參數(shù)資料
型號: LTC4278IDKD#PBF
廠商: Linear Technology
文件頁數(shù): 32/42頁
文件大?。?/td> 0K
描述: IC PD IEEE 802.3AT 25.5W 32-DFN
產(chǎn)品培訓模塊: Power over Ethernet
標準包裝: 52
控制器類型: 供電設備接口控制器(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
供應商設備封裝: 32-DFN(7x4)裸露焊盤
包裝: 管件
產(chǎn)品目錄頁面: 1338 (CN2011-ZH PDF)
LTC4278
38
4278fc
current will flow through the parasitic body diode of the
internal MOSFET and may cause permanent damage to
the LTC4278.
In order to minimize switching noise and improve output
loadregulation,connecttheGNDpinoftheLTC4278directly
to the ground terminal of the VCC decoupling capacitor,
the bottom terminal of the current sense resistor and the
groundterminaloftheinputcapacitor,usingagroundplane
with multiple vias. Place the VCC capacitor immediately
adjacent to the VCC and GND pins on the IC package. This
capacitor carries high di/dt MOSFET gate drive currents.
Use a low ESR ceramic capacitor.
TakecareinPCBlayouttokeepthetracesthatconducthigh
switching currents short, wide and with minimal overall
loop area. These are typically the traces associated with
the switches. This reduces the parasitic inductance and
also minimizes magnetic field radiation. Figure 19 outlines
the critical paths.
Keep electric field radiation low by minimizing the length
and area of traces (keep stray capacitances low). The drain
of the primary-side MOSFET is the worst offender in this
category. Always use a ground plane under the switcher
circuitry to prevent coupling between PCB planes.
Check that the maximum BVDSS ratings of the MOSFETs
are not exceeded due to inductive ringing. This is done by
viewingtheMOSFETnodevoltageswithanoscilloscope.If
it is breaking down, either choose a higher voltage device,
add a snubber or specify an avalanche-rated MOSFET.
Placethesmall-signalcomponentsawayfromhighfrequency
switching nodes. This allows the use of a pseudo-Kelvin
connection for the signal ground, where high di/dt gate
driver currents flow out of the IC ground pin in one direction
(to the bottom plate of the VCC decoupling capacitor) and
small-signal currents flow in the other direction.
Keep the trace from the feedback divider tap to the FB pin
short to preclude inadvertent pick-up.
For applications with multiple switching power converters
connected to the same input supply, make sure that the
input filter capacitor for the LTC4278 is not shared with
other converters. AC input current from another converter
could cause substantial input voltage ripple which could
interfere with the LTC4278 operation. A few inches of PC
trace or wire (L
@ 100nH) between the CIN of the LTC4278
and the actual source VIN, is sufficient to prevent current
sharing problems.
APPLICATIONS INFORMATION
T2
T1
CR
CVIN
MS
MP
GATE
TURN-ON
GATE
TURN-ON
RSENSE
CVCC
SG
VCC
PG
VCC
VIN
GATE
TURN-OFF
GATE
TURN-OFF
Q4
Q3
COUT
4278 F19
OUT
+
Figure 19. Layout Critical High Current Paths
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