,0000 0000,0000 1000,0000 0000,0000 0000,0000 0000,0000 1000,0000 0000,0000 0000,0000 1111,1111 0000,1111 1111,1111 00A3A2,A1A001 0000,0000 0000,0000 0000,0000 0000,0000 0000,0000 0011,0000** 0000,0000 0000,0000 0000,0000 0000,0000 1110,0100 1000,0000 Auto Pin High RESET STATE ** the start-up state of the VEE UVLO and Osc Fail bits depend on the order in which the VDD and VEE supplies are brought up Reset Port 1 Power On 1 Restart Detect 1 Reserved tDIS0 Reserved Detect Enable 1 DC Discon En 1 Port 1 Mode 0 Auto Pin Status Power Enable 1 Detect Status 0 Detect Status 0 Detect Status 0 Detect Status 0 Reserved tSTART Fault 1 tICUT Fault 1 Key: RO = Read Only R/W = Read/Write CoR = Clear on Read WO = Write Only Reset Port 2 Power On 2 Restart Detect 2 Reserved tDIS1 Reserved Detect Enable 2 DC Discon En 2 Port 1 Mode 1 Reserved Power Enable 2 Detect Status 1 Detect Status 1 Detect Status 1 Detect Status 1 Reserved tSTART Fault 2 tICUT Fault 2 4321 Class Complete 4 Class Complete 3 Class Complete 2 Class Complete 1 Detect Complete 4 Detect Complete 3 Detect Complete 2 Detect Complete 1 4321 Mask 7 Encoding CLASS STATUS DETECT STATUS 000 Class Status Unknown 000 Detect Status Unknown 001 Class 1 001 Short Circuit (<1V) 010 Class 2 010 Reserved 011 Class 3 011 RLOW 100 Class 4 100 Detect Good 101 Undefined—Read as Class 0 101 RHIGH 110 Class 0 110 Open Circuit 111 Overcurrent 111 Reserved WO WO 18h Det/Class Restart PB 19h Power Enable PB Pushbuttons R/W 12h Operating Mode Configuration RO 0Ch Port 1 Status 0Dh Port 2 Status Status RO CoR 09h tSTART Event CoR 07h Fault Event CoR 08h tSTART Event RO CoR 06h Fault Event RO CoR 05h Detect Event CoR 03h Power Event CoR 04h Detect Event RO CoR 02h Power Event Events Global tICUT Fault BIT 3 R/W tSTART Fault BIT 4 01h Int Mask Supply Event BIT 5 1000,0000 Global BIT 6 RO BIT 7 00h Interrupt PORT Auto Pin Low R/W Interrupts TABLE 1. REGISTER W ADDRESS REGISTER NAME LTC4258 AP 4258p 9 LTC4258 U U REGISTER FU CTIO S Interrupt Registers Interrupt (Address 00h): Interrupt Register, Read Only. A transition to logical 1 of any bit in this register will assert the INT pin (Pin 3) if the corresponding bit in the Int Mask register is set. Each bit is the logical OR of the corresponding bits in the Event registers. The Interrupt register is Read Only and its bits cannot be cleared directly. To clear a bit in the Interrupt register, clear the corresponding bits in the appropriate Status or Event registers or set bit 7 in the Reset Pushbutton register (1Ah). Int Mask (Address 01h): Interrupt Mask, Read/Write. A logic 1 in any bit of the Int Mask register allows the corresponding Interrupt register bit to assert the INT pin if it is set. A logic 0 in any bit of the Int Mask register prevents the corresponding Interrupt bit from affecting the INT pin. The actual Interrupt register bits are unaffected by the state of the Int Mask register. Event Registers Power Event (Address 02h): Power Event Register, Read Only. The lower four bits in this register indicate that the corresponding port Power Enable status bit has changed; the logical OR of these four bits appears in the Interrupt register as the Pwr Enable Event bit. The upper four bits indicate that the corresponding port Power Good status bit has changed; the logical OR of these four bits appears in the Interrupt register as the Pwr Good Event bit. The Power Event bits latch high and will remain high until cleared by reading from address 03h. Power Event CoR (Address 03h): Power Event Register, Clear on Read. Read this address to clear the Power Event register. Address 03h returns the same data as address 02h and reading address 03h clears all bits at both addresses. Detect Event (Address 04h): Detect Event Register, Read Only. The lower four bits in this register indicate that at least one detection cycle for the corresponding port has completed; the logical OR of these four bits appears in the Interrupt register as the Detect Complete bit. The upper four bits indicate that at least one classification cycle for the corresponding port has completed; the logical OR of these four bits appears in the Interrupt register as the Class Complete bit. In Manual mode, this register indicates that the requested detection/classification cycle has completed and the LTC4258 is awaiting further instructions. In Semiauto or Auto modes, these bits indicate that the Detect Status and Class Status bits in the Port Status registers are valid. The Detect Event bits latch high and will remain high until cleared by reading from address 05h. Detect Event CoR (Address 05h): Detect Event Register, Clear on Read. Read this address to clear the Detect Event register. Address 05h returns the same data as address 04h, and reading address 05h clears all bits at both addresses. Fault Event (Address 06h): Fault Event Register, Read Only. The lower four bits in this register indicate that a tICUT fault has occurred at the corresponding port; the logical OR of these four bits appears in the Interrupt register as the tICUT Fault bit. The upper four bits indicate that a Disconnect event has occurred at the corresponding port; the logical OR of these four bits appears in the Interrupt register as the Disconnect bit. The Fault Event bits latch high and will remain high until cleared by reading from address 07h. Fault Event CoR (Address 07h): Fault Event Register, Clear on Read. Read this address to clear the Fault Event register. Address 07h returns the same data as address 06h and reading address 07h clears all bits at both addresses. tSTART Event (Address 08h): tSTART Event Register, Read Only. The lower four bits in this register indicate that a tSTART fault has occurred at the corresponding port; the logical OR of these four bits appears in the Interrupt register as the tSTART Fault bit. The tSTART Event bits latch high and will remain high until cleared by reading from address 09h. The upper four bits in this register are reserved and will always read as 0. tSTART Event CoR (Address 09h): tSTART Event Register, Clear on Read. Read this address to clear the Fault Event register. Address 09h returns the same data as address 08h and reading address 09h clears all bits at both addresses. Supply Event (Address 0Ah): Supply Event Register, Read Only. Bit 4 indicates that VEE has dropped below the VEE UVLO level (typically –28V). Bit 5 signals that the VDD supply has dropped below the VDD UVLO threshold. Bit 7 indicates that the LTC4258 die temperature has exceeded its thermal shutdown limit (see Note 4 under Electrical Characteristics). The logical OR of bits 4, 5 and 7 appears in the Inter4258p 10 LTC4258 U U REGISTER FU CTIO S rupt register as the Supply Fault bit. The remaining bits in the register are reserved and will always read as 0. The Supply Event bits latch high and will remain high until cleared by reading from address 0Bh. LTC4258 configuration registers at start-up but has no effect after start-up and can be used as a general purpose input if desired, as long as it is guaranteed to be in the appropriate state at start-up. Supply Event CoR (Address 0Bh): Supply Event Register, Clear on Read. Read this address to clear the Fault Event register. Address 0Bh returns the same data as address 0Ah, and reading address 0Bh clears all bits at both addresses. Configuration Registers Status Registers Port 1 Status (Address 0Ch): Port 1 Status Register, Read Only. This register reports the most recent detection and classification results for port 1. Bits 0-2 report the status of the most recent detection attempt at the port and bits 4-6 report the status of the most recent classification attempt at the port. If power is on, these bits report the detection/ classification status present just before power was turned on. If power is turned off at the port for any reason, all bits in this register will be cleared. See Table 1 for detection and classification status bit encoding. Port 2 Status (Address 0Dh): Port 2 Status Register, Read Only. See Port 1 Status. Port 3 Status (Address 0Eh): Port 3 Status Register, Read Only. See Port 