LTC4267CDHC Linear Technology, LTC4267CDHC Datasheet - Page 16

• Manufacturer Part Number: LTC4267CDHC
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC4267CDHC

• Linear Misc Type: Negative Voltage
• Package Type: DFN EP
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Product Depth (mm): 3mm
• Product Length (mm): 5mm
• Mounting: Surface Mount
• Pin Count: 16
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
LTC4267
16
The undervoltage lockout mechanism on P
the LTC4267 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
switching regulator.
The I
0.28V with respect to PGND) to force the LTC4267 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 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 interface.
TURNON
VCC
TH
P
TH
ALL VOLTAGES WITH
RESPECT TO PGND
VCC
TH
exceeds V
/RUN pin voltage is allowed to exceed V
/RUN pin to about 0.7V. In operation, the I
/RUN can be driven below V
< V
TURNOFF
(nominally 8.7V with respect to PGND) at least
TURNOFF
Figure 8. LTC4267 Switching Regulator
Start-Up/Shutdown State Diagram
TURNON
(nominally 5.7V with respect to PGND)
TH
U
/RUN voltage is clamped and is
, the LTC4267 switching regulator
SHUTDOWN
ENABLED
TH
LTC4267
LTC4267
PWM
PWM
U
/RUN pin towards P
V
< V
(NOMINALLY
0.28V)
ITH
ITHSHDN
/RUN
4267 F08
W
VCC
ITHSHDN
V
AND P
(NOMINALLY 8.7V)
VCC
ITH/RUN
pin must exceed
VCC
VCC
voltage must
ITHSHDN
> V
> V
(nominally
U
VCC
ITHSHDN
prevents
TURNON
TH
. When
/RUN
and
Adjustable Slope Compensation
The LTC4267 switching regulator injects a 5µA peak cur-
rent 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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