LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 17

• 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
Dual-Level Current Limit
A PD is permitted to draw up to 15.4W continuously and
up to 400mA for 50ms. The LTC4258 has two correspond-
ing current limit thresholds, I
(425mA typ). These are given by the equations:
R
802.3af compliance. While the LTC4258 allows the port
current to exceed I
timing below), it does not allow the current to exceed I
The current limit circuit monitors the port current by
monitoring the voltage across the sense resistor and re-
duces the MOSFET gate voltage as needed to keep the
current at or below I
I
the MOSFET resistance to a minimum.
t
Whenever more than I
the port’s sense voltage is above V
counts up. If the sense voltage is still above V
t
the port immediately and set the appropriate t
in register 06h/07h. The t
grammed via register 16h, bits 3 and 2 (Table 1).
The t
protect the external MOSFET from thermal stress caused
by repeatedly operating in current limit. The counter
counts up whenever the current is above I
down at 1/16th the rate when it is not. The counter will
bottom out at zero to prevent underflow. Full count indi-
cates that the t
turned off.
This count up/count down behavior implements duty cycle
protection, preventing intermittent current limit faults from
causing cumulative thermal stress in the MOSFET. If the port
enters current limit but then exits before the timer expires,
the count will decrease slowly, giving the I
ability to turn off sooner in the case of a repetitive fault. If
the overcurrent duty cycle is less than 6.3% the t
will be fully reset.
ICUT
ICUT
LIM
S
I
CUT
is the sense resistor and should be 0.5
, the gate voltage is restored to the full value to keep
ICUT
timer expires, the LTC4258 will turn off the power to
Timing
= V
timer is an up/down counter that is designed to
CUT
/R
ICUT
S
, I
CUT
U
timer has expired and the port will be
LIM
LIM
CUT
for a limited time period (see t
= V
. When the current drops below
ICUT
U
= V
LIM
CUT
/R
timer duration can be pro-
CUT
/R
S
CUT
(375mA typ) and I
S
W
flows through a port,
and the t
CUT
CUT
CUT
ICUT
and counts
U
ICUT
ICUT
timer the
when the
for IEEE
Fault bit
timer
timer
ICUT
LIM
LIM
.
If the t
timer will continue to run, counting down at the slow
1/16th rate and preventing the port from being repowered
until the count returns to zero. This protects the MOSFET
from damage due to a faulty PD that may still have a valid
signature, or from errant software that repeatedly writes to
the Power On bit.
The port will not repower until after the t
returns to zero. In manual and semiauto modes the power
enable command must be received after the t
reaches zero. In auto mode the LTC4258 must complete a
valid detection cycle after the t
t
To distinguish between normal turn-on current limit be-
havior and current limit faults which occur after power-up
is complete, the LTC4258 starts a timer (the t
whenever a power-up sequence begins.
The t
most, it allows the user to specify a different current limit
timeout (t
limit duty cycle protection remains functional). Second,
the DC disconnect timer is disabled during this period and
can only begin counting up after the t
expired. Together, these two features let the PD draw the
maximum current I
boot up and begin drawing power without triggering a
t
entire t
instead of a t
the cause of a current fault.
As long as the PD draws less than I
and begins drawing the minimum current within t
t
be indicated.
The t
tion described under t
programmed via register 16h, bits 5 and 4 (Table 1).
START
START
START
START
START
ICUT
fault. Finally, if the device is in current limit for the
expires (if DC disconnect is enabled), no faults will
Timing
START
START
timer expires and causes the port to shut off, the
timer also implements the duty cycle protec-
timer serves three functions. First and fore-
ICUT
period, a t
instead of t
fault. This can be useful for tracking down
INRUSH
ICUT
START
to charge its input capacitance,
timing and its duration can be
ICUT
ICUT
) during turn-on (current
fault will be generated
CUT
counter reaches zero.
at the end of t
LTC4258
START
ICUT
ICUT
START
timer has
DIS
counter
counter
17
timer)
START
after
4258p