LTC4267CGNPBF LINEAR TECH, LTC4267CGNPBF Datasheet - Page 9

• Manufacturer Part Number: LTC4267CGNPBF
• Manufacturer: LINEAR TECH

BLOCK DIAGRAM
V
R
APPLICATIO S I FOR ATIO
OVERVIEW
The LTC4267 is partitioned into two major blocks: a
Powered Device (PD) interface controller and a current
mode fl yback switching regulator. The Powered Device
(PD) interface is intended for use as the front end of a
PD adhering to the IEEE 802.3af standard, and includes
a trimmed 25kΩ signature resistor, classifi cation current
source, and an input current limit circuit. With these
functions integrated into the LTC4267, the signature and
power interface for a PD can be built that meets all the
requirements of the IEEE 802.3af specifi cation with a
minimum of external components.
The switching regulator portion of the LTC4267 is a con-
stant frequency current mode controller that is optimized
for Power over Ethernet applications. The regulator is
designed to drive a 6V N-channel MOSFET and features
soft-start and programmable slope compensation. The
integrated error amplifi er and precision reference give the
PD designer the option of using a nonisolated topology
without the need for an external amplifi er or reference. The
PORTN
CLASS
375mA
140mA
1.237V
CONTROL
CIRCUITS
+
–
BOLD LINE INDICATES HIGH CURRENT PATH
+
–
CLASSIFICATION
CURRENT LOAD
EN
EN
U
INPUT
CURRENT
LIMIT
U
SIGNATURE
RESISTOR
POWER GOOD
25k
V
PORTP
W
9k
16k
SIGDISA
PWRGD
P
U
OUT
REFERENCE
800mV
I
TH
PGND
V
/RUN
FB
REGULATOR
+
–
SHUNT
CLAMP
START
SOFT-
V
LTC4267 has been specifi cally designed to interface with
both IEEE compliant Power Sourcing Equipment (PSE)
and legacy PSEs which do not meet the inrush current
requirement of the IEEE 802.3af specifi cation. By setting
the initial inrush current limit to a low level, a PD using
the LTC4267 minimizes the current drawn from the PSE
during start-up. After powering up, the LTC4267 switches
to the high level current limit, thereby allowing the PD to
consume up to 12.95W if an IEEE 802.3af PSE is present.
This low level current limit also allows the LTC4267 to
charge arbitrarily large load capacitors without exceeding
the inrush limits of the IEEE 802.3af specifi cation. This
dual level current limit provides the system designer with
fl exibility to design PDs which are compatible with legacy
PSEs while also being able to take advantage of the higher
power available in an IEEE 802.3af system.
Using an LTC4267 for the power and signature interface
functions of a PD provides several advantages. The
LTC4267 current limit circuit includes an onboard 100V,
400mA power MOSFET. This low leakage MOSFET is
CC
1.2V
ERROR
AMPLIFIER
P
VCC
0.3µA
0.28V
20mV
–
+
–
+
CURRENT
COMPARATOR
OSCILLATOR
200kHz
SHUTDOWN
COMPARATOR
R
S
Q
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
LTC4267
CURRENT
SLOPE
V
SHUTDOWN
COMP
RAMP
TURNON
P
VCC
<
UNDERVOLTAGE
P
VCC
LOCKOUT
GATE
DRIVER
9
NGATE
SENSE
4267fc
4267 BD