參數(shù)資料
型號(hào)： LTC4274AIUHF-2#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 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
文件頁數(shù)： 9/28頁
文件大小： 313K
代理商： LTC4274AIUHF-2#TRPBF
LTC4274A/LTC4274C
17
4274acf
ApplicAtions inForMAtion
Operating Modes
TheLTC4274A/LTC4274Ccanoperateinoneoffourmodes:
manual, semi-auto, AUTO pin, or shutdown.
Table 2. Operating Modes
MODE
AUTO
PIN
OPMD
DETECT/
CLASS
POWER-UP
AUTOMATIC
ICUT/ILIM
ASSIGNMENT
AUTO Pin
1
11b
Enabled at
Reset
Automatically
Yes
Reserved
0
11b
N/A
Semi-auto
0
10b
Host
Enabled
Upon
Request
No
Manual
0
01b
Once Upon
Request
Upon
Request
No
Shutdown
0
00b
Disabled
No
Inmanualmode,theportwaitsforinstructionsfromthe
host system before taking any action. It runs a single
detection or classification cycle when commanded to
by the host, and reports the result in its Port Status
register. The host system can command the port to turn
on or off the power at any time. This mode should only
be used for diagnostic and test purposes.
In semi-auto mode, the port repeatedly attempts to
detect and classify any PD attached to it. It reports the
status of these attempts back to the host, and waits for
a command from the host before turning on power to
theport.Thehostmustenabledetection(andoptionally
classification) for the port before detection will start.
AUTOpinmodeoperatesthesameassemi-automode
except that it will automatically turn on the power to the
port if detection is successful. In AUTO pin mode, ICUT
and ILIM values are set automatically by the LTC4274A/
LTC4274C.TheAUTOpinmustbehighatresettoensure
proper AUTO pin mode operation.
Inshutdownmode,theportisdisabledandwillnot
detect or power a PD.
Regardlessofwhichmodeitisin,theLTC4274A/LTC4274C
will remove power automatically from a port which gener-
ates a current limit fault. It will also automatically remove
power from any port that generates a disconnect event if
disconnect detection is enabled. The host controller may
also command the port to remove power at any time.
Reset and the AUTO/MID Pins
The initial LTC4274A/LTC4274C configuration depends on
the state of the AUTO and MID pins during reset. Reset
occurs at power-up, or whenever the RESET pin is pulled
low or the global Reset All bit is set. Note that the AUTO pin
is only sampled when a reset occurs. Changing the state
of AUTO or MID after power-up will not change the port
behavior of the LTC4274A/LTC4274C until a reset occurs.
Although typically used with a host controller, the
LTC4274A/LTC4274Ccanalsobeusedinastandalonemode
with no connection to the serial interface. If there is no
host present, the AUTO pin should be tied high so that, at
reset, the port will be configured to operate automatically.
The port will detect and classify repeatedly until a PD is
discovered, set ICUT and ILIM according to the classifica-
tion results, apply power after successful detection, and
remove power when a PD is disconnected.
Table 3 shows the ICUT and ILIM values that will be auto-
matically set in standalone (AUTO pin) mode, based on
the discovered class.
Table 3. ICUT and ILIM Values in AUTO Pin Mode
CLASS
ICUT
ILIM
Class 1
112mA
425mA
Class 2
206mA
425mA
Class 3 or Class 0
375mA
425mA
Class 4
638mA
850mA
The automatic setting of the ICUT and ILIM values only
occurs if the LTC4274A/LTC4274C is reset with the AUTO
pin high.
Ifthestandaloneapplicationisamidspan,theMIDpinshould
be tied high to enable correct midspan detection timing.
DETECTION
Detection Overview
Toavoiddamagingnetworkdevicesthatwerenotdesigned
to tolerate DC voltage, a PSE must determine whether
the connected device is a real PD before applying power.
The IEEE specification requires that a valid PD have a
common-mode resistance of 25k ±5% at any port volt-
age below 10V. The PSE must accept resistances that fall
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