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TPS2493 startup

TERRY COX
Prodigy 40 points
Other Parts Discussed in Thread: TPS2493

I'm checking out a new board which uses the TPS2493PW and ran into a couple issues. The input voltage is 54V and the current limit is set to 1.8A (0.028 ohm sense resistor). My output MOSFET is an IRF540NS. I am using a MOSFET (ZVN3310) to pull the UVEN low so I can disable the output when a switch is pressed acting as a reset.  

If I power up without a load applied, the 2493 starts up. I can activate the reset and the output goes down. I can deactivate the reset and the output comes back. Once the power is up I can apply the 1.2A load and everything is well. I can then increase the load pass the 1.8A level and the 2493 shuts the output off. If I then back the load off to the 1.2A level the output comes back on. All this seems to work well.

Three issues so far:

1) If I apply the reset with the 1.2A load connected, the output goes down. This is what I expected. When I deactivate the reset (with the 1.2A load connected) the 2493 goes into fault. Each time the 2493 retries, it goes into fault.

2) If I apply the input supply voltage with the 1.2A load attached, I get the same result as in issue 1, the 2493 goes into fault and faults on each retry.

Would issues 1 and 2 be related to the power limiting? My PROG resistors are currently 100k and 15k.

3) If I bring the circuit up with out a load, the 2493 starts up as expected. If I then short the output, the 2493 goes into fault as expected, but when I remove the load, the 2493 does not retry. 

I'm not sure why it doesn't retry, what should I check here?

Any inputs are appreciated.  

  • 0
    TI__Genius 9415 points

    There are a number of factors in the parts inability to start every time.

    It can be as you said the Vprog is too low. This is only 9W and may be light depending on the size of the output capacitor.

    The power may be sufficient but the start time needs to be increased by increasing the timer capacitor CT.

    The formulas in the datasheet and the calculation tool on the product page don't consider the load. They just power the output capacitor. The load takes power away from starting the cap.  Not that you can'rt start the load with the cap but it may take more power or time than calculated.

    The preferred way is to start the cap alone and turn on the dc loading with Power Good signal. Usually in downstream DC to DC converters.

    Don't use a power load which is not a good device for hot swap controllers to power into at startup. They are usually constant current devices and even when set uop as resistive may not be representative of the actual load.

     

  • 0 in reply to Bob Kando
    Prodigy 40 points

    Bob,

    I raised Vprog and that does it. But I have another question about restart, my trip point is set at 2A. If the load is say 2.5A the controller goes into fault and then if I remove the load, the output comes back up. If I short the load, then the controller goes into fault, but when the load is removed it doesn't come back up, just stays in fault mode. If I pull the UVEN input low, the controller resets and the output comes back on. What is causing the controller to latch with a shorted output, even staying in fault when the load is removed?

  • 0 in reply to TERRY COX
    TI__Genius 9415 points

    Can you send a schematic? Maybe to my email if you can't post or what are the values of the resistor network on the UV and OV?

    Has either the TPS2493 or FET been damaged in testing?

    Do you have long leads back to the power supply or your LOAD/shorting point?

    Can you scope the gate for proper operation on start-up and re-try with short, and retry without short?