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參數(shù)資料
| 型號： | LTC4274CUHF#PBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 20/28頁 |
| 文件大?。?/td> | 702K |
| 描述： | IC CONTROLLER POE 38-QFN |
| 標(biāo)準(zhǔn)包裝： | 52 |
| 類型： | 以太網(wǎng)供電 PSE 控制器（PoE） |
| 應(yīng)用： | 供電設(shè)備（PSE） |
| 內(nèi)部開關(guān)： | 無 |
| 電流限制： | 可調(diào) |
| 電源電壓： | 3 V ~ 4.3 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 38-QFN |
| 供應(yīng)商設(shè)備封裝： | 38-QFN（5x7）裸露焊盤 |
| 包裝： | 管件 |
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LTC4274
20
4274fd
APPLICATIONS INFORMATION
current reaches the inrush current limit level (typically
450mA), at which point the GATE pin will be servoed to
maintain the specified I
INRUSH
current. During this inrush
period, a timer (t
START
) runs. When output charging is
complete, the port current will fall and the GATE pin will
be allowed to continue rising to fully enhance the MOSFET
and minimize its on-resistance. The final V
GS
is nominally
12V. If the t
START
timer expires before the inrush period
completes, the port will be turned back off and a t
START
fault reported.
Current Limit
The LTC4274 port includes two current limiting thresholds
(I
CUT
and I
LIM
), each with a corresponding timer (t
CUT
and t
LIM
). Setting the I
CUT
and I
LIM
thresholds depends
on several factors: the class of the PD, the voltage of the
main supply (V
EE
), the type of PSE (1 or 2), the sense
resistor (0.5?or 0.25?, the SOA of the MOSFET, and
whether or not the system is required to implement class
enforcement.
Per the IEEE spec, the LTC4274 will allow the port cur-
rent to exceed I
CUT
for a limited period of time before
removing power from the port, whereas it will actively
control the MOSFET gate drive to keep the port current
below I
LIM
. The port does not take any action to limit the
current when only the I
CUT
threshold is exceeded, but
does start the t
CUT
timer. The t
LIM
timer starts when the
I
LIM
threshold is exceeded and current limit is active. If
the current drops below the I
CUT
current threshold before
its timer expires, the t
CUT
timer counts back down, but
at 1/16 the rate that it counts up. This allows the current
limit circuitry to tolerate intermittent overload signals with
duty cycles below about 6%; longer duty cycle overloads
will turn the port off.
I
CUT
is typically set to a lower value than I
LIM
to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4274 will automatically
set I
LIM
to 425mA (shown in bold in Table 5) during in-
rush at port turn-on, and then switch to the programmed
I
LIM
setting once inrush has completed. To maintain IEEE
compliance, I
LIM
should kept at 425mA for all Type 1 PDs,
and 850mA if a Type 2 PD is detected. I
LIM
is automatically
reset to 425mA when a port turns off.
Table 5. Example Current Limit Settings
I
LIM
(mA)
INTERNAL REGISTER SETTING (hex)
R
SENSE
= 0.5?/DIV>
R
SENSE
= 0.25?/DIV>
53
88
106
08
88
159
89
213
80
08
266
8A
319
09
89
372
8B
425
00
80
478
8E
531
92
8A
584
CB
638
10
90
744
D2
9A
850
40
C0
956
4A
CA
1063
50
D0
1169
5A
DA
1275
60
E0
1488
52
49
1700
40
1913
4A
2125
50
2338
5A
2550
60
2975
52
I
LIM
Foldback
The LTC4274 features a two-stage foldback circuit that
reduces the port current if the port voltage falls below
the normal operating voltage. This keeps MOSFET power
dissipation at safe levels for typical 802.3af MOSFETs,
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