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Webench/TPS61236P: TPS61236P Startup sim does not work

tomassetti
Expert 6040 points
Part Number: TPS61236P
Other Parts Discussed in Thread: TINA-TI

Tool/software: Webench

Hi Team,

Please check the webench tool startup simulation for the TPS61236P.  Under the following design conditions:

Vin=3.9V, Vout(design) =5V Iout(design)=3A, the simulation shows the device not starting up after 11ms : Steady State sim looks OK.

Sim download into TINA shows the same thing

thanks,

Steve

  • 0
    TI__Intellectual 2405 points

    Hi Steve,

    Thanks for bringing up this issue to our attention. There seems to be some limitation set with load capacity for this part in both WEBENCH simulation as well as in TINA-TI Sim Exported design. I could simulate it till 2.25A correctly in both WEBENCH and in TINA-TI export design. But above 2.25A the device did not enter into startup mode and no switching observed. I will check with the team internally and get back to you on this.

    Thanks!!

    Best Regards,

    Mahavir Jain.

  • 0 in reply to Mahavir Jain
    TI__Intellectual 2405 points

    Hi Steve,

    While responding to your question, I missed the initial current limit behavior for this part explained in the datasheet (Fig. No.8 which shows the load resistance curve with respect to VIN). Essentially, during startup the device can support a maximum of 2A current for lower VIN values which gradually increases with VIN. Once the steady state is reached, we can increase the current drawn from the device slowly i.e. without hitting the switching current limit. This two-step startup is implemented in the model but it hasn't been enabled in current WEBENCH schematic and hence can not be exported as schematic in TINA-TI. We have to add additional circuitry in exported schematic to test it.

    I could implement the same using switch and voltage source arrangement in TINA-TI schematic. To reduce simulation time, I have connected CC node to ground instead of RCC resistor. Please find attached the TINA-TI testbench and corresponding simulation result for the same. I hope this helps clarify the issue regarding this part.

    Thanks!!

    Best Regards,

    Mahavir Jain.

    webench_design_4220924_250.tsc 

  • 0 in reply to Mahavir Jain
    TI__Expert 6040 points
    Hi Mahavir,

    So the device really can't start up through its normal operating load, even if the load is resistive? Is it safe to say that to operate at currents higher than 2A, the load will have to be switched in after power up?

    thanks for the clarifications!
    Steve
  • 0 in reply to tomassetti
    TI__Intellectual 2405 points

    Hi Steve,

    Yes, that's true. The device will not be able to start switching even with resistive load if it is drawing more than pre-charge limit current. The datasheet section 8.3.2 on Soft Start explains this limitation of the device and about the two-step startup.

    "When the device is enabled, the rectifying switch is turned on to charge the output capacitor to the input voltage. This is called the pre-charge phase. During this phase, the output current is limited to the pre-charge current limit ILIM_pre, which is proportional to the output voltage. The pre-charge current increases when the output voltage gets higher."

    At the end it recommends the point I mentioned in my previous post, about applying load gradually once the device has completed start-up. 

    Thanks!!

    Best Regards,

    Mahavir Jain.

  • 0 in reply to Mahavir Jain
    TI__Expert 6040 points

    Thanks Mahavir!

    Steve