LTC4259A datasheet(17/32 Pages) LINER

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LTC4259A

4259Af

Dual-Level Current Limit

A PD is permitted to draw up to 15.4W continuously and

up to 400mA for 50ms. The LTC4259A has two correspond-

ing current limit thresholds, ICUT (375mA typ) and ILIM

(425mA typ). These are given by the equations:

ICUT = VCUT/RS, ILIM = VLIM/RS

RS is the sense resistor and should be 0.5Ω for IEEE

802.3af compliance. While the LTC4259A allows the port

current to exceed ICUT for a limited time period (see tICUT

timing below), it does not allow the current to exceed ILIM.

The current limit circuit monitors the port current by

monitoring the voltage across the sense resistor and re-

duces the MOSFET gate voltage as needed to keep the

current at or below ILIM. When the current drops below

ILIM, the gate voltage is restored to the full value to keep

the MOSFET resistance to a minimum.

tICUT Timing

Whenever more than ICUT = VCUT/RS flows through a port,

the port’s sense voltage is above VCUT and the tICUT timer

counts up. If the sense voltage is still above VCUT when the

tICUT timer expires, the LTC4259A will turn off the power

to the port immediately and set the appropriate tICUT Fault

bit in register 06h/07h. The tICUT timer duration can be pro-

grammed via register 16h, bits 3 and 2 (Table 1).

The tICUT timer is an up/down counter that is designed to

protect the external MOSFET from thermal stress caused

by repeatedly operating in current limit. The counter

counts up whenever the current is above ICUT and counts

down at 1/16th the rate when it is not. The counter will

bottom out at zero to prevent underflow. Full count indi-

cates that the tICUT timer has expired and the port will be

turned off.

This count up/count down behavior implements duty cycle

protection, preventing intermittent current limit faults from

causing cumulative thermal stress in the MOSFET. If the port

enters current limit but then exits before the timer expires,

the count will decrease slowly, giving the ICUT timer the

ability to turn off sooner in the case of a repetitive fault. If

the overcurrent duty cycle is less than 6.3% the tICUT timer

will be fully reset.

If the tICUT timer expires and causes the port to shut off, the

timer will continue to run, counting down at the slow

1/16th rate and preventing the port from being repowered

until the count returns to zero. This protects the MOSFET

from damage due to a faulty PD that may still have a valid

signature, or from errant software that repeatedly writes to

the Power On bit.

The port will not repower until after the tICUT counter

returns to zero. In manual and semiauto modes the power

enable command must be received after the tICUT counter

reaches zero. In auto mode the LTC4259A must complete

a valid detection cycle after the tICUT counter reaches zero.

tSTART Timing

To distinguish between normal turn-on current limit be-

havior and current limit faults which occur after power-up

is complete, the LTC4259A starts a timer (the tSTART timer)

whenever a power-up sequence begins.

The tSTART timer serves three functions. First and fore-

most, it allows the user to specify a different current limit

timeout (tSTART instead of tICUT) during turn-on (current

limit duty cycle protection remains functional). Second,

the DC disconnect timer is disabled during this period and

can only begin counting up after the tSTART timer has

expired. Together, these two features let the PD draw the

maximum current IINRUSH to charge its input capacitance,

boot up and begin drawing power without triggering a

tSTART fault. Finally, if the device is in current limit for the

entire tSTART period, a tSTART fault will be generated

instead of a tICUT fault. This can be useful for tracking down

the cause of a current fault.

As long as the PD draws less than ICUT at the end of tSTART

and begins drawing the minimum current within tDIS after

tSTART expires (if DC disconnect is enabled), no faults will

be indicated.

The tSTART timer also implements the duty cycle protec-

tion described under tICUT timing and its duration can be

programmed via register 16h, bits 5 and 4 (Table 1).

APPLICATIO S I FOR ATIO

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