Rainbow Electronics MAX5971B - Manual (Page 19)

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MAX5971B
Single-Port, 40W, IEEE 802.3af/at,
PSE Controller with I 2 C
Powered State
When the MAX5971B enters a powered state, the
t
FAULT and t DISC timers are reset. When the startup
timer (t START ) has timed out, the device enters a normal
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.
Overcurrent Protection
The MAX5971B has an internal sense resistor, R SENSE
(see the Functional 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 MAX5971B 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 it increases when
I
RSENSE exceeds I LIM , while in the normal powered 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 overcur-
rent events. When the counter reaches the t
FAULT limit,
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
FAULT .
The timing is software programmable through the timing
register (R16h, Table 23).
After a power-off due to an overcurrent fault, the t
FAULT
timer is not immediately reset but starts decrementing.
The MAX5971B 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. 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
LIM and I CUT thresh-
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
LIM
is always set to 420mA
regardless of the detected class.
The ICUT Register
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
LIM ) and overcurrent (I CUT ) thresholds
(see Tables 36 and 37). The ICUT register can be written
to directly through the I
2 C interface when the automatic
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).
Table 4. Automatic ICUT Programming
CL_DISC
(R17h[2]) PORT CLASSIFICATION
RESULT
ILIM1
SETTING
ILIM2
SETTING
RESULTING ICUT REGISTER
BITS (R2Ah[2:0])
CURRENT LIMIT
(mA)
1 Any — — User programmed —
0 0, 1, 2, 3 — — ICUT = 000 420
0 4 — — ICUT = 001 720
0 5 V EE Unconnected ICUT = 101 850
0 5 Unconnected V EE ICUT = 110 900
0 5 V
EE
V
EE
ICUT = 111 950