LINEAR TECHNOLOGY LTC4269 Technical data

L DESIGN FEATURES

100k

20Ω

3.01k
1%

27.4k
1%

10µF

39k

1µF

BAS21

1.2k

38.3k

33pF

1.5nF

10µF
100V

0.1µF

t

ON

12k

PGDLYV

NEG

SYNC

R

CLASS

SHDN

V

CMP

R

CMP

ENDLY OSC SFST

LTC4269-1

GND

UVLO

V

PORTP

V

CC

T2P

T2P

FB

C

CMP

+

+

V

PORTN

383k
1%

3.01k
1%

10k

1nF

3.3nF

•

•

•

•

33mΩ
1%

FDS2582

10k

15Ω

150Ω

PE-68386

BAT54

100Ω

2200pF

MMBT3906 MMBT3904

1µF

1µF
16V

T1

PA2369NL

SENSE

–

SENSE

+

SG

PG

L1

0.18µH

22pF

100µF

5.1Ω

•

FDS8880

47µF

5V
5A

+

SMAJ58A

30.9Ω

24k

107k

10k

S1B

B1100 s 8 PLCS

2.2µF
100V

10µH

DO1608C-103

0.1µF
100V

36V
PDZ36B

BSS63LT1

V

PORTP

48V

AUXILIARY

POWER

–54V FROM

DATA PAIR

–54V FROM

SPARE PAIR

+

–

PD Controller ICs with Integrated
Flyback or Forward Controllers Meet
Demands of 25.5W PoE+

Introduction

The IEEE 802.3af Power over Ether­net (PoE) standard allows a powered
device (PD), such as an internet
protocol (IP) telephone, to draw up to

12.95W from an Ethernet cable. When
the 802.3af standard was drafted,

12.95W appeared sufficient to cover
the immediately imaginable range of
PD products (primarily IP phones).
Of course, application developers
are always far more innovative than
standards committees anticipate, so
new power-hungry applications for
PoE immediately started to appear,
such as dual-radio IEEE 802.11a/g
and 802.11n wireless access points,
security cameras with pan/tilt/zoom
motors, and color LCD IP video
phones. 12.95W was suddenly not
enough. The IEEE committee re­sponded with the 802.3at standard,
which raises the available PD power

to 25.5W. The new “at” standard, com­monly referred to as PoE+, also adds
a “handshaking” communications
requirement between PDs and power
sourcing equipment (PSEs), while al­lowing backward compatibility with
the legacy “af” standard.

New power control ICs are required
to take advantage of these expanded
requirements. The DC/DC conversion
and control schemes used for legacy
“af” PDs are not optimized for the in­creased power capability and feature
requirements of PoE+. For instance,
in both standards the 37V to 57V PoE
voltage is converted to lower voltages
that digital circuitry can tolerate.
This DC/DC conversion is handled
in the lower power 12.95W standard
with a conventionally rectified (i.e.,
diode rectified) flyback converter. The
higher power 25.5W standard is better

by Ryan Huff

served by a synchronously rectified
(i.e. MOSFET rectified) flyback or a
forward power supply topology.

To meet the new performance
requirements of PoE+, including
handshaking, Linear Technology offers
a new family of PD controller ICs that
integrate a front-end PD controller with
a high performance synchronously
rectified flyback (LTC4269-1) or a
forward (LTC4269-2) power supply
controller.

Features

Both parts combine a PD control­ler—which includes the handshaking
circuitry, Hot Swap™ FET, and input
protection—with a DC/DC power
supply controller. While the power
supply sections of the two parts are
very different, the PD controller in
both is identical.

6

Figure 1. LTC4269-1-based synchronous flyback converter

Linear Technology Magazine • September 2009

+

T2P

T2P

V

NEG

V

PORTN

SHDN

SMAJ58A

R

CLASS

V

PORTP

PGND GND BLANK DELAY

82k

30.9Ω24k

158k 332k

133Ω

BAS516

BAS516

PA2431NL

BAS516

10k

IRF6217

FDS8880

FDS8880

5.1Ω

158k

22.1k

33k

1.5k

50mΩ

2k

5.1Ω

1.2k

TLV431A

PS2801-1-L

V

CC

11.3k

3.65k

22k

0.22µF

0.1µF

R

OSC

V

REF

FB

COMP

I

SENSE

OC

SS_MAXDC

FDS2582

SD_V

SEC

V

IN

S

OUT

LTC4269-2

0.1µF

18V

PDZ18B

10µF

16V

V

CC

33k

237k

107k

10.0k

S1B

B1100 s 8 PLCS

2.2µF

100V

+

10µF

100V

10µH

DO1608C-103

1mH

DO1608C-105

6.8µH
PG0702.682

10.0k

OUT

4.7nF

1nF

5.1Ω

1nF

0.1µF

100V

4.7nF

250V

10nF

+

220µF

6.3V

PSLVOJ227M(12)A

5V

5A

36V

PDZ36B

BSS63LT1

V

PORTP

–48V

AUXILIARY

POWER

–54V FROM

DATA PAIR

–54V FROM

BC857BF

•

•

•

EFFICIENCY (%)

LOAD CURRENT (A)

50.5

95

65

1 1.5 2 2.5 3 3.5 4 4.5

70

75

85

80

90

VIN = 42V
VIN = 50V
VIN = 57V

In the LTC4269, handshaking cir­cuitry, also known as the “High Power
Available,” “Two Finger Detect,” or
“Ping Pong” indicator, allows the PD
to take full advantage of a new PSE’s
full 25.5W of available power. Both
parts include an integrated Hot Swap
MOSFET for a controlled power up of
the PD. The switch has a low 700mΩ
(typical) resistance and a robust 100V
max rating, thus meeting the needs of
a wide range of applications. Auxiliary
power supplies (“wall warts”) can be
accommodated by interfacing to the
SHDN pin to disable the PoE power
path. Setting a programmable clas­sification current allows different
power leveled PDs to be recognized
by the PSE. Achieving this is as easy
as choosing the proper resistor and
placing it from the R
pin. The ICs are chock-full of protec­tion features, including overvoltage,
undervoltage, and overtemperature to
name a few. Finally, complementary
power good indicators signal that the

CLASS

pin to V

PORTN

PD Hot Swap MOSFET is out of the
inrush limit and ready to draw full
power.

The power supply controllers of the
LTC4269s also share some features.
Both offer programmable switching
frequency, which allows the designer
to optimize the trade-off between ef­ficiency and size, or the designer can
choose a specific frequency to meet
application specific EMI require­ments. The power supply soft-start
time is also adjustable to prevent
the PSE from dropping out its power
due to excessive inrush current and
virtually eliminate any power supply

DESIGN FEATURES L

Figure 3. LTC4269-2-based self-driven synchronous forward converter

Figure 2. Efficiency of the circuit in Figure 1

Linear Technology Magazine • September 2009

7