1 Status. Port 4 Status (Address 0Fh): Port 4 Status Register, Read Only. See Port 1 Status. Power Status (Address 10h): Power Status Register, Read Only. The lower four bits in this register report the switch on/off state for the corresponding ports. The upper four bits (the power good bits) indicate that the drop across the power switch and sense resistor for the corresponding ports is less than 2V (typ) and power start-up is complete. The power good bits are latched high and are only cleared when a port is turned off or the LTC4258 is reset. Pin Status (Address 11h): External Pin Status, Read Only. This register reports the real time status of the AUTO (Pin␣ 35) and AD0-AD3 (Pins 7-10) digital input pins. The logic state of the AUTO pin appears at bit␣ 0 and the AD0-AD3 pins at bits 2-5. The remaining bits are reserved and will read as 0. AUTO affects the initial states of some of the Operating Mode (Address 12h): Operating Mode Configuration, Read/Write. This register contains the mode bits for each of the four ports in the LTC4258. See Table 1 for mode bit encoding. At power-up, all bits in this register will be set to the logic state of the AUTO pin (Pin␣ 35). See Operating Modes in the Applications Information section. Disconnect Enable (Address 13h): Disconnect Enable Register, Read/Write. The lower four bits of this register enable or disable DC disconnect detection circuitry at the corresponding port. If the DC Discon Enable bit is set the port circuitry will turn off power if the current draw at the port falls below IMIN for more than tDIS. IMIN is equal to VMIN/ RS, where RS is the sense resistor and should be 0.5Ω for IEEE 802.3af compliance. If the bit is clear the port will not remove power due to low current. Detect/Class Enable (Address 14h): Detection and Classification Enable, Read/Write. The lower four bits of this register enable the detection circuitry at the corresponding port if that port is in Auto or Semiauto mode. The upper four bits enable the classification circuitry at the corresponding port if that port is in Auto or Semiauto mode. In manual mode, setting a bit in this register will cause the LTC4258 to perform one classification or detection cycle on the corresponding port. Writing to the Detect/Class Restart PB (18h) has the same effect without disturbing the Detect/Class Enable bits for other ports. Timing Config (Address 16h): Global Timing Configuration, Read/Write. Bits 0-1 program tDIS, the time duration before a port is automatically tuned off after the PD is removed. The LTC4258 can be programmed to monitor whether port current is below IMIN (DC connect) or port impedance is high (AC disconnect). Bits 2-3 program tICUT, the time during which a port’s current can exceed ICUT without it being turned off. If the current is still above ICUT after tICUT, the LTC4258 will indicate a tICUT fault and turn the port off. Bits 4258p 11 LTC4258 U U REGISTER FU CTIO S 4-5 program tSTART, the time duration before an overcurrent condition during port power-on is considered a tSTART fault and the port is turned off. Note that using the tICUT and tSTART times other than the default is not compliant with IEEE 802.3af and may double or quadruple the energy dissipated by the external MOSFETs during fault conditions. Bits 6-7 are reserved and should be read/written as 0. See Electrical Characteristics for timer bit encoding. Also see the Applications Information for descriptions of tSTART, tICUT and DC disconnect timing. Misc Config (Address 17h): Miscellaneous Configuration, Read/Write. Setting bit 7 enables the INT pin. If this bit is reset, the LTC4258 will not pull down the INT pin in any condition nor will it respond to the Alert Response Address. This bit is set by default. Pushbutton Registers Note Regarding Pushbutton Registers: “Pushbutton” registers are specialized registers that trigger an event when a 1 is written to a bit; writing a 0 to a bit will do nothing. Unlike a standard read/write register, where setting a single bit involves reading the register to determine its status, setting the appropriate bit in software and writing back the entire register, a pushbutton register allows a single bit to be written without knowing or affecting the status of the other bits in the register. Pushbutton registers are writeonly and will return 00h if read. Det/Class Restart PB (Address 18h): Detection/Classification Restart Pushbutton Register, Write Only. Writing a 1 to any bit in this register will start or restart a single detection or classification cycle at the corresponding port in Manual mode. It can also be used to set the corresponding bits in the Detect/Class Enable register (address 14h) for ports in auto or semiauto mode. The lower 4 bits affect detection on each port while the upper 4 bits affect classification. Power Enable PB (Address 19h): Power Enable Pushbutton Register, Write Only. The lower four bits of this register set the Power Enable bit in the corresponding Port Status register; the upper four bits clear the corresponding Power Enable bit. Setting or clearing the Power Enable bits via this register will turn on or off the power in any mode except shutdown, regardless of the state of detection or classification. Note that tICUT, tSTART and disconnect events (if enabled) will still turn off power if they occur. The Power Enable bit cannot be set if the port has turned off due to a tICUT or tSTART fault and the tICUT timer has not yet counted back to zero. See Applications Information for more information on tICUT timing. Clearing the Power Enable bits with this register also clears the detect and fault event bits, the Port Status register, and the Detection and Classification Enable bits for the affected port(s). Reset PB (Address 1Ah): Reset Pushbutton, Write Only. Bits 0-3 reset the corresponding port by clearing the power enable bit, the detect and fault event bits, the status register and the detection and classification enable bits for that port. Bit 4 returns the entire LTC4258 to the power-on reset state; all ports are turned off, the AUTO pin is reread and all registers are returned to their power-on defaults, except VDD UVLO, which remains cleared. Bit 5 is reserved; setting it has no effect. Setting bit 6 releases the Interrupt pin if it is asserted without affecting the Event registers or the Interrupt register. When the INT pin is released in this way, the condition causing the LTC4258 to pull the INT pin down must be removed before the LTC4258 will be able to pull INT down again. This can be done by reading and clearing the event registers or by writing a 1 into bit 7 of this register. Setting bit 7 releases the Interrupt pin, clears all the Event registers and clears all the bits in the Interrupt register. 4258p 12 LTC4258 U W U U APPLICATIO S I FOR ATIO OVERVIEW Over the years, twisted-pair Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has defined an extension to the standard, known as 802.3af, which allows DC power to be delivered simultaneously over the same cable used for data communication. This promises a whole new class of Ethernet devices, including IP telephones, wireless access points, and PDA charging stations, which do not require additional AC wiring or external power transformers, a.k.a. “wall warts.” With about 13W of power available, small data devices can be powered by their Ethernet connections, free from AC wall outlets. Sophisticated detection and power monitoring techniques prevent damage to legacy data-only devices, while still supplying power to newer, Ethernetpowered devices over the twisted-pair cable. A device that supplies power is called Power Sourcing Equipment (PSE); a device that draws power from the wire is called a Powered Device (PD). A PSE is typically an Ethernet switch, router, hub, or other network switching PSE RJ45 4 CAT 5 20Ω MAX ROUNDTRIP 0.05µF MAX 5 0.1µF 100V DGND BYP I2C A PSE is required to provide a nominal 48V DC between either the signal pairs or the spare pairs (but not both) as shown in Figure 10. The power is applied as a voltage between two of the pairs, typically by powering the centertaps of the isolation transformers used to couple the differential data signals to the wire. Since Ethernet data is transformer coupled at both ends and is sent differentially, a voltage difference between the transmit pairs and the receive pairs does not affect the data. A 10base-T/ 100base-TX Ethernet connection only uses 2 of the 4 pairs in the cable. The unused or spare pairs can be powered directly, as shown in Figure 10, without affecting the data. However, 1000base-T uses all 4 pairs and power must be connected to the transformer center taps if compatibility with 1000base-T is required. PD RJ45 4 5 1N4002 ×4 SPARE PAIR GND 3.3V INTERRUPT equipment that is commonly found in the wiring closets where cables converge. PDs can take many forms: digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers, and HVAC thermostats are examples of devices that can draw power from the network. 