LTC4259ACGW Linear Technology, LTC4259ACGW Datasheet - Page 19

• Manufacturer Part Number: LTC4259ACGW
• Description: IC CTRLR POE QUAD AC DISC 36SSOP
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW

• Controller Type: Power over Ethernet Controller (POE)
• Interface: I²C
• Voltage - Supply: 3 V ~ 4 V
• Current - Supply: 2.5mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Linear Misc Type: Negative Voltage
• Family Name: LTC4259A
• Operating Supply Voltage (min): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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 0.5Ω sense resistors, this
limits the short-circuit current to 60mA (typ) instead of the
full 425mA (typ) current limit. When the LTC4259A 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 0.5Ω sense resistor) and pulls
the GATE pin down immediately if such an event occurs,
shutting off the MOSFET in less than 1µs (with no external
capacitor on GATE). Approximately 100µs 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.
ICUT
timer to count up. During a short circuit, I
CURRENT
VOLTAGE
VOLTAGE
20V/DIV
10V/DIV
20A/DIV
PORT
PORT
Figure 15. Rapid Response to 1Ω Short
GATE
ICUT
+15V
GND
0mA
V
V
V
EE
EE
EE
timer is active.
U
LIM
U
current to flow and causing the
SHORT APPLIED
250ns/DIV
FAST PULL-DOWN
ACTIVATED
FET OFF
LIM
W
current limit level
V
V
DD
EE
= –48V
= 3.3V
4258 G04
U
LIM
will be
SENSE
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
then drop to 0V as the PD’s bypass capacitor is charged.
According to the IEEE, the PD can directly connect a 180µF
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 50Ω, in parallel with
a 1µF capacitor. When the PSE has charged this
noncompliant PD up to 20V, the 50Ω 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 LTC4259A 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
Figure 16. Rapid Response to Momentary 100Ω Short
500mA/DIV
CURRENT
VOLTAGE
VOLTAGE
10V/DIV
20V/DIV
PORT
PORT
GATE
+15V
GND
0mA
V
V
V
EE
EE
EE
SHORT APPLIED
FAST PULL-DOWN
DS
CURRENT LIMIT
is high; see the Foldback
100µs/DIV
LTC4259A
SHORT REMOVED
LIM
DS
V
V
DD
EE
current limit)
= –48V
= 3.3V
at 400mA to
4258 G05
19
START
4259afb
.