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LMR50410: max duty cylce and frequency foldback for 700 kHz fixed frequency variant

Ue
Genius 17255 points
Part Number: LMR50410
Other Parts Discussed in Thread: LMR54406, LMR43620

Hi all

My customer has a design with the LMR50410 with these specs:

Vin= 22.8V ... 25.2V

Vout = 5V to 0.98*Vin (PWM controlled on FB Pin via a summation node)

Iout = 0.1 to 0.4 A

And they have this question: 

Does the LMR50410XFDB (fixed 700kHz frequency variant) also have the frequency foldback for high duty cycle of 98% (133kHz) ?

Other comments on the above spec are also appreciated.

Best regards

Ueli

  • 0
    TI__Mastermind 31315 points

    Hi Ueli,

    Unfortunately the LMR50410XFDB does not support frequency foldback. You will want to use the LMR50410XDBVR. 

    However, if the customer is open to it, you may also try the LMR54406, which is set to a higher switching frequency and suitable for applications up to 0.6A. It will allow for a smaller solution size. It also has a PFM mode. 

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    TI__Genius 17255 points

    Hi Richard

    Thank you for the quick reply. 

    As the 98% max duty cycle are a typical value I assume the customer will have to work with a good margin.

    What value would you recommend ?

    The customer prefers fixed frequency as they have also a sensitive analog signal chain on the same PCB.

    Best regards

    Ueli

  • 0 in reply to Ue
    TI__Mastermind 31315 points

    Hi Ueli,

    I did some more digging around and I found a part that might be suitable for your customer. The LMR436x0 should allow for dynamic switching between FPWM and PFM mode. 

    The device comparison table should list all the options.

    One option that fits could be the: LMR43620MSC5RPERQ1

    Regards,

    Richard 

  • 0 in reply to Richard Fu22
    TI__Genius 17255 points

    Hi Richard

    Thank you for the feedback, however this doesn't answer my question.

    Moreover the LMR43620 typical max duty cycle is 96%, meaning lower than the LMR50410XDBVR.

    And the customer doesnt want to switch between FPWM and PFM, they want to operate in FPWM mode only.

    I assume the solution might be to use a PFET DC-DC that can operate at 100% duty cycle but I am not sure we have such a device in our portfolio for the given specs.

    Best regards

    Ueli

  • 0 in reply to Ue
    TI__Mastermind 31315 points

    Hi Ueli,

    Sorry for the misunderstanding. Unfortunately you are right, it looks like we do not have a PFET DC-DC that covers the specs. In this case, you may try with the originally recommended device but see if the max output voltage can be reduced to something like 0.9Vin to make sure that there is some margin. 

    If that is not possible, you may try to use the LMR50410 as is but be careful when operating the part up to 98% DC. 

    Another possible way is to go for an LDO to drop the voltage to 98% of the input voltage, and then buck down as required. You can connect the output of the LDO directly to the load with a load switch to turn on/off as required. 

    Regards,

    Richard