MIC2358 datasheet(13/29 Pages) MICREL

13 / 29 page

Micrel, Inc.

MIC2358YLQ

July 2010

M9999-070810

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 tICUTMAX , 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

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.

MIC2358 Datasheet(PDF) 13 Page - Micrel Semiconductor

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