MAX5971B Maxim Integrated Products, MAX5971B Datasheet - Page 19

• Manufacturer Part Number: MAX5971B
• Description: PSE Controller
• Manufacturer: Maxim Integrated Products

When the MAX5971B enters a powered state, the
t
timer (t
powered condition, allowing power delivery to the PD.
PGOOD (R10h[4], Table 16) is set to 1 when the device
enters the normal Power condition. PGOOD immediately
resets to 0 whenever the power to the port is turned off.
The power-good change bits, PG_CHG ([R02h[4] and
R03h[4], Table 9) are set both when the port powers up
and when it powers down. PWR_EN (R10h, Table 16) is
set to 1 when the port powers up and resets to 0 when a
port shuts down. Set PWR_OFF (R19h[4], Table 26) to 1
to immediately turn off power to the port.
The MAX5971B has an internal sense resistor, R
(see the Functional Diagram), connected between the
source of the internal MOSFET and V
load current. Under normal operating conditions, the
current through R
threshold I
current-limiting circuit regulates the gate voltage of the
internal MOSFET, limiting the current. During transient
conditions, if I
fast pulldown circuit activates to quickly recover from the
current overshoot.
In the normal powered state, the MAX5971B checks for
overcurrent conditions, as determined by I
of I
continuous overcurrent period. This timer is incremented
both in startup and in normal powered state, but under
different conditions. During startup it increases when
I
the counter increases when I
decreases at a slower pace when I
I
allows for detection of repeated short-duration overcur-
rent events. When the counter reaches the t
Table 4. Automatic ICUT Programming
FAULT
RSENSE
LIM
(R17h[2])
CL_DISC
LIM
or I
1
0
0
0
0
0
. The t
START
and t
CUT
exceeds I
LIM
. A slower decrement for the t
) has timed out, the device enters a normal
DISC
FAULT
PORT CLASSIFICATION
RSENSE
. If I
______________________________________________________________________________________
SENSE
LIM
timers are reset. When the startup
RSENSE
counter sets the maximum-allowed
RESULT
0, 1, 2, 3
, while in the normal powered state
exceeds I
Any
4
5
5
5
(I
Overcurrent Protection
RSENSE
exceeds I
RSENSE
LIM
Single-Port, 40W, IEEE 802.3af/at,
) never exceeds the
RSENSE
by more than 2A, a
Powered State
EE
exceeds I
Unconnected
LIM
SETTING
to monitor the
FAULT
ILIM1
, an internal
CUT
V
V
drops below
—
—
—
FAULT
EE
EE
= ~88%
counter
CUT
SENSE
limit,
. It
Unconnected
SETTING
ILIM2
V
V
—
—
—
EE
EE
PSE Controller with I
the MAX5971B powers down the port and asserts the
IMAX_FLT bits (R06h[0] and R07h[0]). For a continuous
overstress, a fault occurs exactly after a period of t
The timing is software programmable through the timing
register (R16h, Table 23).
After a power-off due to an overcurrent fault, the t
timer is not immediately reset but starts decrementing.
The MAX5971B allows the port to be powered on only
when the t
sets an automatic port power duty-cycle protection to
the internal MOSFET to avoid overheating. Through pro-
grammable registers, the MAX5971B allows the rate of
decrement to be adjusted or for the restart timeout to be
disabled entirely (see Tables 23 and 24).
In the normal powered state, the I
olds are set automatically according to the classification
result (see Table 4 for classification results based on
detection current, and the Electrical Characteristics table
for the corresponding thresholds). The thresholds can
also be set manually by programming the ICUT register
(R2Ah[2:0]). During startup, I
regardless of the detected class.
The ICUT register determines the maximum current limit
allowed for the MAX5971B during the powered state.
The ICUT bits (R2Ah[2:0]) allow manual programming of
the current limit (I
(see Tables 36 and 37). The ICUT register can be written
to directly through the I
ICUT programming bit, CL_DISC (R17h[2]), is set to 1
(see Table 4). In this case, the current limit of the port is
configured regardless of the status of the classification.
By setting the CL_DISC bit to 0 (default), the MAX5971B
automatically sets the ICUT register based upon the
classification result (see Tables 4, 36, and 37 in the
Register Map and Description section).
RESULTING ICUT REGISTER
FAULT
User programmed
BITS (R2Ah[2:0])
ICUT = 000
ICUT = 001
ICUT = 101
ICUT = 110
ICUT = 111
LIM
counter reaches zero. This feature
) and overcurrent (I
2
C interface when the automatic
LIM
The ICUT Register
is always set to 420mA
LIM
CURRENT LIMIT
and I
CUT
) thresholds
CUT
(mA)
420
720
850
900
950
—
thresh-
FAULT
FAULT
2
C
19
.