0.1µF AGND CMPD3003 VDD DETECT INT 1/4 SCL LTC4259A SDAIN SDAOUT VEE 1 3 10k –48V IRFM120A S1B 2 3 Rx SENSE GATE OUT 0.5Ω DATA PAIR 0.1µF Tx 6 DATA PAIR 6 5µF ≤ CIN ≤ 300µF SMAJ58A 58V Rx 2 1k 0.47µF 100V X7R 1 Tx 1N4002 ×4 GND RCLASS SMAJ58A 58V PWRGD LTC4257 7 7 6 6 –48VIN –48VOUT DC/DC CONVERTER + VOUT – SPARE PAIR 4258 F10 Figure 10. System Diagram 4258p 13 LTC4258 U W U U APPLICATIO S I FOR ATIO The LTC4258 provides a complete solution for detection and powering of PD devices in an IEEE 802.3af compliant system. The LTC4258 consists of four independent ports, each with the ability to detect, classify, and provide isolated –48V power to a PD device connected to it. The LTC4258 senses removal of a PD with IEEE 802.3af compliant AC or DC methods and turns off –48V power when the PD is removed. An internal control circuit takes care of system configuration and timing, and uses an I2C interface to communicate with the host system. Regardless of which mode it is in, the LTC4258 will remove power automatically from any port that generates a tSTART or tICUT overcurrent fault event (see tICUT Timing and tSTART Timing sections). It will also automatically remove power from any port that generates a disconnect event if the appropriate Disconnect Enable bit is set in the Disconnect Enable register. The host controller may also remove power at any time by setting the appropriate Power Off bit in the Power Enable PB register. Power-On RESET OPERATING MODES Each LTC4258 port can operate in one of four modes: Manual, Semiauto, Auto or Shutdown. The operating mode for a port is set by the appropriate bits in the Operating Mode register. The LTC4258 will power up with all ports in Shutdown mode if the external AUTO pin is tied low; if AUTO is high, all ports will wake up in Auto mode. The operating mode can be changed at any time via the I2C interface, regardless of the state of the AUTO pin. • In Manual mode, a port will wait for instructions from the host system before taking any action. It will run single detection or classification cycles when commanded, and will report results in the Port Status registers. When the host system decides it is time to turn on or off power to a port, it can do so by setting the appropriate Power On/Off bits in the Power Enable PB register regardless of the current status of detection or classification. • In Semiauto mode, the port will repeatedly attempt to detect and classify a PD device␣ attached to the link. It will report this information in its Port Status register, and wait for the host system to set the appropriate Power On bit in the Power Enable PB register before applying power to the port. • In Auto mode, the port will detect and classify a PD device connected to it, then immediately turn on the power if detection was successful regardless of the result of classification. • In Shutdown mode, the port is disabled and will not detect or power a PD. Also, the detect and fault event bits, status bits and enable bits for the port are reset to zero. At turn-on or any time the LTC4258 is reset (either by pulling the RESET pin low or writing to the global Reset All bit), all the ports turn off and all internal registers go to a predefined state, shown in Table 1. Several of the registers assume different states based on the state of the AUTO pin at reset. The default states with AUTO high allow the LTC4258 to detect and power up a PD in Automatic mode, even if nothing is connected to the I2C interface. SIGNATURE DETECTION The IEEE defines a specific pair-to-pair PD signature resistance that identifies a device that can accept Power over Ethernet in accordance with the 802.3af specification. When the port voltage is below 10V, an 802.3af compliant PD will have a 25k signature resistance. Figure 11 illustrates the relationship between the PD signature resistance (white box from 23.75k to 26.25k) and required resistance ranges the PSE must accept (white box) and reject (gray boxes). According to the 802.3af specification, the PSE may or may not accept resistances in the two ranges of 15k to 19k and 26.5k to 33k. Note that the black box in Figure 11 represents the 150Ω pair-to-pair termination used in legacy 802.3 devices like a computer’s network interface card (NIC) that cannot accept power. RESISTANCE 0Ω PD PSE 10k 20k 150Ω (NIC) 15k 30k 23.75k 26.25k 19k 26.5k 33k 4258 F11 Figure 11. IEEE 802.3af Signature Resistance Ranges 4258p 14 LTC4258 U W U U APPLICATIO S I FOR ATIO The LTC4258 checks for the signature resistance by forcing two test currents on the port (via the DETECTn pins) in sequence and measuring the resulting voltages. It then subtracts the two V-I points to determine the resistive slope while removing voltage offset caused by any series diodes or current offset caused by leakage at the port (see Figure 12). The LTC4258 will typically accept any PD resistance between 17k and 29k as a valid PD and report Detect Good (100 binary) in the Detect Status bits (bits 2 through 0) of the corresponding Port Status register. Values outside this range, including open and short circuits, are also reported in the Detect Status bits. Refer to Table 1 for a complete decoding of the Detect Status bits. The first test point is taken by forcing a test current into the port, waiting a short time to allow the line to settle and measuring the resulting voltage. This result is stored and the second current is applied to the port, allowed to settle and the voltage measured. Each point takes 100ms to measure, and an entire detection cycle takes 200ms. The port’s operating mode controls if and when the LTC4258 runs a detection cycle. In manual mode, the port will sit idle until a Restart Detection (register 18h) command is received. It will then run a complete 200ms detection cycle on the selected port, report the results in the Detect Status bits in the corresponding Port Status register and return to idle until another command is received. In Semiauto mode, the LTC4258 autonomously tests valid PDs connected to the ports but it will not apply power until instructed to do so by the host controller. It repeatedly queries the port every 320ms and updates the Detect Status bits at the end of each cycle. If a Detect Good is reported, it will advance to the classification phase and report that result in the Port Status register. Until instructed to do otherwise, the LTC4258 will continue to repeat detection on the port. Behavior in Auto mode is similar to Semiauto; however, after a Detect Good is reported, the LTC4258 performs the classification phase and then powers up the port without further intervention. The signature detection circuitry is disabled when the port is in Shutdown mode, powered up or the corresponding Detect Enable bit is cleared. 275 CURRENT (µA) The LTC4258 will not report Detect Good if the PD has more than 5µF in parallel with its signature resistor. 