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LM2623 – About the calculating formula of the switching frequency

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Guru 21045 points
Other Parts Discussed in Thread: LM2623

Hi Team,

 

Could you please send the calculating formula of the switching frequency to us if you have any information?

We understand that “Frequency vs VIN” is mentioned in datasheet but would like to know the calculating formula.

Because, our customer would like to calculate the switching frequency at the following conditions.

 

[Conditions]

-Vin=0.9V to 1.5V

-Vout=12V

-Iout=1mA

-Switching frequency = they would like to set more than 1MHz

 

Regards,

Kanemaru

  • 0
    TI__Expert 4565 points

    Hi,

    This is very old design boost device and I don't have any info of the original design team of this. I will check with our current design team of there is any info left.

    Are you using 0.9V for VDD as well? According to spec, VDD should be at least 1.1V to start.

    For SW freq, chance of freq higher than 1MHz using low voltage seems very low looking at Figure 5 in datasheet.

    And there is no max duty spec in datasheet, but usually this is around 0.9 which conversion ration is ~10. Using 0.9V input to generate output 12V seems to high.

  • 0 in reply to Sung Ho Yoon
    Guru 21045 points

    Hi Yoon-san,

     

    Thank you for your prompt reply.

    According to our customer, the startup input voltage is more than 1.1V at the startup.

    So, the specifications of this device do not have any problem.

    Therefore, I would appreciate it if you could contact me once you have checked.

     

    And, I can’t find the other suitable TI device.

    Could you please let us know if you have a recommended other device?

     

    Regards,

    Kanemaru

  • 0 in reply to Hide
    TI__Expert 4565 points

    Hi Kanemaru-san,

    Sorry, I've checked our design team and internal database for this device, but couldn't find any info to answer your questions.

    The original team members are not in TI any more.

    But my comments on this are still same.

    Boost SW freq using that low input voltage will remain very low and high conversion ratio is not recommended.

    I got same opinion from other system engineer for your target application and this device is the only one supporting this low voltage we could find as well.

    Thanks,

  • 0 in reply to Sung Ho Yoon
    Guru 21045 points

    Hi Yoon-san,

     

    Thank you for your reply.

    I understand. Thank you very much.

     

    Regards,

    Kanemaru

  • 0 in reply to Sung Ho Yoon
    Prodigy 10 points
    -Vin=0.9V to 4.5V
    -Vout=6V
    -Iout=1A
    could you please tell me if LM2623 can work in this condition, thanks.
  • 0 in reply to user3517979
    TI__Expert 4565 points

    Hi,

    First, Vin should be higher than 1.1V to start normal operation(boost).

    And max switch current is 2.2A for LM2623A. If Vin = 0.9V and Vout = 6V, then required switch current will be 6*1/0.9 = 6.7A even for 100% efficiency and 0% ripple current.

    It seems input condition is too wide to support for this kind of small boost device.

    And I'm not supporting this device anymore, so I will redirect you if you have further questions.

    Hope this helps,