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LM3409-Q1: Vcst logic and tolerance under different Vadj

Manu Chang
Intellectual 1990 points
Part Number: LM3409-Q1
Other Parts Discussed in Thread: LM3409

Tool/software:

Hi,

My customers are curious about the detail logics of peak current control and the tolerance of Vcst when adding an external voltage on Iadj pin:

1. Can you explain in details that how the two comparators is triggered and the each output status when the Vsns is hitting Vcst and peak current threshold ?

2. What is the tolerance of the Vcst when adding an external Vadj on Iadj pin (the datasheet only listed the min and max value with 1V Vadj, how to calculate when the min and max value with like 0.6V Vadj? Is it linear like the typical value) ? 

3. What internal parameters might have affects on the tolerance of the Vcst when adding an external Vadj (Customers have concerns that the max value of Vcst might lead to too big current)?

BR,

Manu

  • 0
    TI__Expert 5730 points

    Hi Manu, 

    Why do you need this information? Who is the customer and what is the PPM ID for this project?

    The LM3409 is not the best part to select if accuracy is your top priority. What exactly is your customer trying to achieve? I can provide recommendations even if that means selecting a different part that is better for their use case.

    Best,
    Daniel

  • 0 in reply to Daniel Louis
    TI__Intellectual 1990 points

    Hi Daniel,

    I submitted this question in E2E to expect a quick response on explanations. Since you might need customers info, I will move this to email and we can discus in the email.

    Thanks.

    BR,

    Manu

  • 0 in reply to Manu Chang
    TI__Expert 5730 points

    Manu, 

    As dictated by TI, we try and respond within 24 hours. You can see that I am responding within that window and I will continue to do so

  • 0 in reply to Manu Chang
    TI__Expert 5730 points

    Hi Manu, 

    After confirming with the designer, the difference in tolerance for V_CST at different IADJ voltages is because the V_ADJ = 1V specification was determined using an external supply, whereas the V_ADJ = V_ADJ-OC is using the internal voltage reference of the device which will have a worse tolerance



    This is a constant off-time controller and we set the frequency based on COFF/ROFF and the inductor value and we set the peak current based on V_ADJ and the current sense resistor. Because the converter regulates the average current through the inductor, the inductor will take a longer to ramp up and ramp down for larger current set points.

    Best,
    Daniel