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LM3478: Start Current Requirement

Emanuel Marinescu
Intellectual 270 points
Part Number: LM3478
Other Parts Discussed in Thread: LM5013

Tool/software:

I made a circuit with the LM3478 in boost configuration, Vin 4-10.5V, Vo required 18V @ 600-900mA. The current is not changing, I mentioned 600-900mA as the right level will be decided later.

I followed the WB design which looks like in the schematics below except for the parts in red which are close to the needed values:

I added the MOSFET and Diode subbers as a habit, the circuit was tested with and without them.

The long introduction in short.... not working.... at the output I get the Vin less the drop on the diode.

As the circuit is powered by a DC-DC based on the LM5013, at some point I started to suspect a possible startup current limitation problem.

Yesterday I played with the Excel Tool and found that the Iset resistor can be increased, lowering the peak current. Is this true?

I will appreciate if you can assist with a solution to lower the startup current to minimum.

  • 0
    TI__Mastermind 30280 points

    Hi Emanuel,

    Thanks for using the e2e forum.
    You mentioned you are already using our quickstart calculation tool for LM3478, which is a good starting point for optimizing the design.
    If you see the device is triggering overcurrent protection during startup and the output voltage cannot be reached, you can indeed reduce the sense resistor R54 to increase the current limit. Please also check if the compensation is suitable for the design parameters and the system runs stable.

    Unfortunately, the LM3478 device has a fixed startup time of 4ms, which cannot be increased, so some inrush currents may not be avoidable.

    Please also note that due to christmas time, further answers may be delayed due to vacation leaves.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 270 points

    Hi Niklas,

    One thing I need to mention is that there are enormous differences between the WB design and the quickstart calculation tool, totally unexplainable.

    For example, the WB specified Rsen 12mohm while the excel calculates 39.5mohm.

    Increasing the resistor value decreases the inductor peak current. 

    And the last thing, the compensation network returns serious changes in the resistor and capacitors. Which is more reliable, the WB or the Excel?

    Let me describe what we need:

    The circuit is supposed to work in 2 modes, from a DC-DC converter supplying 10.5V and from a backup battery with a nominal voltage of 4.8V.

    In the first case the current is 600mA and with battery 450mA. I see some problems at the low voltage with the compensation network.

    While the circuit according to the calculator has a nice 95% efficiency at 10.5V, but at 4V it drops. Most of the time it is supposed to work at 10.5V, but in battery mode we are interested to have the best efficiency and what is happening is exactly the opposite, at 4V the efficiency drops.

  • 0 in reply to Emanuel Marinescu
    TI__Mastermind 30280 points

    Hi Emanuel,

    In general, I would recommend the quickstart calculator over Webench, as it takes more inputs from the users.
    However, the quickstart calculator for LM3478 was just released recently, so we are still collecting feedback from users if all recommended values fit their design, or if further fine tuning on the calculations is necessary.
    If you see that overcurrent protection is triggered with a 40mOhm sense resistor, I would recommend to go with the Webench recommendation of 12mOhm and check if better results are achieved.
    I would appreciate feedback here, as it may require an update of the excel calculator.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 270 points

    It seems that I don't understand or misinterpreting some parameter.

    With 12m Rsen the current is very high compared to what it is with a 50m resistor - see resulta below.

    But what puzzles me is how there is such a big difference between the WB and Excel

  • 0 in reply to Emanuel Marinescu
    TI__Prodigy 630 points

    Hi Emanuel,

    Thanks for using e2e. Due to the Christmas holidays, our engineer is out of office, and will return on the 7th of January. Please expect some delays in our response. Thanks for your understanding.

    Kind regards,

    Johannes

  • 0 in reply to Johannes Braun
    TI__Mastermind 30280 points

    Hello Emanuel,

    Happy new year and thank you very much for your patience.
    The difference between WB and Excel might come from the equations used to calculate the "actual inductor peak current limit"
    There might be something wrong with Excel formula here, so I will check with a designer if all parameters are implemented correctly. (For example, if the internal slope compensation is not considered, the sense resistor recommendation will be too large)

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 270 points

    Hi Niklas,

    Thank you for the reply.

    The slop compensation shows in the Excel so I assume is calculated.

    I tried to replace the 12mohm (WB) sense resistor to 50mohm (Excel).

    Then I noticed that in many circuits I see the feedback resistors in range of >100K. The WB recommended 13.3K/1K for 18V output.

    I replaced them with 133K/10K and it started to work. I didn't made complete testing yet, but the voltage looks stable. I am still with the 50mohm.

    Very weird

  • 0 in reply to Emanuel Marinescu
    TI__Mastermind 30280 points

    Hi Emanuel,

    Thanks for the update.
    So you see a difference just by changing the feedback resistor sum?
    This would mean the issue is not only related to the current sensing circuit.

    It is definitely possible that the stability of system can suffer by the wrong choice of feedback resistor values. Here is an app note that give more explanations on this:
    https://www.ti.com/lit/an/slyt469/slyt469.pdf

    As the system runs stable with the new values, I would continue testing with this state.

    Please let me know if any new findings come up.
    Best regards,
    Niklas