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LM61440-Q1: WEBENCH Trouble with LM61440

Alex Khramov23
Intellectual 270 points
Part Number: LM61440-Q1
Other Parts Discussed in Thread: LM61440

I'm trying to create BUCK regulator using TI LM61440 with the next input

Input voltage 9 -16V

Case 1: 

output : 5V@2A

At 2MHz switching frequency it works

When I'm trying to reduce the switching frequency from 2,2MHz to 1,2 MHz ( or around) WEBENCH generates error message "Unstable design"

output 5V@1A

switching frequency 2MHz

 WEBENCH generates error message "Unstable design"

Question : Is it LM61440 Model trouble or it reflect the actual behaviour? 

  • 0
    TI__Mastermind 47060 points

    Hi Alex,

    This may be a modeling issue. I have contacted the WEBENCH design support team to look into this issue.

    Notice on the datasheet Table 3 some recommended component values for different output voltage and switching frequency. 

    The input voltage range is typical 13.5V for automotive but should accommodate 5V - 36V input voltage range and 0A-4A output current range.

    What should have happened is the WEBENCH model resizing the inductor value but still providing a stable design since as the switching frequency goes lower, there will be more inductor ripple current and a need for a larger inductor value.

    Regards,

    Jimmy 

  • 0
    TI__Expert 8130 points

    Hello Alex,

    I understand your confusion. I got the same results on my end. This is a model "trouble".
    For example, it would not let me generate the example application circuit Figure 30 in the datasheet.

    Regards,
    Alejandro

  • 0 in reply to Jimmy Hua
    Intellectual 270 points

    Hi Jimmy,

    The my PCB space available is quite limited and expected switching frequency is around 800KHz , meanwhile WEBENCH offer 200KHz or even lower. Lower frequency require bigger coil and this not a case for me. I also want to mention the WEBENCH stability report depend from output current , for example 3,5A is OK but 2,5A already generate instability report.

     

  • 0 in reply to Alex Khramov23
    TI__Mastermind 47060 points

    Hi Alex,

    Yes this sounds like a modeling issue. If you are able to generate a 3.5A output current application, you should still be able to do 2.5A given the same BOM components. 

    Refer to Figure 32 for a rough understanding of operating at higher switching frequency for 5V output. This will allow you to pick smaller sized inductor and output capacitor at the cost of slightly lower maximum ambient temperature based on output current. 

    There is no reason why for the same BOM components the 3.5A simulation can be generated by 2.5A result in instability.

    I've addressed this issue to the WEBENCH support team and they will be reviewing the model to check for this abnormality.

    Regards,

    Jimmy