LTC4263 datasheet(23/32 Pages) LINER

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LTC4270/LTC4271

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Port Current Policing

The LTC4270/LTC4271 can augment tCUT current moni-

toring with a policing function to track the one second

current averages. A port violating the user-specified Port

Police Threshold will be shut off with both a tCUT and

Police event recorded. A port current Police event can be

differentiated from a port tCUT violation by reading both

events bits; both bits are set for a Police violation while

only the tCUT bit is set for tCUT timer violations.

Port Voltage Readback

The LTC4270/LTC4271 measures the output voltage at

each port with an internal A/D converter. Port data is

only valid when the port power is on and reads zero at

all other times.

Disconnect

The LTC4270/LTC4271 monitors powered ports to ensure

thePDcontinuestodrawtheminimumspecifiedcurrent.A

disconnecttimercountsupwheneverportcurrentisbelow

7.5mA(typ),indicatingthatthePDhasbeendisconnected.

If the tDIS timer expires, the port will be turned off and the

disconnect bit in the fault event register will be set. If the

current returns before the tDIS timer runs out, the timer

resets. As long as the PD exceeds the minimum current

level more often than tDIS, it will remain powered.

Although not recommended, the DC disconnect feature

can be disabled by clearing the corresponding enable bits.

Note that this defeats the protection mechanisms built

into the IEEE specification, since a powered port will stay

powered after the PD is removed. If the still-powered port

is subsequently connected to a non-PoE data device, the

device may be damaged.

The LTC4270/LTC4271 does not include AC disconnect

circuitry, but includes AC Disconnect Enable bits to main-

tain compatibility with the LTC4259A. If the AC Disconnect

Enable bits are set, DC disconnect will be used.

ApplicAtions inForMAtion

Masked Shutdown

The LTC4270/LTC4271 provides a low latency port shed-

ding feature to quickly reduce the system load when

required. By allowing a pre-determined set of ports to

be turned off, the current on an overloaded main power

supply can be reduced rapidly while keeping high priority

devices powered. Each port can be configured to high or

low priority; all low-priority ports will shut down within

6.5μs after the MSD pin is pulled low, high priority ports

will remain powered. If a port is turned off via MSD, the

correspondingDetectionandClassificationEnablebitsare

cleared, so the port will remain off until the host explicitly

re-enables detection.

In the LTC4270/LTC4271 chipset the active level of MSD

is register configurable as active high or low. The default

is LTC4266-compatible active low behavior.

VEE Readback

The LTC4270/LTC4271 measures the VEE voltage with an

internal 12-bit A/D converter.

General Purpose IO

Two sets of general purpose IO pins are available in the

LTC4270/LTC4271chipset.Thefirstsetofgeneralpurpose

IO are GP1 and GP0. These fully bidirectional IO are 3.3V

CMOS IO on the LTC4271 chip.

The second set of general purpose IO pins are XIO1 and

XIO0. These fully bidirectional IO are 4.3V CMOS IO on

the LTC4270 chip.

Code Download

LTC4271 firmware is field-upgradable by downloading

and executing RAM images. RAM images are volatile

and must be re-downloaded after each VDD power cycle,

but will remain valid during reset and VEE power events.

Contact Linear Technology for code download procedures

and RAM images.

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