LTC4257IS8 Linear Technology, LTC4257IS8 Datasheet - Page 11

• Manufacturer Part Number: LTC4257IS8
• Description: IC CONTROLLER POE INTERFAC 8SOIC
• Manufacturer: Linear Technology
• Type: Power over Ethernet Switch (PoE)r

Specifications of LTC4257IS8

• Applications: IP Phones, Power over LAN, Network Routers and Switches
• Internal Switch(s): Yes
• Current Limit: 350mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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 800µF
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
U
U
TURN ON
UVLO
W
Figure 7. Smart Thermal Protection State Diagram
T < 120°C
CURRENT
100%
U
CURRENT
T > 120°C
T < 120°C
0%
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 200µF can be charged
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.
CURRENT
50%
T > 140°C
4257 F07
LTC4257
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