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TLV1117LV: Pspice model current limiter not working?

Anthony Cataldo
Prodigy 50 points
Part Number: TLV1117LV
Other Parts Discussed in Thread: TLV1117, TPS7A91

I have a 1.2V fixed output TLV1117LV with 3.3V in, and its output is feeding the core voltage for two FPGAs. The FPGA decoupling capacitance is ~1400 uF total.

The input slew rate is around 5V/ms, so the 3.3V input comes up in around 660 uS.

The input and output capacitors are both 1 uF.

When I put all these parameters in the Pspice model, I get a huge current spike of around 3.5A as soon as the input reaches around 2V. It stays like that until the decoupling caps are charged up to 1.2V.

When I change the input ramp to a 1 ns step the current spike gets worst. The output ramps as fast as the input, which is not realistic.

It's as if there is no current limiting at all during startup for the model. This does not seem to agree with the data sheet, and it does not agree with the results shown in your eval board documentation.

It's pretty obvious that the startup behavior of the Pspice model is off. But my questions is by how much? I'm relying on the device's current limiter to work as advertised during startup, otherwise my 3.3V power supply is going to drop out if the current demand is too high.

The model's current limiter does seem to work after the device is powered up and stable. Not so during the Vin ramp, however.

Can confirm if the current limiter will operate at start up?

Thank you

  • 0
    TI__Mastermind 48665 points

    Hi Anthony,

    It looks to me that the model does not have current limit modeled. It is likely that the model was not developed to be a true-to-life representation of the part behavior but rather a better representation than using an ideal voltage source. 

    TLV1117LV is an old part so I would surprised if it starts up in current limit. With 1.4mF output capacitance there will surely be inrush current on startup before current limit kicks in if you are using a part like TLV1117 that does not have many features such as soft start.

    What is your expected load during normal operation? If the normal load is not large you may be able to get away with putting a resistor at the input to slow the startup transient to limit the inrush spike.

    You may want to consider using a newer part that does have soft start so that you can more finely control the startup behavior. TPS7A91 and TPS748 should be able to satisfy your technical needs. However, they are currently out of stock. You can sign up for notifications when they come back in stock if that is the route you would like to take.

    Regards,

    Nick

  • 0
    Prodigy 50 points

    The eval board documentation suggests there is some delay from when Vin is applied to when the output starts to ramp. See Figure 1 that I attached. The output voltage/current at startup looks well controlled to me. Also the comment in 4.1 Turn-on Sequence says "The output voltage startup ramp is not load dependent." There's no mention of inrush. Suggest you check with the design engineers to see if there is a soft start or inrush limiter at startup.

    See the TLV1117LVxxEVM-714 Evaluation Module User's Guide, May 2011

  • 0 in reply to Anthony Cataldo
    TI__Mastermind 48665 points

    Hi Anthony,

    I've reached out to a designer that was on the project. Note that the datasheet does not make any mention of soft-start, so I suspect that there is no soft-start feature present. 

    Notice that the current shown in Figure 1 is the output current, i.e. load current. You will not see inrush current if you monitor the current through the load because the inrush current doesn't go through the load. The statement that the output voltage ramp is not load dependent is a slight oversimplification. When COUT is not large there will be very little discernable difference in the startup ramp rate. However, when there is a large COUT like you are working with there will be a discernable difference because the TLV1117LV will be operating in current limit until the output cap is charged to the target output voltage. Since there is no output current headroom available in this scenario, any load current takes away from the output cap charging. 

    Regards,

    Nick

  • +1 in reply to Nick Butts
    TI__Mastermind 48665 points

    Hi Anthony,

    I talked to a designer about this. There is no soft start or inrush current limiter. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 50 points

    Thank you, I'll find a way to limit the inrush.