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LM61430-Q1: SW waveform

kura
Genius 4840 points
Part Number: LM61430-Q1

Tool/software:

Our customer is considering to use LM61430AAS-Q1.

- Output voltage = 5.0V or 3.3V (They would like to use 3.3V if possible)

- Oscillation frequency = 2041kHz output

(See attached file for circuit diagram)

LM61430AAS-Q1.xlsx

<Symptoms>

- When the product is operated with 3.3V output, the oscillation frequency changes and the waveform breaks up.

- When the product is operated with 5.0V output, the waveform only expands slightly, and the oscillation frequency does not change.

(See attached file for detailed waveform)

<Questions>

About the symptoms

Is it correct to assume that the above symptoms are caused by the AUTO mode of the part?

In that case, is the attached waveform normal operation?

If it is not normal, please tell me the possible causes.

About AUTO mode

If the above symptoms are caused by the AUTO mode of the M61430AAS-Q1,

will changing from LM61430AAS-Q1 to LM61430AFS-Q1 solve the problem?

Also, please let me know if there are any points to be aware of when making changes.

Regards,

Kura

  • 0
    TI__Mastermind 23685 points

    Hi Kura,

    I've assigned an expert to your question.

    Thanks

    Andrew

  • 0 in reply to Andrew Kutzler
    TI__Genius 11245 points

    Hi Kura,

    I have a few questions:

    1. What is the input voltage? 

    2. The excel file has two different waveforms for each 3.3V and 5V. What is the output load during these two times (You mention "Sleep Mode" and "In operation").

    Looks like the device may be getting into PFM and DEM within AUTO Mode but I would like to know the operating conditions. 

    3. Do you need a fixed frequency operation? (If yes then the FPWM variant - LM61430AFS-Q1 - will be the right choice, it will have a poorer light load efficiency when compared to AUTO Mode). 

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Andrew Kutzler
    TI__Genius 11245 points

    Hi Kura,

    I have a few questions:

    1. What is the input voltage? 

    2. The excel file has two different waveforms for each 3.3V and 5V. What is the output load during these two times (You mention "Sleep Mode" and "In operation").

    Looks like the device may be getting into PFM and DEM within AUTO Mode but I would like to know the operating conditions. 

    3. Do you need a fixed frequency operation? (If yes then the FPWM variant - LM61430AFS-Q1 - will be the right choice, it will have a poorer light load efficiency when compared to AUTO Mode). 

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Genius 4840 points

    Taru-san

    Thank you for your reply.

    1.Vin:13.5V

    2.output load    

       Sleep Mode:1.2mA

       In operation:360ｍA

    3.Yes,they need a fixed frequency operation(around 2MHz)

     Please let me know if there are any disadvantages to changing from LM61430AAS-Q1 to LM61430AFS-Q1.

    regards,

    kura

  • 0 in reply to Taru Kapoor
    Genius 4840 points

    Taru-san

    Is there any updated information?

  • 0 in reply to kura
    TI__Genius 11245 points

    Hi Kura,

    Sincere apologies for the delay. 

    Schematic review - Overall the schematic looks okay - I do not see any issues with the design. 

    Coming to the observations - 

    If the customer needs fixed frequency operation - they have to shift to LM61430AFS-Q1 - the only major drawback is the efficiency at light loads - it will be much worse compared to auto mode. 

    If you see the comparison in the above screenshots (taken from the datasheet) - AUTO mode maintains efficiency above 80% across different operating voltages and output currents - while FPWM efficiency is much worse when the output load is in the mA range. 

    Regarding the waveform - frequency foldback is expected which can be seen in the capture that you have shared. However, for the sleep mode - I would expect a severely reduced frequency. Is the customer checking this after the soft start is over and the output voltage has stabilized at 5V and 3.3V? 

    Thanks,

    Best Regards,

    Taru