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| 型號(hào)： | LTC4278IDKD#PBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 10/42頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PD IEEE 802.3AT 25.5W 32-DFN |
| 產(chǎn)品培訓(xùn)模塊： | Power over Ethernet |
| 標(biāo)準(zhǔn)包裝： | 52 |
| 控制器類型： | 供電設(shè)備接口控制器（PD） |
| 接口： | IEEE 802.3af |
| 電源電壓： | 4.5 V ~ 20 V |
| 電流 - 電源： | 6.4mA |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 32-WFDFN 裸露焊盤 |
| 供應(yīng)商設(shè)備封裝： | 32-DFN（7x4）裸露焊盤 |
| 包裝： | 管件 |
| 產(chǎn)品目錄頁面： | 1338 (CN2011-ZH PDF) |
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LTC4278
18
4278fc
PWRGD PIN WHEN SHDN IS INVOKED
InPDapplicationswhereanauxiliarypowersupplyinvokes
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
TheIEEE802.3af/atspecificationrequiresaPDtowithstand
any applied voltage from 0V to 57V indefinitely. However,
there are several possible scenarios where a PD may
encounter excessive heating.
During classification, excessive heating may occur if the
PSEexceedsthe75msprobingtimelimit.Atturn-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-temper-
ature 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
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
CONTACT INFORMATION
Bel Fuse Inc.
206 Van Vorst Street
Jersey City, NJ 07302
Tel: 201-432-0463
www.belfuse.com
Coilcraft Inc.
1102 Silver Lake Road
Gary, IL 60013
Tel: 847-639-6400
www.coilcraft.com
Halo Electronics
1861 Landings Drive
Mountain View, CA 94043
Tel: 650-903-3800
www.haloelectronics.com
PCA Electronics
16799 Schoenborn Street
North Hills, CA 91343
Tel: 818-892-0761
www.pca.com
Pulse Engineering
12220 World Trade Drive
San Diego, CA 92128
Tel: 858-674-8100
www.pulseeng.com
Tyco Electronics
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: 800-227-7040
www.circuitprotection.com
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.
APPLICATIONS INFORMATION
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