TPS2379: Operation

Tyson Craven

Prodigy 30 points

Part Number: TPS2379

If the class resistor does not match the load, is the PSE supposed to shut down the power to the PD?

over 4 years ago

0 Michael Pahl over 4 years ago

TI__Mastermind 21320 points

Hello Tyson,

Table 1 in the datasheet defines the values for the class resistor. It does not relate to the load, but to the IEEE802.3 standard.

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Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Tyson Craven over 4 years ago in reply to Michael Pahl

Prodigy 30 points

But according to Table 1 the class resistor corresponds to a minimum power at PD and a maximum power at PD. Suppose the class does not correspond to the range of minimum to maximum power. For example suppose the class resistor is 243 Ohms which corresponds to a class 1 but the load is 100 ohms which would draw 25 Watts at 50 Volts which is a class 4. Does the PSE shut off the power. Similarly suppose the class resistor is 63.4 Ohms which is for a class 4 but the load is 833 Ohms which would draw 3 Watts at 50 Volts which would be a class 1. What does the PSE do?

0 Michael Pahl over 4 years ago in reply to Tyson Craven

TI__Mastermind 21320 points

Hello,

It is hard to say for ALL PSE's, but I will explain for IEEE 802.3 compliant PSE's.

If the power consumed by the PD is less than the class the PSE will continue to power the device. If you look at any of our test results for a design, you will see we check efficiency, startup and load regulation at multiple load levels (starting from no load to full load).

If the power consumed by the PD is more than the class allocation, then the PSE will shut off power.

In the new IEEE802.3.bt standard, there is also something called power demotion, which is explained in this video:

https://training.ti.com/understanding-power-demotion-tis-ieee8023bt-powered-devices-pd