LTC4257_15 datasheet(11/20 Pages) LINER

Part #	LTC4257

Description	IEEE 802.3af PD Power over Ethernet Interface Controller

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Manufacturer	LINER [Linear Technology]

Direct Link	http://www.linear.com

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LTC4257

11

4257fb

Thermal Protection

The LTC4257 includes smart thermal protection in order

to provide full device functionality in a miniature package

while maintaining safe operating temperatures. Several

factors create the possibility for tremendous power

dissipation within the LTC4257. IEEE 802.3af mandates

that inrush current be limited to less than 400mA while

standard telecom power can be as high as 57V. At turn on,

before the load capacitor has charged up, the instanta-

neous power dissipated by the LTC4257 can be over 20W.

As the load capacitor charges up, the power dissipation in

the LTC4257 will decrease until it reaches a steady-state

value dependent on the DC load current. The size of the

load capacitor determines how fast the power dissipation

in the LTC4257 subsides. At room temperature, the

LTC4257 can handle load capacitors as large as 800

µF

without going into thermal shutdown. With a large load

capacitor like this, the LTC4257 die temperature will

increase by about 50

°C during a single turn-on sequence.

If for some reason power were removed from the part and

then quickly reapplied so that the LTC4257 had to charge

up the load capacitor again, the temperature rise would be

excessive if safety precautions were not implemented.

The LTC4257 protects itself from thermal damage by

monitoring the die temperature. If the die temperature

exceeds the overtemperature trip point, the part switches

to a half-power mode where the current limit is set to 50%

of its normal level. This reduces power dissipation and

helps prevent further heating. If the part continues to heat

up and reaches the shutdown temperature, the current is

reduced to zero and very little power is dissipated in the

part until it cools below the overtemperature set point. The

LTC4257 current limit will continue switching between

0%, 50% and 100% current levels (Figure 7) until the load

capacitor is fully charged.

If the PD is designed to operate at a high ambient tempera-

ture and with the maximum allowable supply (57V), there

will be a limit to the size load capacitor that can be charged

up before the LTC4257 reaches the overtemperature trip

point. Hitting the overtemperature trip point intermittently

does not harm the LTC4257, but it will delay completion of

capacitor charging. Capacitors up to 200

µFcanbecharged

without a problem.

During classification, excessive heating of the LTC4257

can occur if the PSE violates the 75ms probing time limit.

To protect the LTC4257, the thermal protection circuitry

will disable classification current if the die temperature

exceeds the overtemperature trip point. When the die

cools down below the trip point, classification current is

re-enabled.

Once the LTC4257 has charged up to the load capacitor

and the PD is powered and running, there will be some

residual heating due to the DC load current of the PD

flowing through the internal MOSFET. In some applica-

tions, the LTC4257 power dissipation may be significant

and if dissipated in the S8 package, excessive package

heating could occur. This problem can be solved with the

use of the DD package which has superior thermal perfor-

mance. The DD package includes an exposed pad that

should be soldered to an isolated heatsink on the printed

circuit board.

Figure 7. Smart Thermal Protection State Diagram

APPLICATIO S I FOR ATIO

4257 F07

T < 120

°C

T < 120

°C

T > 120

°C

T > 140

°C

100%

CURRENT

UVLO

TURN ON

50%

CURRENT

0%

CURRENT

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