______________________________________________________________________________________ 17
Single-Port, 40W, IEEE 802.3af/802.3at PSE
Controller with Integrated MOSFET
Overcurrent Protection
The MAX5971A has an internal sense resistor, R
SENSE
 
(see the Simplified Diagram), connected between the
source of the internal MOSFET and V
EE
 to monitor the
load current. Under normal operating conditions, the
current through R
SENSE
  (I
RSENSE
) never exceeds the
threshold I
LIM
. If I
RSENSE
  exceeds I
LIM
, an internal
current-limiting circuit regulates the gate voltage of the
internal MOSFET, limiting the current. During transient
conditions, if I
RSENSE
 exceeds I
LIM
 by more than 2A, a
fast pulldown circuit activates to quickly recover from the
current overshoot.
In the normal powered state, the MAX5971A checks for
overcurrent conditions, as determined by I
CUT
 = ~88%
of I
LIM
. The t
FAULT
 counter sets the maximum allowed
continuous overcurrent period. This timer is incremented
both in startup and in normal powered state, but under
different conditions. During startup, the counter increas-
es when I
RSENSE
 exceeds I
LIM
, while in the normal pow-
ered state the counter increases when I
RSENSE
 exceeds
I
CUT
. It decreases at a slower pace when I
RSENSE
 
drops below I
LIM
 or I
CUT
. A slower decrement for the
t
FAULT
 counter allows for detection of repeated short-
duration overcurrent events. When the counter reaches
the t
FAULT
 limit, the MAX5971A powers down the port.
For a continuous overstress, a fault occurs exactly after
a period of t
FAULT
.
After a power-off due to an overcurrent fault, the t
FAULT
 
timer is not immediately reset but starts decrementing.
The MAX5971A allows the port to be powered on only
when the t
FAULT
 counter reaches zero. This feature sets
an automatic port power duty-cycle protection to the
internal MOSFET to avoid overheating.
In the normal powered state, the I
LIM
 and I
CUT
 thresh-
olds are set automatically according to the classification
result (see Table 2 for classification results based on
detection current, and the Electrical Characteristics table
for the corresponding thresholds). During startup, I
LIM
is
always set to 420mA regardless of the detected class.
Foldback Current
During startup and normal operation, an internal circuit
senses the port voltage and reduces the current-limit
value and the overcurrent threshold when (V
AGND
  -
V
OUT
) < 27V. The foldback function helps to reduce the
power dissipation on the internal MOSFET. The current
limit eventually reduces down to I
TH_FB
 when (V
AGND
 -
V
OUT
) < 10V (see Figure 3).
Figure 3. Foldback Current Characteristics
V
AGND
- V
OUT
I
LIM
I
TH_FB
10V
27V
I
RSENSE
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