TPS2383

SLUS559B − APRIL 2003 − REVISED JULY 2004

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www.ti.com

FUNCTIONAL DESCRIPTION

The TPS2383 architecture has been designed to work efficiently with simple low-cost controllers such as those

in the MSP430 family of devices. Reference design and code examples for complete PSE management are

available from TI for the TPS2383/MSP430 chipset solution.

The PSEPM discovery method, as defined in the IEEE 802.3af Standard, uniquely identifies a powered device

25-k

Ω resistor. Use of low-level probe signals during discovery prevents damage to non-802.3 devices. The use

of a point-to-point slope detection method for the PD 25-k

Ω resistor measurement allows accurate detection,

even if series steering diodes are present at the PD. For legacy loads, capacitive detection can be enabled. In

this mode the TPS2383 A/D is used to measure the loads capacitive value.

After a successful discovery of the PD, the TPS2383 has a classification feature to identify the expected PD

power level based on a current signature from the PD. The classification current level is measured at a reduced

terminal voltage of 17.5 V and classified with 12 bits of resolution. The controller can then use this information

to classify per the levels of the IEEE 802.3af standard or use levels custom to the application. Knowledge of

the expected load power allows the power sourcing equipment to be built with a smaller and less expensive

system power supply. For installations where classification is not needed, and reduced power-up time is

desired, classification can be bypassed by setting the appropriate bits in the per-port write register.

In classification and powered modes, the PSEPM drives an external low side N-channel MOSFET for control

to minimize board power dissipation in enclosures that may be controlling 100’s of ports. Current sensing is

performed with a low value resistor, again minimizing board power dissipation. In discovery mode, due to the

very low current used, an internal N-channel MOSFET is utilized in conjunction with an external, high value,

current sense resistor.

The TPS2383 identifies all fault conditions defined in the PoE IEEE 802.3af Standard. The monitored conditions

include input undervoltage lockout (UVLO), output undervoltage (UV) and overvoltage (OV), average and peak

overcurrent detection, average undercurrent detection, and run current. If a fault condition is detected during

power ramp or at any other time, the PSEPM circuit breaks by disconnecting the 48-V return line and then

updates the fault status of the corresponding port register. When the TPS2383 is disabled the PSEPM ramps

the current down at a controlled rate and the PSEPM changes states to a lower power sleep mode.

To maintain full compliance to Underwriters Laboratory (UL), IEC950 SELV and NEMKO safety standards an

optional, low cost self-resetting PTC fuse can be added to the unswitched +48-V line to protect the system

supply and wiring infrastructure from secondary building wiring faults.

When the PSEPM is enabled and a PD is discovered and optionally classified, power is ramped to the PD at

a controlled current ramp rate to reduce EMI. Upon completion of the current ramp up, the port current remains

limited at less than 400 mA. Upon startup the port can remain in current limit for a timed value of 50 ms which

allows the bulk filter capacitance of the PD to charge. Once a PD is successfully powered and the external

N-channel MOSFET is fully on, the average and peak current to the PD is continuously monitored. A

disconnected load is detected if the average current falls below 10 mA. An overcurrent is detected if the average

current exceeds 375 mA and/or the peak current exceeds 425 mA. If any of these conditions exist, status bits

are set in the per-port read register set and the power is removed from the load.

For maximum rejection of external wiring and power supply noise sources during the measurement of line

current, voltage, resistance or capacitance in discovery, classification and powered modes, a proprietary low

noise A/D converter is used. Converted measurements are processed and compared with digital set-points for

limit compliance. The 12-bit conversion of this data-acquisition system is available through the read register

enabling measurement of the discovery resistance, classification current and powered mode port running

current and voltage. This is a valuable feature in ethernet switch management as it allows monitoring of real-time

parameters across the system network.

The TPS2383 is available in a full function 64 pin LQFP package.