25kΩ SLOPE 165 VALID PD 0V-2V OFFSET FIRST DETECTION POINT CLASSIFICATION SECOND DETECTION POINT A PD has the option of presenting a “classification signature” to the PSE to indicate how much power it will draw when powered up. This signature consists of a specific constant current draw when the PSE port voltage is between 15.5V and 20.5V, with the current level indicating the power class to which the PD belongs. Per the IEEE 802.3af specification, the LTC4258 identifies the five classes of PD listed in Table 2. During classification, the LTC4258 controls and VOLTAGE 4258 F12 Figure 12. PD Detection Table 2. IEEE 802.3af Powered Device Classes IEEE 802.3af CLASS CLASSIFICATION CURRENT AT PSE MAXIMUM PD POWER MINIMUM PSE OUTPUT POWER 0 0mA to 5mA 12.95W 15.4W 1 8mA to 13mA 3.84W 4W Low Power PD 2 16mA to 21mA 6.49W 7W Medium Power PD 3 25mA to 31mA 12.95W 15.4W High or Full Power PD 4 35mA to 45mA 12.95W 15.4W Reserved, Power as Class O CLASS DESCRIPTION PD Does Not Implement Classification, Unknown Power 4258p 15 LTC4258 U W U U APPLICATIO S I FOR ATIO measures the port voltage through the DETECTn pin. Note that class 4 is presently specified by the IEEE as reserved for future use. Figure 13 shows a PD load line, starting with the shallow slope of the 25k signature resistor below 10V, then drawing the classification current (in this case, class␣ 3) between 14.5V and 20.5V. The LTC4258’s load line for classification is also shown in Figure 13. It has low impedance until current limit at 65mA (typ). The LTC4258 will classify a port immediately after a successful detection cycle in Semiauto or Auto modes, or when commanded to in Manual mode. It measures the PD classification signature current by applying 18V (typ) to the port and measuring the resulting current. It reports the detected class in the Class Status bits in the corresponding Port Status register. Note that in Auto mode, the port will power up regardless of which class is detected. The classification circuitry is disabled when the port is in Shutdown mode, powered up, or the corresponding Class Enable bit is cleared. 60 PSE LOAD LINE OVER CURRENT 50 CURRENT (mA) 48mA 40 CLASS 4 30 CLASS 3 20 TYPICAL CLASS 3 PD LOAD LINE 10 0 0 5 No External Capacitors No external capacitors are required on the GATE pins for active current limit stability, lowering part count and cost. This also allows the fastest possible turn-off under severe overcurrent conditions, providing maximum safety and protection for the MOSFETs, load devices and board traces. Connecting capacitors to the external MOSFET gates can adversely affect the LTC4258’s ability to respond to a shorted port. Inrush Control 23mA The 802.3af standard lists two separate maximum current limits, ILIM and IINRUSH. Because they have identical values, the LTC4258 implements both as a single current limit using VLIM (described below). Their functions are differentiated through the use of tICUT and tSTART, respectively (see tICUT Timing and tSTART Timing sections). To maintain consistency with the standard, the IINRUSH term is used when referring to an initial tSTART power-up event. 14.5mA CLASS 1 10 15 VOLTAGE (VCLASS) Once the decision has been made to turn on power to a port, the LTC4258 uses a 50µA current source to pull up on the GATE pin. Under normal power-up circumstances, the MOSFET gate will charge up rapidly to VT (the MOSFET threshold voltage), the MOSFET current will rise quickly to the current limit level and the GATE pin will be servoed to maintain the proper IINRUSH charging current. When output charging is complete, the MOSFET current will fall and the GATE pin will be allowed to continue rising to fully enhance the MOSFET and minimize its on resistance. The final VGS is nominally 13V. When a port is turned off, a 50µA current source pulls down on the GATE pin, turning the MOSFET off in a controlled manner. 33mA CLASS 2 CLASS 0 Gate Currents 6.5mA 20 25 4258 F13 Figure 13. PD Classification POWER CONTROL The primary function of the LTC4258 is to control the delivery of power to the PSE port. It does this by controlling the gate drive voltage of an external power MOSFET while monitoring the current via a sense resistor and the output voltage at the OUT pin. This circuitry serves to couple the raw isolated –48V input supply to the port in a controlled manner that satisfies the PD’s power needs while minimizing disturbances on the –48V backplane. When the LTC4258 turns on a port, it turns on the MOSFET by pulling up on the gate. The LTC4258 is designed to power up the port in current limit, limiting the inrush current to IINRUSH. The port voltage will quickly rise to the point where the PD reaches its input turn-on threshold and begins to draw current to charge its bypass capacitance, slowing the rate of port voltage increase. 4258p 16 LTC4258 U W U U APPLICATIO S I FOR ATIO Dual-Level Current Limit A PD is permitted to draw up to 15.4W continuously and up to 400mA for 50ms. The LTC4258 has two corresponding current limit thresholds, ICUT (375mA typ) and ILIM (425mA typ). These are given by the equations: ICUT = VCUT/RS, ILIM = VLIM/RS RS is the sense resistor and should be 0.5Ω for IEEE 802.3af compliance. While the LTC4258 allows the port current to exceed ICUT for a limited time period (see tICUT timing below), it does not allow the current to exceed I LIM. The current limit circuit monitors the port current by monitoring the voltage across the sense resistor and reduces the MOSFET gate voltage as needed to keep the current at or below ILIM. When the current drops below ILIM, the gate voltage is restored to the full value to keep the MOSFET resistance to a minimum. tICUT Timing Whenever more than ICUT = VCUT/RS flows through a port, the port’s sense voltage is above VCUT and the tICUT timer counts up. If the sense voltage is still above VCUT when the tICUT timer expires, the LTC4258 will turn off the power to the port immediately and set the appropriate tICUT Fault bit in register 06h/07h. The tICUT timer duration can be programmed via register 16h, bits 3 and 2 (Table 1). The tICUT timer is an up/down counter that is designed to protect the external MOSFET from thermal stress caused by repeatedly operating in current limit. The counter counts up whenever the current is above ICUT and counts down at 1/16th the rate when it is not. The counter will bottom out at zero to prevent underflow. Full count indicates that the tICUT timer has expired and the port will be turned off. This count up/count down behavior implements duty cycle protection, preventing intermittent current limit faults from causing cumulative thermal stress in the MOSFET. If the port enters current limit but then exits before the timer expires, the count will decrease slowly, giving the ICUT timer the ability to turn off sooner in the case of a repetitive fault. If the overcurrent duty cycle is less than 6.3% the tICUT timer will be fully reset. If the tICUT timer expires and causes the port to shut off, the timer will continue to run, counting down at the slow 1/16th rate and preventing the port from being repowered until the count returns to zero. This protects the MOSFET from damage due to a faulty PD that may still have a valid signature, or from errant software that repeatedly writes to the Power On bit. The port will not repower until after the tICUT counter returns to zero. In manual and semiauto modes the power enable command must be received after the tICUT counter reaches zero. In auto mode the LTC4258 must complete a valid detection cycle after the tICUT counter reaches zero. tSTART Timing To distinguish between normal turn-on current limit behavior and current limit faults which occur after power-up is complete, the LTC4258 starts a timer (the tSTART timer) whenever a power-up sequence begins. The tSTART timer serves three functions. First and foremost, it allows the user to specify a different current limit timeout (tSTART instead of tICUT) during turn-on (current limit duty cycle protection remains functional). Second, the DC disconnect timer is disabled during this period and can only begin counting up after the tSTART timer has expired. Together, these two features let the PD draw the maximum current IINRUSH to charge its input capacitance, boot up and begin drawing power without triggering a tSTART fault. Finally, if the device is in current limit for the entire tSTART period, a tSTART fault will be generated instead of a tICUT fault. This can be useful for tracking down the cause of a current fault. As long as the PD draws less than ICUT at the end of tSTART and begins drawing the minimum current within tDIS after tSTART expires (if DC disconnect is enabled), no faults will be indicated. The tSTART timer also implements the duty cycle protection described under tICUT timing and its duration can be programmed via register 16h, bits 5 and 4 (Table 1). 