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TPS2372: Inrush issue

howard banks
Intellectual 580 points
Part Number: TPS2372
Other Parts Discussed in Thread: TPS54560,

I am developing a PCB  that allows us to select an output voltage 12-42VDC that is powered from PoE using the TPS2372 as the PD interface.

The circuit is follows the standard TPS2372 which makes available RTN and PG along with VDD. The TPS2372 is set for CLASS 4 device using 63R4 resistors on CLS1 and CLS2 pins.

My 12-42VDC output is generated from a TPS54560. The EN line of the TPS54560 is connected to the TPS2372 PG pin. There is a slow start delay on the TPS54560 which is approx 2.3mSec.

I am having issues connecting a load to the TPS54560 that tries to draw the maximum amount of current at switch on. At 12V output i would be expecting to be able to draw approx 2Amps but this isnt the case.

I have reverted to using the TPS2372-4EVM-006 EVM and undertaken the following tests-

  1. set electronic load to 0.32A, turn load on, connect to TPS2372-4EVM, connect to PSE, detect, classify and switch supply to load. I cant repeat this with any higher currents.
  2. set electronic load to 0.38A, turn load off, connect to TPS2372-4EVM, connect to PSE, detect, classify, turn load on, provides voltage and current. turn load off. Cant repeat with any higher currents.
  3. set electronic load to 0.38A, turn load off, connect to TPS2372-4EVM, connect to PSE, detect, classify, turn load on, provides voltage and current. increase load to 0.5A (and above) without problem.

With the above, when i turn the load on its manually done and happens a couple of seconds after PG goes high.

I have been pointed to section 7.4.10 in the TPS2372 datasheet and i do see the V(rtn-vss) waveform as seen in figure 21

From this it appears that i can just load the output of the TPS2372 to take the maximum current at switch on but need to stage it.

From the designs and documentation i've looked at most have the PG pin tied to the switcher start up, yet this doesn't work for me using the TPS54560A.

Can i have some guidance as to what is going on here, why i cant draw more than 0.38A and what switcher part i might be best looking at for the variable output?

Thanks.

  • 0
    TI__Mastermind 41790 points
    Hi Howard,

    The TPS2372-4EVM-006 was designed to pair with a DCDC converter to evaluate the converter's performance with the TPS2372's inrush delay feature.

    The TPS2372 has an inrush current between 275mA and 395mA. Adding a load to the EVM will prevent the bulk cap from fully charging and thus the IC will never get out of inrush (internal pass FET will remain in linear region and possibly heat up/thermal cycle) after detection and class.

    This is why when the load is off first, the bulk cap will charge to the PSE voltage, PD will go out of inrush current limit (395mA) and jump to normal current limit (2.5A). So when you enable the load, it should have no issues. This explains #1 and #3.

    For #2, timing is important when turning the load back ON. It should be after 80ms from the time the PSE applies 48V. If you are applying the load after 80ms and remove the load at the later time, what is the failure that you're describing? If the TPS2372 is not applying autoMPS, the PSE should simply remove power to the PD after the load is removed. But there shouldn't be a fault on the PSE. If there is a fault on the PSE, can you try with a resistive load and not an eload?

    For the TPS54x design, I saw the schematic you're trying to implement. I noticed the PG is pulled up to VDD (48V) when the enable pin of the TPS54 has an abs max of 8V. At a high level, it should work if the DCDC is not pulling any load during inrush and the DCDC is shutdown. This is how we implement it on the TPS2373-3EVM-757 (PD controller with a separate PWM buck converter).

    Regards,
    Darwin
  • 0 in reply to Darwin Fernandez
    Intellectual 580 points

    Hi Darwin,

    I placed a resistive load onto the output of the TPS2372EVM and all was well. I let the board detect and classify and then waited a couple of seconds before connecting the resistor load. I could pull the expected current.

    So, as everyday is a school day, why doesnt an Eload work??

    I then turned my attention to my PCB and isolated the tps2372 circuit from the tps54560 circuit. I then connected the tps54560 circuit supply to the output supply of the tps2372eval board. I also put a wire to the enable line of the tps54560 so that i could enable/disable it manually.

    Initially i used the Eload on the output of the tps54560 circuit and went through a cycle of 

    1. take EN of tps54560 low in readiness for power up.
    2. connect load to output of tps54560 and set to required current draw
    3. connect tps2372 to PSE
    4. allow tps2372 to be detected and classified. Wait a further couple of seconds
    5. release EN of tps545460

    At the lower voltage output of the tps54560 i saw the expected current, but it began to tail off as i called for higher voltages (>28V). My tps54560 circuit supplies a range of output voltage between 12V and 42V in 2V steps. I think this drop off is down to the tps54560 circuit and i need to investigate further.

    So it looks like the circuit does work. I made the mistake of using an Eload on the output of the tps2372 which cost me some time. (I tried the Eload in constant current and constant resistance mode)

    I'm going to try and load the output of the tps54560 with a resistive load but i think i will see the same issues.

    When i try to take too much current, the PSE and PD cycle in a detect/classify loop. As my load is connected and im not using PG from the tps2372 to control the tps54560 then i believe it remains in inrush.

    My goal is to use tps2372 PG to control the tps54560 enable pin as there is the inbuilt inrush delay, though i might opt to put a micro in there to allow a longer delay to be programmed and other parts of the circuit monitored.  However, its extra cost that i really hoped i didn't need.

    Howard 

  • 0 in reply to howard banks
    TI__Mastermind 41790 points
    Hi Howard,

    Glad to hear that the PD controller is operating as expected. My experience with an eload has always been to use it for load transients and efficiency. The eload is an active element always trying to pull current even if the DCDC is off so I seldom use it for startup/shutdown.

    Please let me know after solving the TPS54x converter debugging if you need further assistance with the PD controller.

    Thanks!