LTC4278CDKD#PBF Linear Technology, LTC4278CDKD#PBF Datasheet - Page 18

IC PD IEEE 802.3AT 25.5W 32-DFN

LTC4278CDKD#PBF

Manufacturer Part Number
LTC4278CDKD#PBF
Description
IC PD IEEE 802.3AT 25.5W 32-DFN
Manufacturer
Linear Technology
Datasheet

Specifications of LTC4278CDKD#PBF

Controller Type
Powered Device Interface Controller (PD)
Interface
IEEE 802.3af
Voltage - Supply
4.5 V ~ 20 V
Current - Supply
6.4mA
Operating Temperature
0°C ~ 70°C
Mounting Type
Surface Mount
Package / Case
32-DFN
Input Voltage
60V
Supply Current
1.35mA
Digital Ic Case Style
DFN
No. Of Pins
32
Duty Cycle (%)
88%
Frequency
100kHz
Operating Temperature Range
°C To +70°C
Msl
MSL 1 - Unlimited
Rohs Compliant
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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APPLICATIONS INFORMATION
LTC4278
PWRGD PIN WHEN SHDN IS INVOKED
In PD applications where an auxiliary power supply invokes
the SHDN feature, the PWRGD pin becomes high imped-
ance. This prevents the PWRGD pin that is connected to
the UVLO pin from interfering with the DC/DC converter
operations when powered by an auxiliary power supply.
OVERVOLTAGE LOCKOUT
The LTC4278 includes an overvoltage lockout (OVLO)
feature (Figure 6) which protects the LTC4278 and its load
from an overvoltage event. If the input voltage exceeds the
OVLO threshold, the LTC4278 discontinues PD operation.
Normal operations resume when the input voltage falls
below the OVLO threshold and when C1 is charged up.
THERMAL PROTECTION
The IEEE 802.3af/at specifi cation requires a PD to withstand
any applied voltage from 0V to 57V indefi nitely. However,
there are several possible scenarios where a PD may
encounter excessive heating.
During classifi cation, excessive heating may occur if the
PSE exceeds the 75ms probing time limit. At turn-on, when
the load capacitor begins to charge, the instantaneous
power dissipated by the PD interface can be large before
it reaches the line voltage. And if the PD experiences a
fast input positive voltage step in its operational mode
(for example, from 37V to 57V), the instantaneous power
dissipated by the PD Interface can be large.
The LTC4278 includes a thermal protection feature which
protects the LTC4278 from excessive heating. If the
LTC4278 junction temperature exceeds the over-tempera-
ture threshold, the LTC4278 discontinues PD operations
and power good becomes inactive. Normal operation
resumes when the junction temperature falls below the
overtemperature threshold and when C1 is charged up.
EXTERNAL INTERFACE AND COMPONENT SELECTION
Transformer
Nodes on an Ethernet network commonly interface to the
outside world via an isolation transformer. For PDs, the
18
isolation transformer must also include a center tap on
the RJ45 connector side (see Figure 7).
The increased current levels in a Type 2 PD over a Type
1 increase the current imbalance in the magnetics which
can interfere with data transmission. In addition, proper
termination is also required around the transformer to
provide correct impedance matching and to avoid radiated
and conducted emissions. Transformer vendors such as
Bel Fuse, Coilcraft, Halo, Pulse, and Tyco (Table 4) can
assist in selecting an appropriate isolation transformer
and proper termination methods.
Table 4. Power over Ethernet Transformer Vendors
VENDOR
Bel Fuse Inc.
Coilcraft Inc.
Halo Electronics
PCA Electronics
Pulse Engineering
Tyco Electronics
Input Diode Bridge
Figure 2 shows how two diode bridges are typically con-
nected in a PD application. One bridge is dedicated to the
data pair while the other bridge is dedicated to the spare
pair. The LTC4278 supports the use of either silicon or
Schottky input diode bridges. However, there are tradeoffs
in the choice of diode bridges.
CONTACT INFORMATION
206 Van Vorst Street
Jersey City, NJ 07302
Tel: 201-432-0463
www.belfuse.com
1102 Silver Lake Road
Gary, IL 60013
Tel: 847-639-6400
www.coilcraft.com
1861 Landings Drive
Mountain View, CA 94043
Tel: 650-903-3800
www.haloelectronics.com
16799 Schoenborn Street
North Hills, CA 91343
Tel: 818-892-0761
www.pca.com
12220 World Trade Drive
San Diego, CA 92128
Tel: 858-674-8100
www.pulseeng.com
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: 800-227-7040
www.circuitprotection.com
4278fa

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