Datasheet

LTC4267-1
9
42671fa
For more information www.linear.com/4267-1
block DiagraM
42671 BD
V
PORTN
BOLD LINE INDICATES HIGH CURRENT PATH
P
OUT
–
+
PGND
R
CLASS
PWRGD
CONTROL
CIRCUITS
INPUT
CURRENT
LIMIT
POWER GOOD
CLASSIFICATION
CURRENT LOAD
1.237V
EN
375mA
140mA
9k
16k
–
+
EN
25k
SIGNATURE
RESISTOR
V
PORTP
SIGDISA
–
+
–
+
SLOPE
COMP
CURRENT
RAMP
P
VCC
GATE
DRIVER
NGATE
SENSE
200kHz
OSCILLATOR
UNDERVOLTAGE
LOCKOUT
Q
R
CURRENT
COMPARATOR
SHUTDOWN
COMPARATOR
SHUTDOWN
S
20mV
I
TH
/RUN
ERROR
AMPLIFIER
V
FB
SOFT-
START
CLAMP
V
CC
SHUNT
REGULATOR
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
0.28V
P
VCC
<
V
TURNON
0.3µA
P
VCC
–
+
1.2V
800mV
REFERENCE
applicaTions inForMaTion
OVERVIEW
The LTC4267-1 is partitioned into two major blocks: a
Powered Device (PD) interface controller and a current
mode flyback 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 25k signature resistor, classification current
source, and an input current limit circuit. With these
functions integrated into the LTC4267-1, the signature
and power interface for a PD can be built that meets all
the requirements of the IEEE 802.3af specification with a
minimum of external components.
The switching regulator portion of the LTC4267-1 is a
constant frequency current mode controller that is opti-
mized 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 amplifier and precision reference give the
PD designer the option of using a nonisolated topology
without the need for an external amplifier or reference. The
LTC4267-1 has been specifically 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 specification. By setting
the
initial inrush current limit to a low level, a PD using
the LTC4267-1 minimizes the current drawn from the PSE
during start-up. After powering up, the LTC4267-1 switches
to the high level current limit, thereby allowing the PD to
consume up to 13W if an IEEE 802.3af PSE is present.
This low level current limit also allows the LTC4267-1 to
charge arbitrarily large load capacitors without exceeding
the inrush limits of the IEEE 802.3af specification. This
dual level current limit provides the system designer with
flexibility 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.