PD77728 datasheet(8/44 Pages) MICROCHIP

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PD77728

Functional Descriptions

Data Sheet

DS00003920E - 8

1.6.2

Short-Circuit Protection

If the port enters the current limit and the port voltage (VPORT) drops below a configurable value, the

port is considered to be in a short-circuit condition. In this case, the port is turned off within 100

µs (typical) to minimize the power dissipation on the MOSFET during such a harsh condition. The

voltage below which the port is considered to be in a short-circuit condition is also configurable.

1.6.3

Inrush (Power-Up) Protection

During the port inrush phase, the PD capacitor is charged with a constant current for up to 75 ms. If

the PD is not limiting the current, the PSE uses its current limit to limit the capacitor inrush current

(for most cases, ILIM is 0.425A). Inrush current is also configurable. A failed PD capacitor, or too large

a capacitor value, might result in either a true short-circuit condition on the port during inrush (leads

to very high-power dissipation, equal to 0.425A × VMAIN), or a very high-power dissipation due to

the slow increase in the capacitor voltage. A dedicated inrush protection mechanism is provided to

protect the device from such events, assuring that the port’s MOSFET does not exceed its SOA under

any condition.

1.6.4

Over-Temperature (OVT) Protection

OVT protection adds an additional layer of protection to the device, and protects the device from

overheating and damaging in parallel to the other protection mechanisms. An example for OVT

protection is a slow increase in ambient temperature (for example, a failed fan), resulting in an

elevated junction temperature which exceeds the maximum operating junction temperature. In this

case, the OVT real-time protection either limits the number of ports that can be turned on at such

temperature, or turns off the port (s) with the highest junction temperature.

1.6.5

TCUT/ICUT Protection

The firmware selects TCUT and ICUT threshold levels based on the assigned class. The port is turned

off if the port current exceeds ICUT for a cumulative time of TCUT (typical 65 ms). Both TCUT and ICUT

values are configurable.

1.6.6

Maintain Power Signature (MPS)

Although this condition does not endanger the PSE device, the MPS signature is required to keep a

PD powered and to disconnect its power if PD is removed. The PD77728 device incorporates MPS

protection, in which the port is turned off if the PD current does not comply with the required hold

current and time, as defined in the IEEE 802.3bt standard. Both the hold current (IHOLD/IHOLD-2P) and

the duration (TMPS, TMPDO) are configurable parameters.

1.7

Over Supply Shutdown (OSS)

OSS is a control pin required to turn off ports of a certain priority due to failure in one of the power

supplies. A dedicated, fast shut-down bus is located between the PoE controller and the digital block

to allow fast shut-down response to the OSS signal. Both 1-bit signal priority and 3-bit signal priority

are supported.

1.8

VMAIN Under-Voltage Lockout (VMAIN_UVLO)

VMAIN under-voltage lockout (VMAIN_UVLO) turns off ports when VMAIN drops below a set threshold.

1.9

Surge

The PD77728 device supports up to 2 kV per IEC-61000-4-5-2014 without external protection

components.

For a higher level of surge, request

AN4813 Surge Protection for Systems Based on PD777xx 8-Port

PSE PoE Manager Application note.

PD77728 Datasheet(PDF) 8 Page - Microchip Technology

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