MIC2358YLQ Datasheet, PDF (13/29 Pages) Micrel Semiconductor – IEEE 802.3af Octal Power Sourcing Equipment Controller

Micrel, Inc.
Powered State
Assuming sufficient power budget is available, after a
successful PD detection and optional classification
cycle. The MIC2358YLQ enters POWER_UP state
and begins applying 48V power to the port in an
inrush-controlled manner. If the sense terminal
exceeds VSUOC, an internal current-limiting circuit
regulates the gate drive voltage, limiting the current to
Iinrush = VSUOC / RS. If the current-limit condition
persists, when the startup timer, tILIMMAX, times out,
the port shuts off and fault condition is reported as
short event
After a fault free power-up, the MIC2358YLQ enters
POWER_ON state and maintains 48V power to the
port in a steady-state manner. During steady-state
power delivery, the MIC2358YLQ constantly checks
for overload conditions by monitoring ICUT = VSSOC
/ RS. If the overload condition persists for a time
period greater than t_ICUTMAX , the overload timer times
out, the port shuts off and an overload event is
reported. During steady-state power delivery, if the
sense terminal exceeds VSC, an internal current-
limiting circuit regulates the gate drive voltage, limiting
the current to Ilim = VSC / RS. If the current-limit
condition persists, when the timer, tILIMMAX, times
out, the port shuts off and fault condition is reported
as short event. Under all circumstances neither Ilim
nor Iinrush are allowed to exceed their respective
maximum threshold.
In the TEST_MODE state, the MIC2358YLQ checks
the selected port for short or overload faults with the
same method used during POWER_UP and
POWER_ON states.
Disconnect Detection
Disconnect Detection ensures that the MIC2358YLQ
PSE shuts off power delivery when a PD is
disconnected from a port. The IEEE 802.3af standard
specifies two methods for determining that a PD has
disconnected from the PSE referred to as DC or AC
Maintain Power Signature (MPS).
With the DCMPS pin high during reset or enabled by
host, the MIC2358YLQ DC disconnect detection
function starts at beginning of POWER_UP state. If
the sense terminal falls below VDCMPSMIN for time
out period of tMPSDLY, the port shuts off and fault
condition is reported as DCMPS absent event.
With the ACMPS pin high during reset or enabled by
host, the MIC2358YLQ AC disconnect detection
function starts at beginning of POWER_UP state. The
MIC2358YLQ produces a low frequency square wave
on the ACDRIVE pin. This signal is level-shifted
through a single external 47µF capacitor, then injected
on to the positive terminal of each port through an
July 2010
MIC2358YLQ
external 1.96kΩ AC current sense resistor. The
voltage amplitude of the AC probing signal is
measured by the ACSENSE pin of the MIC2358YLQ.
The AC current into each port is measured by sensing
the voltage drop across the external AC sense
resistor. By combining the sensed voltage and
current, the MIC2358YLQ can then determine if the
low-frequency AC impedance on the port is within the
limits specified by 802.3af. If the impedance falls
outside of the AC maintain power signature limits for
longer than tMPSDLY, the port shuts off and fault
condition is reported as an ACMPS absent event.
If both DC and AC disconnect are enabled on a port,
then per section 33.2.10.1 of the 802.3af standard,
both the DC and AC maintain power signatures must
be present on a port to keep it powered. If either the
DC or the AC maintains power signature does not
exist for longer than tMPSDLY, then the port will be
shut off.
A port with neither, DC or AC disconnect enabled will
not power off automatically when the PD is removed.
Shared Power Management
In Managed Mode, an external host management
applications manages the available power budget,
and maintains which ports are powered up and at
what classification. For systems where no host
management application is present or where it is not
desired to burden, the controller with power budget
management tasks, the MIC2358YLQ performs
Shared Power Management to manage the power
budget in Autonomous mode.
The power budget is established by selecting the
appropriate values of a pair of resistors. A 250kΩ
resistor is placed between the SPMREF pin and
ground. The MIC2358YLQ will apply 2.5V to this
resistor to establish a per-watt-of-power reference
current (nominally 10µA per Watt). Another resistor is
placed from the SPMBUS pin-to-ground, with a value
that establishes the size of the system power budget.
The value of the resistor is determined by dividing
2.5V by the reference current (10µA), then dividing by
the total power budget in Watts.
For example, if the total power budget is 100W, then
the SPMBUS resistor should be 2.5kΩ. Then, when
the MIC2358YLQ powers up a port, it will also source
a current out of the SPMBUS pin that is proportional
to the maximum power for that port’s classification.
As more ports power up, these currents all sum
together, and the voltage at SPMBUS will increase
accordingly. Once the voltage on SPMBUS has
reached 2.5V, the total power budget has been
allocated, and the MIC2358YLQ will not allow
additional ports to power up until an already powered
port is powered off.
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