參數(shù)資料
型號(hào)： LTC4274AIUHF-1#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 電源管理
英文描述： POWER SUPPLY MANAGEMENT CKT, PQCC38
封裝： 7 X 5 MM, LEAD FREE, PLASTIC, MO-220WHKD, QFN-38
文件頁(yè)數(shù)： 14/28頁(yè)
文件大小： 313K
代理商： LTC4274AIUHF-1#TRPBF
LTC4274A/LTC4274C
21
4274acf
ApplicAtions inForMAtion
Current Limit
The LTC4274A/LTC4274C port includes two current limit-
ing thresholds (ICUT and ILIM), each with a corresponding
timer (tCUT and tLIM). Setting the ICUT and ILIM thresholds
depends on several factors: the class of the PD, the volt-
age of the main supply (VEE), the type of PSE (Type 1 or
Type 2), the sense resistor (0.5Ω or 0.25Ω), the SOA of
the MOSFET, and whether or not the system is required
to implement class enforcement.
Per the IEEE specification, the LTC4274A/LTC4274C will
allow the port current to exceed ICUT 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 ILIM. The port does not take any action to
limit the current when only the ICUT threshold is exceeded,
but does start the tCUT timer. If the current drops below
the ICUT current threshold before its timer expires, the
tCUT timer counts back down, but at 1/16 the rate that it
counts up. If the tCUT timer reaches 60ms (typical) the
port is turned off and the port tCUT fault is set. This allows
the current limit circuitry to tolerate intermittent overload
signalswithdutycyclesbelowabout6%;longerdutycycle
overloads will turn the port off.
The ILIM current limiting circuit is always enabled and
actively limiting port current. The tLIM timer is enabled
only when the programmable tLIM field is non-zero. This
allows tLIM to be set to a shorter value than tCUT to provide
more aggressive MOSFET protection and turn off a port
before MOSFET damage can occur. The tLIM timer starts
when the ILIM threshold is exceeded. When the tLIM timer
reaches 1.7ms (typ) times the programmable tLIMfieldthe
port is turned off and the port tLIM fault is set. When the
tLIM field is zero, tLIM behaviors are tracked by the tCUT
timer, which counts up during both ILIM and ICUT events.
ICUT is typically set to a lower value than ILIM to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4274A/LTC4274C will
automatically set ILIM to 425mA (shown in bold in Table 6)
during inrush at port turn-on, and then switch to the
programmed ILIM setting once inrush has completed.
TomaintainIEEEcompliance,ILIMshouldbekeptat425mA
for all Type 1 PDs, and 850mA if a Type 2 PD is detected.
ILIM is automatically reset to 425mA when a port turns off.
Table 6. Example Current Limit Settings
ILIM (mA)
INTERNAL REGISTER SETTING (hex)
RSENSE = 0.5Ω
RSENSE = 0.25Ω
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
ILIM Foldback
The LTC4274A/LTC4274C features a two-stage foldback
circuit that reduces the port current if the port voltage
falls below the normal operating voltage. This keeps
MOSFETpowerdissipationatsafelevelsfortypical802.3af
MOSFETs, even at extended 802.3at power levels. Current
limit and foldback behavior are programmable. Table 6
gives examples of recommended ILIM register settings.
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