LTC4267CDHC-3#PBF Linear Technology, LTC4267CDHC-3#PBF Datasheet - Page 16

• Manufacturer Part Number: LTC4267CDHC-3#PBF
• Description: IC POE 802.3AF W/REG 16-DFN
• Manufacturer: Linear Technology

Specifications of LTC4267CDHC-3#PBF

• Controller Type: Power over Ethernet Controller (POE)
• Interface: IEEE 802.3af
• Current - Supply: 3mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -

APPLICATIO S I FOR ATIO
LTC4267-3
16
The undervoltage lockout mechanism on P
the LTC4267-3 switching regulator from trying to drive
the external N-Channel MOSFET with insuffi cient gate-to-
source voltage. The voltage at the P
V
momentarily to enable operation. The P
fall to V
before the undervoltage lockout disables the switching
regulator. This wide UVLO hysteresis range supports
applications where a bias winding on the fl yback trans-
former is used to increase the effi ciency of the LTC4267-3
switching regulator.
The I
with respect to PGND) to force the LTC4267-3 switching
regulator into shutdown. An internal 0.3µA current source
always tries to pull the I
the I
P
begins to operate and an internal clamp immediately pulls
the I
pin voltage will vary from roughly 0.7V to 1.9V to represent
current comparator thresholds from zero to maximum.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC4267-3 switching regulator comes out of shutdown.
Specifi cally, the I
prevented from reaching maximum until 1.4ms have
passed. This allows the input current of the PD to rise in a
smooth and controlled manner on start-up and stay within
the current limit requirement of the LTC4267-3 interface.
TURNON
VCC
TH
TH
TH
exceeds V
/RUN pin voltage is allowed to exceed V
/RUN can be driven below V
/RUN pin to about 0.7V. In operation, the I
P
ALL VOLTAGES WITH
RESPECT TO PGND
VCC
TURNOFF
(nominally 8.7V with respect to PGND) at least
Figure 8. LTC4267-3 Switching Regulator
Start-Up/Shutdown State Diagram
< V
TURNOFF
TURNON
(nominally 5.7V with respect to PGND)
TH
U
/RUN voltage is clamped and is
, the LTC4267-3 switching regulator
SHUTDOWN
TH
LTC4267-3
LTC4267-3
ENABLED
U
PWM
PWM
/RUN pin towards P
V
< V
(NOMINALLY
0.28V)
ITH
ITHSHDN
/RUN
ITHSHDN
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W
VCC
V
AND P
(NOMINALLY 8.7V)
ITH/RUN
VCC
pin must exceed
(nominally 0.28V
VCC
voltage must
> V
VCC
> V
ITHSHDN
ITHSHDN
U
VCC
TURNON
prevents
TH
. When
/RUN
and
Adjustable Slope Compensation
The LTC4267-3 switching regulator injects a 5µA peak
current ramp out through its SENSE pin which can be
used for slope compensation in designs that require it.
This current ramp is approximately linear and begins at
zero current at 6% duty cycle, reaching peak current at
80% duty cycle. Programming the slope compensation
via a series resistor is discussed in the External Interface
and Component Selection section.
EXTERNAL INTERFACE AND COMPONENT SELECTION
Input Interface Transformer
Nodes on an Ethernet network commonly interface to the
outside world via an isolation transformer (Figure 9). For
PoE devices, the isolation transformer must include a
center tap on the media (cable) side. Proper termination
is required around the transformer to provide correct
impedance matching and to avoid radiated and conducted
emissions. Transformer vendors such as Bel Fuse, Coil-
craft, Pulse and Tyco (Table 3) can provide assistance with
selection of an appropriate isolation transformer and proper
termination methods. These vendors have transformers
specifi cally designed for use in PD applications.
Table 3. Power over Ethernet Transformer Vendors
VENDOR
Bel Fuse Inc.
Coilcraft, Inc.
Pulse Engineering
Tyco Electronics
CONTACT INFORMATION
206 Van Vorst Street
Jersey City, NJ 07302
Tel: [phone]
FAX: [phone]
http://www.belfuse.com
1102 Silver Lake Road
Cary, IL 60013
Tel: [phone]
FAX: [phone]
http://www.coilcraft.com
12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
FAX: [phone]
http://www.pulseeng.com
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: [phone]
FAX: [phone]
http://www.circuitprotection.com
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