4258p 17 LTC4258 U W U U APPLICATIO S I FOR ATIO Foldback Foldback is designed to limit power dissipation in the MOSFET during power-up and momentary short-circuit conditions. At low port output voltages, the voltage across the MOSFET is high, and power dissipation will be large if significant current is flowing. Foldback monitors the port output voltage and reduces the VLIM current limit level linearly from its full value (212.5mV typ) at a port voltage of 18V to approximately 1/7th of the full value (30mV typ) at a port voltage of 0V. With 0.5Ω sense resistors, this limits the short-circuit current to 60mA (typ) instead of the full 425mA (typ) current limit. When the LTC4258 is in foldback, the tICUT timer is active. Short-Circuit Protection If a port is suddenly shorted out, the MOSFET power dissipation can rise to very high levels, jeopardizing the MOSFET even before the normal current limit circuit can respond. A separate short-circuit current limit circuit watches for significant overcurrent events (VSENSE >275mV, >550mA with a 0.5Ω sense resistor) and pulls the GATE pin down immediately if such an event occurs, shutting off the MOSFET in less than 1µs (with no external capacitor on GATE). Approximately 100µs later, GATE is allowed to rise back up and the normal current limit circuit will take over, allowing ILIM current to flow and causing the tICUT timer to count up. During a short circuit, ILIM will be reduced by the foldback feature to 1/7th of the nominal value. Choosing External MOSFETs Power delivery to the ports is regulated with external power MOSFETs. These MOSFETs are controlled as previously described to meet the IEEE 802.3af specification. Under normal operation, once the port is powered and the PD’s bypass capacitor is charged to the port voltage, the external MOSFET dissipates very little power. This suggests that a small MOSFET is adequate for the job. Unfortunately, other requirements of the IEEE 802.3af mandate a MOSFET capable of dissipating significant power. When the port is being powered up, the port voltage must reach 30V or more before the PD turns on. The port voltage can then drop to 0V as the PD’s bypass capacitor is charged. According to the IEEE, the PD can directly connect a 180µF capacitor to the port and the PSE must charge that capacitor with a current limit of 400mA to 450mA for at least 50ms. An even more extreme example is a noncompliant PD that provides the proper signature during detection but then behaves like a low valued resistor, say 50Ω, in parallel with a 1µF capacitor. When the PSE has charged this noncompliant PD up to 20V, the 50Ω resistor will draw 400mA (the minimum IEEE prescribed ILIM current limit) keeping the port voltage at 20V for the remainder of tSTART. The external MOSFET sees 24V to 37V VDS at 400mA to 450mA, dissipating 9.6W to 16.7W for 60ms (typ). The LTC4258 implements foldback to reduce the current limit when the MOSFET VDS is high; see the Foldback section. Without foldback, the MOSFET could see as much as 25.7W for 60ms (typ) when powering a shorted or a noncompliant PD with only a few ohms of resistance. With foldback, the MOSFET sees a maximum of 18W for the duration of tSTART. The LTC4258’s duty cycle protection enforces 15 times longer off time than on time, preventing successive attempts to power a defective PD from damaging the MOSFET. System software can enforce even longer wait times. When the LTC4258 is operated in semiauto or manual mode—described in more detail under Operating Modes—it will not power on a port until commanded to do so by the host controller. By keeping track of tSTART and tICUT faults, the host controller can delay turning on the port again after one of these faults even if the LTC4258 reports a Detect Good. In this way the host controller implements a MOSFET cooling off period which may be programmed to protect smaller MOSFETs from repeated thermal cycling. The LTC4258 has built-in duty cycle protection for tICUT and tSTART (see tICUT Timing and tSTART Timing sections) that is sufficient to protect the MOSFETs shown in Figure 1. Before designing a MOSFET into your system, carefully compare its safe operating area (SOA) with the worst case conditions (like powering up a defective PD) the device will 4258p 18 LTC4258 U W U U APPLICATIO S I FOR ATIO face. Using transient suppressors, polyfuses and extended wait times after disconnecting a PD are effective strategies to reduce the extremes applied to the external MOSFETs. Surge Suppressors and Circuit Protection IEEE 802.3af Power over Ethernet is a challenging Hot Swap application because it must survive the (probably unintentional) abuse of everyone in the building. While hot swapping boards in a networking or telecom card cage is done by a trained technician or network administrator, anyone in the building can plug a device into the network. Moreover, in a card cage the physical domain being powered is confined to the card cage. With Power over Ethernet, the PSE supplies power to devices up to 100 meters away. Ethernet cables could potentially be cut, shorted together, and so on by all kinds of events from a contractor cutting into walls to someone carelessly sticking a screwdriver where it doesn’t belong. Consequently, the Power over Ethernet power source (PSE) must be designed to handle these events. The most dramatic of these is shorting a powered port. What the PSE sees depends on how much CAT-5 cable is between it and the short. If the short occurs on the far end of a long cable, the cable inductance will prevent the current in the cable from increasing too quickly and the LTC4258’s built-in short-circuit protection will take control of the situation and turn off the port. Some energy is stored in the cable, but the transient suppressor on the port clamps the port voltage when the cable inductance causes the voltage to fly back after the MOSFET is turned off. Because the cable only had 600mA or so going through it, an SMAJ58A or equivalent device can easily control the port voltage during flyback. With no cable connected at all, a powered port shorted at the PSE’s RJ-45 connector can reach high current levels before the port is shut down. There is no cable inductance to store energy so once the port is shut down the situation is under control. A short—hence low inductance—piece of CAT-5 will not limit the rapid increase of current when the port is shorted. Even though the LTC4258 short-circuit shutdown is fast, the cable may have many amps flowing through it before the MOSFET can be turned off. Due to the high current, this short piece of cable flies back with significant energy behind it and must be controlled by the transient suppressor. Choosing a surge suppressor that will not develop more than a few volts of forward voltage while passing more than 10A is important. A positive port voltage may forward bias the detect diode (D DET n), bringing the LTC4258’s DETECTn pin positive as well and engaging the DETECTn clamps. This will generally not damage the LTC4258 but extreme cases can cause the LTC4258 to reset. When it resets, the LTC4258 signals an interrupt, alerting the host controller which can then return the LTC4258 to normal operating mode. A substantial transient surge suppressor can typically protect the LTC4258 and the rest of the PSE from these faults. Placing a polyfuse between the RJ-45 connector and the LTC4258 and its associated circuitry can provide additional protection. To meet safety requirements, place the polyfuse in the ground leg of the PSE’s output. DC DISCONNECT DC disconnect monitors the sense resistor voltage whenever the power is on to make sure that the PD is drawing the minimum specified current. The disconnect timer counts up whenever port current is below 7.5mA (typ). If the tDIS timer runs out, the corresponding port will be turned off and the disconnect bit in the fault register will be set. If the undercurrent condition goes away before the tDIS timer runs out, the timer will reset. The timer will start counting from the beginning if the undercurrent condition occurs again. The undercurrent circuit includes a glitch filter to filter out noise. The DC disconnect feature can be disabled by clearing the corresponding DC Discon Enable bits in the Disconnect register (13h). The tDIS timer duration can be programmed by bits 1 and 0 of register 16h. The LTC4258 implements a variety of current sense and limit thresholds to control current flowing through the port. Figure 14 is a graphical representation of these thresholds and the action the LTC4258 takes when currrent crosses the thresholds. 