Fully Autonomous IEEE 802.3af Power over Ethernet Midspan PSE Requires No Microcontroller – Design Note 350 Dilian Reyes Introduction The IEEE802.3af Power over Ethernet (PoE) standard defines how power will be delivered over CAT5 lines. Despite the differences between legacy devices and those that adhere to the new standard, there is no need to completely replace existing systems. The nominal 48V required by powered devices (PDs) can be delivered by midspan power sourcing equipment (PSE), which is connected to the front end in series with legacy routers and switches. The LTC ®4259A is a quad PSE controller designed for both endpoint and midspan PSEs that integrates PD signature detection, power level classification, AC and DC disconnect detection and current limit without the need for a microcontroller. VDD A PSE’s Duties The responsibilities of the PSE are to correctly detect if a compliant PD has been connected to a port, optionally classify the PD and properly apply power to the PD while protecting the port from fault conditions. Once a PD is powered on, the PSE monitors a PD’s presence and switches off power when the device is removed. A PSE must also provide overcurrent protection to prevent damage to the PSE and PD. Traditional PSE solutions use a microcontroller to perform the detection measurements and calculations and control additional circuitry that switches power to L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. RESET 10k 160Ω OSCIN 3 4 6 5 10 9 8 7 17 18 19 20 INT SHDN1 SCL SDAIN SDAOUT AD0 AD1 AD2 AD3 SHDN2 SHDN3 SHDN4 35 2 VDD AUTO 16 BYP + 10k 10μF 100V 1 RESET LTC4259A DGND AGND VEE SENSE1 GATE1 OUT1 SENSE2 GATE2 OUT2 15 21 28 32 33 34 29 30 31 VIN –48V TO –57V 0.1μF 0.1μF 50V 0.1μF SENSE3 GATE3 OUT3 25 26 27 10k 36 OSCIN DETECT1 DETECT2 DETECT3 DETECT4 11 12 13 14 SENSE4 GATE4 OUT4 22 23 24 10k 10k 0.50Ω VEE 1μF 100V IRFM120A tBACKOFF 2N7002 1k CMPD3003 0.47μF 100V X7R 2N7002 1k CMPD3003 0.47μF 100V X7R 2N7002 1k CMPD3003 0.47μF 100V X7R 2N7002 1k CMPD3003 0.47μF 100V X7R 8 7 SMAJ58A S1B 0.1μF 100V X7R 0.50Ω IRFM120A SMAJ58A S1B 0.1μF 100V X7R 0.50Ω IRFM120A SMAJ58A S1B 0.1μF 100V X7R 0.50Ω IRFM120A 8 7 RJ45 5 4 8 7 RJ45 5 4 8 7 RJ45 SMAJ58A S1B 18M VDD 1.1M 18M VDD 1.1M 18M VDD 1.1M 18M VDD 0.1μF 100V X7R 1.1M Si2343DS Si2343DS Si2343DS Si2343DS 300Ω 300Ω 300Ω 300Ω DN350 F01 Figure 1. Autonomous 4-Port Power over Ethernet Midspan PSE 11/04/350_conv RJ45 5 4 5 4 a PD. The LTC4259A in Figure 1, by contrast, requires no microcontroller and runs autonomously carrying out signature detection. It automatically interprets the loading conditions and powers on a valid PD. The midspan PSE also must not interfere with an endpoint’s operation. An endpoint PSE applies power on either the signal pairs or the spare pairs of the CAT5 cable, while the midspan PSE must apply power to only the spare pairs. To avoid conflict if the two were to be connected at the same time, the circuit in Figure 2 implements an LTC1726 watchdog timer to periodically disable the LTC4259A’s detection scheme for two seconds. Midspan devices are required to have a back off capability after a failed attempt of detection to allow for a potentially present endpoint PSE to detect and power on a port. After the back off interval is complete, LTC4259A detection is re-enabled for at least one full detection cycle. If a midspan or endpoint PSE is able to detect a valid signature 25kΩ (RSIG) and power up the PD, a compliant PD would no longer display the RSIG to prevent any further good signature detects and power ups from a second PSE. Hardware implementation of the back off timer eliminates the need for a microcontroller software timing routine. VDD The LTC4259A auto mode uses the AC disconnect method by default. The LT®1498 in Figure 3 is a dual rail-to-rail op amp used to output a sine wave to drive OSCIN of the LTC4259A. The LTC4259A applies the AC signal to the lines and detects its presence when a PD has been removed and the port power is to be switched off. VDD 590k 1% 750k 2200pF NPO 2 3 10k 0.1μF 6 0.1μF – 1/2 LTC1498CS8 10k 1 2 1 3 4.99k VC1 VC2 VCA 22k 6 RT RST LTC1726-2.5 7 5 150k WT WDI GND C1, C2 330nF C3, C4 12nF 4 1k Supplying 3.3V from –48V A 3.3V supply powers the digital portion of the LTC4259A. The LTC3803 circuit in Figure 4 converts –48V to 3.3V eliminating the need for a second power supply. This boost regulator circuit achieves a tight 2% regulation and outputs 400mA, enough for up to twelve LTC4259As and port indicator LEDs in a 48-port application. + 0.22μF 100V Data Sheet Download www.linear.com Linear Technology Corporation 0.22μF 100V 10μF 63V 10k 1% 10μF 16V 5 FDC2512 VCC 6 ITH/RUN NGATE LTC3803ES6 3 4 1k SENSE VFB GND 0.1Ω 1W 2 1% B1100 10μH VDD 3.3V 400mA 10μF 6.3V 3.32k 1% 100μF 6.3V 1 5Ω DN350 F02 DC disconnect measures a minimum current drawn from a port to determine if a PD is present and requires power. While this is easier to implement, AC disconnect is considered a more accurate detection of a PD’s presence. AC disconnect measures the PD impedance and would keep a port powered even for PDs that idle at low power. DN350 F03 Figure 3. Sinewave Circuit for AC Disconnect VEE Disconnect Detection When a PD is unplugged from a powered port, the IEEE standards specify that a PSE must implement at least one of two power disconnect detections modes for port power removal: DC disconnect and/or AC disconnect. + OSCIN 300k Figure 2. Midspan PSE Backoff Timer 7 2.4k VSIN = 2.1VP-P 1.2VOS 105Hz MMBT2907 VEE 5 2200pF + 100μH tBACKOFF – 1/2 LTC1498CS8 681k 1% 0.1μF 8 2200pF NPO 56k 2.2nF FMMT723 FMMT723 806Ω 1% 47.5k 1% DN350 F04 Figure 4. –48V to 3.3V Boost Converter LTC4259A Options The LTC4259A also allows flexibility when designing an endpoint or midspan PSE. Internal registers can be accessed via I2C for additional control and settings, including the option of DC disconnect. The LTC4259A aids in IEEE-compliant power management by providing PD classification—a better method than guessing via monitoring current—of devices that present a class, such as an LTC4257 PD Interface Controller. 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