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UNDERVOLTAGE LOCKOUT (UVLO)

PROGRAMMABLE INRUSH CURRENT LIMIT AND FIXED OPERATIONAL CURRENT LIMIT

TPS2375

TPS2376

TPS2377

SLVS525A – APRIL 2004 – REVISED SEPTEMBER 2004

APPLICATION INFORMATION (continued)

The TPS2375 incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine when

to apply power to the downstream load and allow the PD to power up. The IEEE 802.3af specification dictates a

maximum PD turnon voltage of 42 V and a minimum turnoff voltage of 30 V (Figure 19). The IEEE 802.3af

standard assumes an 8-V drop in the cabling based on a 20-

Ω feed resistance and a 400-mA maximum inrush

limit. Because the minimum PSE output voltage is 44 V, the PD must continue to operate properly with input

voltages as low as 36 V. The TPS2375 UVLO limits are designed to meet the turnon, turnoff, and hysteresis

requirements.

Various legacy PSE systems in the field do not meet the minimum output voltage of 44 V. The TPS2377 UVLO

limits are designed to support these systems with a lower turnon voltage and smaller hysteresis. Although the

TPS2377 works with compliant PSEs, it could potentially exhibit startup instabilities if the PSE output voltage

rises slowly. The TPS2375 is recommended for applications with compliant PSEs.

In order to provide flexibility for noncompliant designs, the TPS2376 allows the designer to program the turnon

thresholds with a resistor divider. The hysteresis of the TPS2376, measured as a percentage of the turnon

voltage, is similar to that of the TPS2375. To use the TPS2376, connect a resistor divider between VDD and

the combination of the two should equal 24.9 k

Ω. The divider ratio should be chosen to obtain 2.5 V at the UVLO

The TPS2375 uses the UVLO function to control the load through an onboard MOSFET switch. Figure 19

graphically shows the relationships of the UVLO thresholds defined in the Electrical Characteristics section to the

TPS2375 operational states.

Inrush limiting is beneficial for a number of reasons. First, it provides a mechanism to keep the inrush current

below the 400 mA, 50 ms, maximum inrush allowed by the standard. Second, by reducing the level of the current

limit below the PSE operational limit, which can be as low as the classification power divided by the PSE voltage,

it allows an arbitrarily large bulk capacitor to be charged. Third, some legacy PSEs may not tolerate large inrush

currents while powering their outputs up.

The TPS2375 operational current limit protects the internal power switch from instantaneous output faults and

current surges. The minimum operational current limit level of 405 mA is above the maximum PSE output current

limit of 400 mA. This current limit allows the PD to draw the maximum available power and also allows the PSE

to detect fault conditions.

The TPS2375 incorporates a state machine that controls the inrush and operational current limit states. When

current limit set to the programmed inrush value. The load capacitor charges and the RTN pin voltage falls from

switches to the internal 450-mA operational level after a 150-

µs delay. This switchover can be seen in the

operation of PG, which goes active (open drain) after inrush terminates as seen in Figure 1. The internal power

MOSFET is disabled if the input voltage drops below the lower UVLO threshold.

When in the operating current-limit state, a fault on the output or a large input transient can cause the internal

MOSFET to limit current. The RTN voltage rises above its normal operating level of less than 0.5 V while in

voltage must drop below the lower UVLO threshold to clear this latch. If the RTN voltage does not exceed 10 V

while in current-limit state, but the condition persists long enough to overheat the TPS2375, the thermal limit

circuit activates, as described in the thermal protection section.

systems. This same inrush current level suffices for many TPS2375 applications.

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