4258p 19 LTC4258 U W U U APPLICATIO S I FOR ATIO 300mV 600mA 250mV 500mA 200mV 400mA 150mV 300mA 100mV 200mA 50mV 100mA 0mV SENSEn VOLTAGE 0mA CURRENT DC DISCUT RS = 0.5Ω CONNECT (ICUT) CURRENT LIMIT IN 1µs PORT OFF IN tICUT OR tSTART CURRENT LIMIT NORMAL OPERATION PORT OFF IN tDIS LIMIT (ILIM) SHORT CIRCUIT EFFECT 4258 F14 Figure 14. LTC4258 Current Sense and Limits SERIAL DIGITAL INTERFACE The LTC4258 communicates with a host (master) using the standard 2-wire interface as described in the SMBus Specification Version 2.0 (available at http://smbus.org). The SMBus is an extension of the I2C bus, and the LTC4258 is also compatible with the I2C bus standard. The Timing Diagrams (Figures 5 through 9) show the timing relationship of the signals on the bus. The two bus lines, SDA and SCL, must be high when the bus is not in use. External pull-up resistors or current sources, such as the LTC1694 SMBus accelerator, are required on these lines. If the SDA and SCL pull-ups are absent, not connected to the same positive supply as the LTC4258’s VDD pin, or are not activated when the power is applied to the LTC4258, it is possible for the LTC4258 to see a START condition on the I2C bus. The interrupt pin (INT) is only updated between I2C transactions. Therefore if the LTC4258 sees a START condition when it powers up because the SCL and SDA lines were left floating, it will not assert an interrupt (pull INT low) until it sees a STOP condition on the bus. In a typical application the I2C bus will immediately have traffic and the LTC4258 will see a STOP so soon after power up that this momentary condition will go unnoticed. Isolating the Serial Digital Interface IEEE 802.3af requires that network segments be electrically isolated from the chassis ground of each network interface device. However, the network segments are not required to be isolated from each other provided that the segments are connected to devices residing within a single building on a single power distribution system. For simple devices such as small powered Ethernet switches, the requirement can be met by using an isolated power supply to power the entire device. This implementation can only be used if the device has no electrically conducting ports other than twisted-pair Ethernet. In this case, the SDAIN and SDAOUT pins of the LTC4258 can be connected together to act as a standard I2C/SMBus SDA pin. If the device is part of a larger system, contains serial ports, or must be referenced to protective ground for some other reason, the Power over Ethernet subsystem including the LTC4258s must be electrically isolated from the rest of the system. The LTC4258 includes separate pins (SDAIN and SDAOUT) for the input and output functions of the bidirectional data line. This eases the use of optocouplers to isolate the data path between the LTC4258s and the system controller. Figure 15 shows one possible implementation of an isolated interface. The SDAOUT pin of the LTC4258 is designed to drive the inputs of an optocoupler directly, but a standard I2C device typically cannot. U1 is used to buffer I2C signals into the optocouplers from the system controller side. Schmitt triggers must be used to prevent extra edges on transitions of SDA and SCL. Bus Addresses and Protocols The LTC4258 is a read-write slave device. The master can communicate with the LTC4258 using the Write Byte, Read Byte and Receive Byte protocols. The LTC4258’s primary serial bus address is (010A3A2A1A0)b, as designated by pins AD3-AD0. All LTC4258s also respond to the address (0110000)b, allowing the host to write the same command into all of the LTC4258s on a bus in a single transaction. If the LTC4258 is asserting (pulling low) the INT pin, it will also acknowledge the Alert Response Address (0001100)b using the receive byte protocol. 4258p 20 LTC4258 U W U U APPLICATIO S I FOR ATIO 0.1µF VDD INT SCL SDAIN SDAOUT AD0 LTC4258 AD1 AD2 AD3 DGND AGND BYP 0.1µF 0.1µF VDD CPU U1 SCL 2k 200Ω 0100000 0.1µF VDD INT SCL SDAIN SDAOUT AD0 LTC4258 AD1 AD2 AD3 DGND AGND BYP 2k U2 200Ω I2C ADDRESS 0100001 0.1µF SDA 0.1µF HCPL-063L TO CONTROLLER U3 VDD INT SCL SDAIN SDAOUT AD0 LTC4258 AD1 AD2 AD3 DGND AGND BYP 200Ω 200Ω SMBALERT 0.1µF 0.1µF GND CPU U1: FAIRCHILD NC7WZ17 U2, U3: AGILENT HCPL-063L 0.1µF HCPL-063L 0100010 • • • VDD INT SCL SDAIN SDAOUT AD0 LTC4258 AD1 AD2 AD3 DGND AGND BYP 0101110 0.1µF 0.1µF ISOLATED 3.3V + 10µF ISOLATED GND VDD INT SCL SDAIN SDAOUT AD0 LTC4258 AD1 AD2 AD3 DGND AGND BYP 0.1µF 0101111 4258 F15 Figure 15. Optoisolating the I2C Bus 4258p 21 LTC4258 U W U U APPLICATIO S I FOR ATIO The START and STOP Conditions When the bus is idle, both SCL and SDA must be high. A bus master (typically the host controller) signals the beginning of communication with a slave device (like the LTC4258) by transmitting a START condition. A START condition is generated by transitioning SDA from high to low while SCL is high. When the master has finished communicating with the slave, it issues a STOP condition. A STOP condition is generated by transitioning SDA from low to high while SCL is high. The bus is then free for communication with another SMBus or I2C device. Acknowledge The Acknowledge signal is used for handshaking between the master and the slave. An Acknowledge (active LOW) generated by the slave lets the master know that the latest byte of information was received. The corresponding SCL clock pulse is always generated by the master. The master releases the SDA line (HIGH) during the Acknowledge clock pulse. The slave must pull down the SDA line during the Acknowledge clock pulse so that it remains a stable LOW during the HIGH period of this clock pulse. When the master is reading from a slave device, it is the master’s responsibility to acknowledge receipt of the data byte in the bit that follows unless the transaction is complete. In that case the master will decline to acknowledge and issue the STOP condition to terminate the communication. Write Byte Protocol The master initiates communication to the LTC4258 with a START condition and a 7-bit bus address followed by the Write Bit (Wr) = 0. If the LTC4258 recognizes its own address, it acknowledges and the master delivers the command byte, signifying to which internal LTC4258 register the master wishes to write. The LTC4258 acknowledges and latches the lower five bits of the command byte into its Register Address register. Only the lower five bits of the command byte are checked by the LTC4258; the upper three bits are ignored. The master then delivers the data byte. The LTC4258 acknowledges once more and latches the data into the appropriate control register. Finally, the master terminates the communication with a STOP condi- tion. Upon reception of the STOP condition, the Register Address register is cleared (see Figure␣ 6). Read Byte Protocol The master initiates communication from the LTC4258 with a START condition and the same 7-bit bus address followed by the Write Bit (Wr) = 0. If the LTC4258 recognizes its own address, it acknowledges and the master delivers the