DN361 - Simple Battery Circuit Extends Power over Ethernet (PoE) Peak Current

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
+
5
C11
10μF
6.3V
R11
1k
PS2911
5
1
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
+
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
Data Sheet Download
www.linear.com
Linear Technology Corporation
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.
For applications help,
call (408) 432-1900
dn361f_conv LT/TP 0405 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005
Similar pages