Simple Battery Circuit Extends Power over Ethernet (POE) Peak Current – Design Note 361 Mark Gurries Introduction Power over Ethernet (PoE) is a new development that allows for the delivery of power to Ethernet-based devices via standard Ethernet CAT5 cable, precluding the need for wall adapters or other external power sources. The PoE specification defines a hardware detection protocol where Power Sourcing Equipment (PSE) is able to identify PoE Powered Devices (PDs), thus allowing full backwards compatibility with non-PoE-aware (legacy) Ethernet

devices. The PoE specification also sets an upper limit on the power that can be drawn by a PD. The problem is: what happens when a PD must draw more power than allowed by the PoE standard? Examples may be the spin up of a disk drive or a period of sustained transmission of data from an RF transmitter. If the average power load of these applications is less than L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.

L1 3.3μH C2 1μF 100V X7R D5 1N5237B 8.2V

ETHERNET AC1 BR2

R2 220k Q2 MMBTA42

D3 BAS516

6

2

7

1

5

C9 220μF 6.3V OS-CON

C14 100μF 6.3V X5R w2 Q1 Si3440DV

LTC4267 C1 0.1μF 100V X7R

AC1 BR1 FROM SPARE PAIR

+

C5 4.7μF 100V

AC2 HDO1

4 6 13 5

D2 SMAJ58A 12

7 1,8,9,16

PVCC

NGATE

NC

SENSE

VPORTP

VFB

RCLASS

ITH/RUN

SIGDISA

PWRGD

VPORTIN

POUT

3 14 15 2 11

PGND

R4 0.125Ω 1% 1/4W C12 1μF 6.3V

R5 4.7k Q3 2N7002

10

D4 BAS516

R8 R9 P 100k VCC 6.8k

6 7 8

WALL SHDN SUSP HPWR

OUT BAT TIMER CLPROG

13

ACPR

PROG

14

CHRG

GND

3 2 12

1-CELL 4.2V Li-Ion

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5

C11 10μF 6.3V

R11 1k

PS2911 5

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4

2

TO DC/DC CONVERTERS LTC4055

R21 470Ω

D7 BAS516 R1 10k

D9 UPS340

R16 10k 1%

R6 510Ω

3

D1 SMB540

C4 4.7μF 10V X5R

AC2 HDO1

5V POE R15 34.8k

R7 100Ω 1/8W

C6 470pF

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FROM DATA PAIR

C3 68pF 200V R3 220k

R10 10Ω

T1 PA1134 4 8

R12 47k C7 27nF D8 ZRL431

C8 2.2nF 250V AC YCAP

C13 33nF R13 100k 1%

R14 33.2k

DN361 F01

9 11 10

R18 53.6k

R19 105k 1%

C10 0.1μF

TO HOST

Figure 1. Simple Battery Charger/PowerPath™ Controller (LTC4055) Augments PoE Regulator’s (LTC4267) Peak Output Power to Overcome PoE Power Constraints 04/05/361_conv

5V POE 5V 1.8A

the available PoE power, one solution is to store power in the PD when power consumption is low and then tap the reserve to augment PoE power when needed. For many applications, a rechargeable battery fits the bill.

of regulation, the LTC4055 automatically switches the battery power over to the OUT pin using its internal ideal diode circuit. There is no delay in the switchover, so power is never lost.

Of course, one can’t just throw a battery and a battery charger into the mix. The power path must be able to change seamlessly, on the fly, from PoE-powers-deviceand-charges-battery, to PoE-and-battery-power-device, to battery-powers-device. Figure 1 shows a complete and compact solution.

When 5V PoE power is restored, the battery is disconnected from the load and charging is permitted. The LTC4055 charge current is adjustable and in Figure 1, the circuit is limited to 900mA which is drawn from the OUT pin. That leaves 900mA to run the system while charging. Powered devices connected to the OUT pin must be compatible with the Li-Ion voltage range. The ACPR pin of the LTC4055 can be used to indicate which power source is providing power, allowing the PD to configure itself accordingly.

The PoE Circuit By default, power over the Ethernet is not available. The standard calls for a protocol to be implemented that allows the Ethernet hub to identify the device needing power. The LTC ®4267 simplifies the design of PDs by providing wholesale implementation of the protocol and power management functions. PoE power comes in the form of –48V at 350mA. If the PoE current is allowed to exceed 400mA, the standard calls for the PSE to break the circuit. This is a problem for devices that occasionally need a little more juice than PoE will offer. Another problem is that –48V does not easily convert to commonly used positive voltage supply rails. Designers are forced to provide DC isolation along with the inverted down conversion to a more usable voltage. To meet these requirements, the LTC4267 used in Figure 1’s circuit implements an input current limited DC input isolated flyback converter, providing a user-settable regulated low voltage. The LTC4267 circuit in Figure 1 supplies 5V at 1.8A. 5V is a popular supply voltage to run logic, interface with other devices such as USB, and of primary concern in this application, to charge a single Li-Ion cell to its target termination voltage of 4.2V. PowerPath and Charger Circuit In Figure 1, the LTC4055 provides triple PowerPath control and Li-Ion battery charging. One path is created by connecting an external Schottky diode to the LTC4055’s OUT pin and the built-in wall adapter detection circuits. In this case, the “wall adapter” power comes from the LTC4267 5V power supply called 5V PoE. The second path is for USB power, not used in this application. The third path is the battery discharge path. When the 5V PoE power goes away or drops out

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High Transient Load or Continuous Current Load Operation When the power limit of the 5V PoE supply is reached, the voltage drops and the battery charger shuts down to relieve the PSE of the charge current load. If the voltage continues to collapse, the battery automatically is placed into parallel operation with the 5V PoE power supply, thus increasing the available peak load current. The LTC4055 ACPR signal is active high during the overload. Battery charging automatically resumes once the overload goes away and the 5V PoE voltage has risen enough to show recovery. Optimization Options If sustained currents approaching 1.8A are expected from the 5V PoE and there are thermal management issues related to the diode’s heat dissipation, the diode D9 can be replaced with the LTC4411 ideal diode for more efficient operation. Recommended DC/DC converters to generate logic supplies in this application include the LTC3443 buck-boost and/or the LTC3407-2 dual buck regulators. Conclusion The highly integrated LTC4267 and LTC4055 simplify the design of compact, simple and complete batterybased power systems that run from Ethernet power. More importantly, seamless PowerPath control enables circuits that can use a battery to augment Ethernet power when an application momentarily demands more than the PoE standard allows.

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