Rainbow Electronics MAX5971А - Manual (Page 17)

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MAX5971A
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