command byte, signifying which internal LTC4258 register it wishes to read from. The LTC4258 acknowledges and latches the lower five bits of the command byte into its Register Address register. At this time the master sends a REPEATED START condition and the same 7-bit bus address followed by the Read Bit (Rd) = 1. The LTC4258 acknowledges and sends the contents of the requested register. Finally, the master declines to acknowledge and terminates communication with a STOP condition. Upon reception of the STOP condition, the Register Address register is cleared (see Figure 7). Receive Byte Protocol Since the LTC4258 clears the Register Address register on each STOP condition, the interrupt register (register 0) may be read with the Receive Byte Protocol as well as with the Read Byte Protocol. In this protocol, the master initiates communication with the LTC4258 with a START condition and a 7-bit bus address followed by the Read Bit (Rd) = 1. The LTC4258 acknowledges and sends the contents of the interrupt register. The master then declines to acknowledge and terminates communication with a STOP condition (see Figure 8). Alert Response Address and the INT Pin In a system where several LTC4258s share a common INT line, the master can use the Alert Response Address (ARA) to determine which LTC4258 initiated the interrupt. The master initiates the ARA procedure with a START condition and the 7-bit ARA bus address (0001100)b followed by the Read Bit (Rd) = 1. If an LTC4258 is asserting the INT pin, it acknowledges and sends its 7-bit bus address (010A3A2A1A0)b and a 1 (see Figure 9). While 4258p 22 LTC4258 U W U U APPLICATIO S I FOR ATIO it is sending its address, it monitors the SDAIN pin to see if another device is sending an address at the same time using standard I2C bus arbitration. If the LTC4258 is sending a 1 and reads a 0 on the SDAIN pin on the rising edge of SCL, it assumes another device with a lower address is sending and the LTC4258 immediately aborts its transfer and waits for the next ARA cycle to try again. If transfer is successfully completed, the LTC4258 will stop pulling down the INT pin. When the INT pin is released in this way or if a 1 is written into the Clear Interrupt pin bit (bit 6 of register 1Ah), the condition causing the LTC4258 to pull the INT pin down must be removed before the LTC4258 will be able to pull INT down again. This can be done by reading and clearing the event registers or by writing a 1 into the Clear All Interrupts bit (bit 7 of register 1Ah). The state of the INT pin can only change between I2C transactions, so an interrupt is cleared or new interrupts are generated after a transaction completes and before new I2C bus communication commences. Periodic polling of the alert response address can be used instead of the INT pin if desired. If any device acknowledges the alert response address, then the INT line, if connected, would have been low. System Software Strategy Control of the LTC4258 hinges on one decision, the LTC4258’s operating mode. The three choices are described under Operating Modes. In Auto mode the LTC4258 can operate autonomously without direction from a host controller. Because LTC4258s running in Auto mode will power every valid PD connected to them, the PSE must have 15.4W/port available. To reduce the power requirements of the –48V supply, PSE systems can track power usage, only turning on ports when sufficient power is available. The IEEE describes this as a power allocation algorithm and places two limitations: the PSE shall not power a PD unless it can supply the guaranteed power for that PD’s class (see Table 2) and power allocation may not be based solely on a history of each PD’s power consumption. In order for a PSE to implement power allocation, the PSE’s processor/controller must control whether ports are powered—the LTC4258 cannot be allowed to operate in Auto mode. Semiauto mode fits the bill as the LTC4258 automatically detects and classifies PDs, then makes this information available to the host controller, which decides to apply power or not. Operating the LTC4258 in Manual mode also lets the controller decide whether to power the ports but the controller must also control detection and classification. If the host controller operates near the limit of its computing resources, it may not be able to guide a Manual mode LTC4258 through detect, class and port turn-on in less than the IEEE mandated maximum of 950ms. In a typical PSE, the LTC4258s will operate in Semiauto mode as this allows the controller to decide to power a port without unduly burdening the controller. With an interrupt mask of F4h, the LTC4258 will signal to the host after it has successfully detected and classed a PD, at which point the host can decide whether enough power is available and command the LTC4258 to turn that port on. Similarly, the LTC4258 will generate interrupts when a port’s power is turned off. By reading the LTC4258’s interrupt register, the host can determine if a port was turned off due to overcurrent (tSTART or tICUT faults) or because the PD was removed (Disconnect event). The host then updates the amount of available power to reflect the power no longer consumed by the disconnected PD. Setting the MSB of the interrupt mask causes the LTC4258 to communicate fault conditions caused by failures within the PSE, so the host does not need to poll to check that the LTC4258s are operating properly. This interrupt driven system architecture provides the controller with the final say on powering ports at the same time, minimizing the controller’s computation requirements because interrupts are only generated when a PD is detected or on a fault condition. 4258p 23 LTC4258 U W U U APPLICATIO S I FOR ATIO voltage to the –48V rail, improving the regulation tolerance over the more traditional large resistor voltage divider. This approach achieves high accuracy with a transformerless design. The LTC4258 can also be used to power older powered Ethernet devices that are not 802.3af compliant and may be detected with other methods. Although the LTC4258 does not implement these older detection methods automatically, if software or external circuitry can detect the noncompliant devices, the host controller may command the LTC4258 to power the port, bypassing IEEE compliant detection and classification and sending power to the noncompliant device. IEEE 802.3af COMPLIANCE AND EXTERNAL COMPONENT SELECTION The LTC4258 is designed to control power delivery in IEEE 802.3af compliant Power Sourcing Equipment (PSE). Because proper operation of the LTC4258 may depend on external signals and power sources, like the –48V supply (VEE) or the OSCIN oscillator source, external components such as the sense resistors (RS), and possibly software running on an external microprocessor, using the LTC4258 in a PSE does not guarantee 802.3af compliance. Using an LTC4258 does get you most of the way there. This section discusses the rest of the elements that go along with the LTC4258 to make an 802.3af complaint PSE. Each paragraph below addresses a component which is critical for PSE compliance as well as possible pitfalls that can cause LOGIC LEVEL SUPPLY In additon to the 48V used to source power to each port, a logic level supply is required to power the digital portion of the LTC4258. To simplify design and meet voltage isolation requirements, the logic level supply can be generated from the isolated – 48V supply. Figure 16 shows an example method using an LT®1619 to control a –48V to 3.3V current mode supply. This boost converter topology uses the LT1619 current mode controller and a current mirror which reflects the 3.3V output B1100 100µH ISOLATED GND 910k + 1µF 100V 510Ω 4.7µH 3.32k 1% Si2328DS + 47µF 10V 10µF 16V 8 7 DRV GATE 5 SENSE LT1619 2 FB S/S GND VC 4 ISOLATED GND 6 VIN 1 FMMT593 FMMT593 0.100Ω 1% 1W 3 47k 1.24k 1% 100k 100pF 4700pF VEE ISOLATED –48V 47µF 10V 100Ω Si2328DS CMPZ4702B + VDD 3.3V 300mA 10Ω 4258 F16 Figure 16. –48V to 3.3V Boost Converter 4258p 24 LTC4258 U W U U APPLICATIO S I FOR ATIO a PSE to be noncompliant. For