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LM62460: Design tool, Duty cycle and Vin equal to Vout condition

Kai Lai
Expert 6646 points
Part Number: LM62460
Other Parts Discussed in Thread: LM61495, , LM61480, LM3489, LM25148

Hi Team, 

My customer is designing a 12~24Vin, 12Vout power, when 12Vin = 12Vout, they hope that the controller can close 100% duty(H side SW fully turn on) as possible.

Here is some of my questions : 

  1. Does LM62460, LM61480, and LM61495 have the quick design calculation tool now?
  2. For 12Vin = 12Vout/5.5A design, do you think LM62460 can handle it without concern ?
  3. For the duty cycle parameter, how to define frequency foldback condition for DMAX 98% ? Does it need to set auto mode for frequency foldback?
  4. How to define the condition for DMAX(typ) 87%? 
  5. Customer prefer FPWM mode, may I your suggestion about this application? Lout value, Fsw setting…etc
    I think fixed_2.2MHz, Lout=1.5uH should be good.

BR,

Kai

  • 0
    TI__Guru 50120 points

    Hello Kai,

    The LM62460 will be in dropout condition where the output voltage will scale down with as the input voltage approaches the set output voltage. Here is an example of dropout for a 5Vout| 2.2MHz application in the datasheet. Note that the output voltage (y axis) begins to linearly reduce as input voltage (x axis) decreases. The dropout point occurs at higher input voltage for higher load current. Because of this, the LM62460/LM61480/LM61495 cannot be used in a true 100% duty cycle application.

    I would suggest you use a 100% duty cycle controller such as the LM3489 because of the PFET architecture:

    Regards,

    Jimmy

     

  • 0 in reply to Jimmy Hua
    TI__Expert 6646 points

    Hi Jimmy,

    So which mean for Vin = Vout application, we must to choose the device what can support 100% duty right ?

    If I choose LM25148 (it can support max. 99% duty) will still have voltage drop issue like you mentioned, am I right ?

    Thanks!

    Kai

  • 0 in reply to Kai Lai
    TI__Guru 50120 points

    Hi Kai,

    For devices that operate at near 100% mode like the LM25148, it would not support full 100% mode for a lengthy time but does support near 100% mode where the device will be switching with some pulses. 

    See app note SLYT747 for an example of a 100% mode device vs a near 100% mode device. 

    Again if you want a 100% mode device, I would suggest looking at the LM3489. This device is in a PFET architecture and does not require external bootstrap capacitor, which will help to realize 100% duty cycle applications.

    Regards,

    Jimmy