LTC4263CS datasheet(15/24 Pages) LINER

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LTC4263

4263fe

APPLICATIONS INFORMATION

condition is removed before the tOVLD timer expires, the

port stays powered and the timer is reset.

Foldback

Foldback is designed to limit power dissipation in the

LTC4263 during power-up and momentary short-circuit

conditions. At low port output voltages, the voltage

across the internal MOSFET is high, and power dissipa-

tion will be large if signiﬁcant current is ﬂowing. Foldback

monitors the port output voltage and reduces the ILIM

current limit level for port voltages of less than 28V, as

shown in Figure 7.

Figure 7. Current Limit Foldback

Thermal Protection

The LTC4263 includes thermal overload protection in

order to provide full device functionality in a miniature

package while maintaining safe operating temperatures.

Several factors create the possibility for very large power

dissipation within the LTC4263. At port turn-on, while

ILIM is active, the instantaneous power dissipated by the

LTC4263 can be as high as 12W. This can cause 40ºC or

more of die heating in a single turn-on sequence. Similarly,

excessive heating can occur if an attached PD repeatedly

pushes the LTC4263 into ILIM by drawing too much cur-

rent. Excessive heating can also occur if the VDD5 pin is

shorted or overloaded.

The LTC4263 protects itself from thermal damage by

monitoring die temperature. If the die temperature exceeds

the overtemperature trip point, the LTC4263 removes port

power and shuts down all functions including the internal

5V regulator. Once the die cools, the LTC4263 waits four

seconds, then restarts detection.

DC Disconnect

The DC disconnect circuit monitors port current whenever

power is on to detect continued presence of the PD. IEEE

802.3af mandates a minimum current of 10mA that the

PD must draw for periods of at least 75ms with optional

dropouts of no more than 250ms. The tMPDO disconnect

timer increments whenever port current is below 7.5mA

(typ). If the timer expires, the port is turned off and the

LTC4263 waits 1.5 seconds before restarting detection. If

the undercurrent condition goes away before tMPDO(350ms

(typ)

), the timer is reset to zero. The DC disconnect circuit

includes a glitch ﬁlter to prevent noise from falsely resetting

the timer. The current must be present for a period of at

least 20ms to guarantee reset of the timer. To enable DC

disconnect, tie the OSC pin to VSS.

AC Disconnect

AC disconnect is an alternate method of sensing the pres-

ence or absence of a PD by monitoring the port impedance.

The LTC4263 forces an AC signal from an internal sine wave

generator on to the port. The ACOUT pin current is then

sampled once per cycle and compared to IACDMIN. Like DC

disconnect, the AC disconnect sensing circuitry controls the

tMPDO disconnect timer. When the connection impedance

rises due to the removal of the PD, AC peak current falls

below IACDMIN and the disconnect timer increments. If the

impedance remains high (AC peak current remains below

IACDMIN), the disconnect timer counts to tMPDO and the

port is turned off. If the impedance falls, causing AC peak

current to rise above IACDMIN for two consecutive samples

before the maximum count of the disconnect timer, the

timer resets and the port remains powered.

The AC disconnect circuitry senses the port via the ACOUT

pin. Connect a 0.47μF 100V X7R capacitor (CDET) and

a 1kΩ resistor (RDET) from the DETECT pin to the port

output as shown in Figure 8. This provides an AC path for

sensing the port impedance. The 1kΩ resistor, RDET, limits

current ﬂowing through this path during port power-on and

power-off. An AC blocking diode (DAC) is inserted between

the OUT pin and the port to prevent the AC signal from

VDD48 – VOUT (V)

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