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LMR61428 3.7v to 12V 2A design issue

li wang101153
Prodigy 90 points
Other Parts Discussed in Thread: LMR61428, LM3481

Hello, I am trying to design a DCDC module which employs LMR61428. As I reading the datasheet:

" However if the VOUT is outside this range, the VDD pin should be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (VIN),"

Can I just tie VDD down to the battery? How about the boot and freq pins? Could you show me a schematic that will work for 12V output? Thank you.

  • 0
    TI__Genius 15810 points

    Hello Li,

    The LMR61428 is a boost regulator with an integrated FET. The FET/switch current limit is 2.85A. In a boost topology, the switch current limit is derived as follows:

    Isw = Iout/(1-DutyCycle)

    The duty cycle is D = (Vout-Vin)/Vout = 70%

    Therefore, Isw = 6.5A for a load current of 2A. This is much above the switch current limit of 2.85A and therefore the LMR61428 is not a right choice for your design. For your design consideration, you would benefit by using the LM3481. You could use the Webench panel on the product folder (http://www.ti.com/product/LM3481) to start with a design. 

    To answer your other question, Vdd could be tied to an external battery.

    I hope this is helpful.

    Regards,
    Akshay