further assistance please contact Linear Technology’s Applications department. Sense Resistors The LTC4258 is designed to use a 0.5Ω sense resistor, RS, to monitor the current through each port. The value of the sense resistor has been minimized in order to reduce power loss and as a consequence, the voltage which the LTC4258 must measure is small. Each port may be drawing up to 450mA with this current flowing through the sense resistor and associated circuit board traces. To prevent parasitic resistance on the circuit board from obscuring the voltage drop across the sense resistor, the LTC4258 must Kelvin sense the resistor voltage. One way to achieve Kelvin sensing is “star grounding,” shown pictorially in Figure 1. Another option is to use a – 48V power plane to connect the sense resistor and the LTC4258 VEE pin. Either of these strategies will prevent voltages developed across parasitic circuit board resistances from affecting the LTC4258 current measurement accuracy. The precision of the sense resistor directly affects the measurement of the IEEE parameters IINRUSH, ILIM, ICUT and IMIN. Therefore, to maintain IEEE compliance, use a resistor with 0.5% or better accuracy. Power MOSFETs The LTC4258 controls power MOSFETs in order to regulate current flow through the Ethernet ports. Under certain conditions these MOSFETs have to dissipate significant power. See the Choosing External MOSFETs section for a detailed discussion of the requirements these devices must meet. Common Mode Chokes Both nonpowered and powered Ethernet connections achieve best performance (for data transfer, power transfer and EMI) when a common mode choke is used on each port. In the name of cost reduction, some designs share a common mode choke between two adjacent ports. Even for nonpowered Ethernet, sharing a choke is not recommended. With two ports passing through the choke, it cannot limit the common mode current of either port. Instead, the choke only controls the sum of both ports’ common mode current. Because cabling from the ports generally connects to different devices up to 200m apart, a current loop can form. In such a loop, common mode current flows in one port and out the other, and the choke will not prevent this because the sum of the currents is zero. Another way to view this interaction between the paired ports is that the choke acts as a transformer coupling the ports’ common modes together. In nonpowered Ethernet, common mode current results from nonidealities like ground loops; it is not part of normal operation. However, Power over Ethernet sends power and hence significant current through the ports; common mode current is a byproduct of normal operation. As described in the Choosing External MOSFETs section and under the Power Supplies heading below, large transients can occur when a port’s power is turned on or off. When a powered port is shorted (see Surge Suppressors and Circuit Protection), a port’s common mode current may be excessive. Sharing a common mode choke between two ports couples start-up, disconnect and fault transients from one port to the other. The end result can range from momentary noncompliance with 802.3af to intermittent behavior and even to excessive voltages that may damage circuitry (in both the PSE and PD) connected to the ports. Detect, AC Blocking and Transient Supressor Diodes During detection and classification, the LTC4258 senses the port voltage through the detect diodes DDET. Excessive voltage drop across DDET will corrupt the LTC4258’s detect and classification results. Select a diode for DDET that will have less than 0.7V of forward drop at 0.4mA and less than 0.9V of forward drop at 50mA. 4258p 25 LTC4258 U W U U APPLICATIO S I FOR ATIO Power Supplies The LTC4258 must be supplied with 3.3V (VDD) and –48V (VEE). Poor regulation on either of these supplies can lead to noncompliance. The IEEE requires a PSE output voltage between 44V and 57V. When the LTC4258 begins powering an Ethernet port, it controls the current through the port to minimize disturbances on VEE. However, if the VEE supply is underdamped or otherwise unstable, its voltage could go outside of the IEEE specified limits, causing all ports in the PSE to be noncompliant. This scenario can be even worse when a PD is unplugged because the current can drop immediately to zero. In both cases the port voltage must always stay between –44V and –57V. In addition, the 802.3af specification places specific ripple, noise and load regulation requirements on the PSE. Among other things, disturbances on either VDD or VEE can adversely affect detection, classification and the AC disconnection sensing. Proper bypassing and stability of the VDD and VEE supplies is important. Another problem that can affect the VEE supply is insufficient power, leading to the supply voltage drooping out of the specified range. The 802.3af specification states that if a PSE powers a PD it must be able to provide the maximum power level requested by the PD based on the PD’s classification. The specification does allow a PSE to choose not to power a port because the PD requires more power than the PSE has left to deliver. If a PSE is built with a VEE supply capable of less than 15.4W • (number of PSE’s Ethernet ports), it must implement a power allocation algorithm that prevents ports from being powered when there is insufficient power. Because the specification also requires the PSE to supply 400mA at up to a 5% duty cycle, the VEE supply capability should be at least a few percent more than the maximum total power the PSE will supply to PDs. Finally, the LTC4258s draw current from VEE. If the VDD supply is generated from VEE, that power divided by the switcher efficiency must also be added to the VEE supply’s capability. 4258p 26 LTC4258 U PACKAGE DESCRIPTIO GW Package 36-Lead Plastic SSOP (Wide .300 Inch) (Reference LTC DWG # 05-08-1642) 1.143 ±0.127 10.668 MIN 15.290 – 15.544* (.602 – .612) 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 7.416 – 7.747 10.160 – 10.414 (.400 – .410) 0.520 ±0.0635 0.800 TYP RECOMMENDED SOLDER PAD LAYOUT 7.417 – 7.595** (.292 – .299) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 2.286 – 2.387 (.090 – .094) 2.463 – 2.641 (.097 – .104) 0.254 – 0.406 × 45° (.010 – .016) 0° – 8° TYP 0.231 – 0.3175 (.0091 – .0125) 0.610 – 1.016 (.024 – .040) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 0.800 (.0315) BSC 0.304 – 0.431 (.012 – .017) *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.152mm (0.006") PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE 0.127 – 0.305 (.005 – .0115) GW36 SSOP 0502 4258p 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. 27 LTC4258 U TYPICAL APPLICATIO ISOLATED 3.3V ISOLATED GND 0.1µF DGND AGND 2k U2 200Ω VDD 0.1µF BYP DETECT SCL 1/4 SDAIN LTC4258 SDAOUT INT VDD CPU U1 SCL 2k 200Ω VEE L1 SENSE GATE OUT 0.1µF RS 0.5Ω TO CONTROLLER 0.1µF 100V X7R SDA HCPL-063L –48V ISOLATED U3 DDET CMPD3003 DTSS 58V SMAJ58A 10k Q1 IRFM120A RJ45 CONNECTOR 1/2 PULSE H2009 200Ω 0.01µF 200V 75Ω 0.01µF 200V 75Ω 0.01µF 200V 75Ω 75Ω 200Ω SMBALERT PHY 0.1µF GND CPU HCPL-063L (NETWORK PHYSICAL LAYER CHIP) DDET: CENTRAL SEMI CMPD3003 DTSS: DIODES INC SMAJ58A L1: PULSE ENG PO473 Q1: FAIRCHILD IRFM120A RS: VISHAY WSL2010 0.5Ω 0.5% T1: PULSE ENG H2009 U1: FAIRCHILD NC7WZ17 U2, U3: AGILENT HCPL-063L T1 1:1 1 2 3 4 5 6 7 8 0.01µF 200V 4258 F17 T1 1:1 1000pF 2000V Figure 17. One Complete Isolated Powered Ethernet Port RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1619 Low Voltage Current Mode PWM Controller –48V to 3.3V at 300mA, MSOP Package LTC1694 SMBus/I2C Accelerator Improved I2C Rise Time, Ensures Data Integrity LTC4255 Quad Network Power Controller Non-IEEE 802.3af Compliant Current Levels LTC4257 IEEE 802.3af PD Interface Controller 100V 400mA Internal Switch, Programmable Classification LTC4257-1 IEEE 802.3af PD Interface Controller 100V 400mA Dual Current Limit LTC4259A Quad IEEE 802.3af Power Over Ethernet Controller With AC Disconnect 4258p 28 Linear Technology Corporation LT/TP 0104 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2004