LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 18

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC4258
Foldback
Foldback is designed to limit power dissipation in the
MOSFET during power-up and momentary short-circuit
conditions. At low port output voltages, the voltage across
the MOSFET is high, and power dissipation will be large if
significant current is flowing. Foldback monitors the port
output voltage and reduces the V
linearly from its full value (212.5mV typ) at a port voltage
of 18V to approximately 1/7th of the full value (30mV typ)
at a port voltage of 0V. With 0.5 sense resistors, this
limits the short-circuit current to 60mA (typ) instead of the
full 425mA (typ) current limit. When the LTC4258 is in
foldback, the t
Short-Circuit Protection
If a port is suddenly shorted out, the MOSFET power
dissipation can rise to very high levels, jeopardizing the
MOSFET even before the normal current limit circuit can
respond. A separate short-circuit current limit circuit
watches for significant overcurrent events (V
>275mV, >550mA with a 0.5 sense resistor) and pulls
the GATE pin down immediately if such an event occurs,
shutting off the MOSFET in less than 1 s (with no external
capacitor on GATE). Approximately 100 s later, GATE is
allowed to rise back up and the normal current limit circuit
will take over, allowing I
t
reduced by the foldback feature to 1/7th of the nominal
value.
Choosing External MOSFETs
Power delivery to the ports is regulated with external
power MOSFETs. These MOSFETs are controlled as previ-
ously described to meet the IEEE 802.3af specification.
Under normal operation, once the port is powered and the
PD’s bypass capacitor is charged to the port voltage, the
external MOSFET dissipates very little power. This sug-
gests that a small MOSFET is adequate for the job. Unfor-
tunately, other requirements of the IEEE 802.3af mandate
a MOSFET capable of dissipating significant power. When
the port is being powered up, the port voltage must reach
30V or more before the PD turns on. The port voltage can
18
ICUT
timer to count up. During a short circuit, I
ICUT
timer is active.
U
LIM
U
current to flow and causing the
LIM
W
current limit level
U
LIM
will be
SENSE
then drop to 0V as the PD’s bypass capacitor is charged.
According to the IEEE, the PD can directly connect a 180 F
capacitor to the port and the PSE must charge that
capacitor with a current limit of 400mA to 450mA for at
least 50ms.
An even more extreme example is a noncompliant PD that
provides the proper signature during detection but then
behaves like a low valued resistor, say 50 , in parallel with
a 1 F capacitor. When the PSE has charged this
noncompliant PD up to 20V, the 50 resistor will draw
400mA (the minimum IEEE prescribed I
keeping the port voltage at 20V for the remainder of t
The external MOSFET sees 24V to 37V V
450mA, dissipating 9.6W to 16.7W for 60ms (typ).
The LTC4258 implements foldback to reduce the current
limit when the MOSFET V
section. Without foldback, the MOSFET could see as much
as 25.7W for 60ms (typ) when powering a shorted or a
noncompliant PD with only a few ohms of resistance. With
foldback, the MOSFET sees a maximum of 18W for the
duration of t
The LTC4258’s duty cycle protection enforces 15 times
longer off time than on time, preventing successive at-
tempts to power a defective PD from damaging the
MOSFET. System software can enforce even longer wait
times. When the LTC4258 is operated in semiauto or
manual mode—described in more detail under Operating
Modes—it will not power on a port until commanded to do
so by the host controller. By keeping track of t
t
port again after one of these faults even if the LTC4258
reports a Detect Good. In this way the host controller
implements a MOSFET cooling off period which may be
programmed to protect smaller MOSFETs from repeated
thermal cycling. The LTC4258 has built-in duty cycle
protection for t
Timing sections) that is sufficient to protect the MOSFETs
shown in Figure 1.
Before designing a MOSFET into your system, carefully
compare its safe operating area (SOA) with the worst case
conditions (like powering up a defective PD) the device will
ICUT
faults, the host controller can delay turning on the
START
ICUT
.
and t
START
DS
(see t
is high; see the Foldback
ICUT
Timing and t
LIM
DS
current limit)
at 400mA to
START
START
START
4258p
and
.