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lm2678 variable power supply for dc motor

Other Parts Discussed in Thread: LM2678, LM5020, LM5022, LM3481

Hi,

I want to design a variable power supply to power a drv8432 and opted to use the lm2678.The maximum input voltage to the converter is 25V and the Imax is 5A. The output should be able to go up to about 20v and go down as low as possible (preferably 0V).

1. According to what I gleaned from the datasheet of the lm2678 I need an inductor of 10uH. Is this correct? I can put a higher value if needed. What are the advantages / disadvantages of that ?

2. The drv 8432 recommends a 1000uF input cap. Is this going to badly effect the stability of the circuit?

regards 

  • Hi Gehan,

    We've received your request and will get back to you shortly.

    Thanks,

    Anston

  • Hi,

    I would really love a reply right about now before I connect anything and probably damage the part!!!!!!!!!!! :D :D :D

    Thanks

  • Hello

     

    1.  A inductor works in a Pulsed Wave Circuit like this,, It stores a determined amount of current during the off cycle and during the rise time of the cycle then you have Voltage stored + applied voltage,, so the higher uh the more storage you have. the disadvantage's of a higher inductor is its resonant frequency (the higher inductor  lower the frequency) unless you have one that is made up of several thinner wires which gives more surface area which allows higher resonant frequency,, or a magnetic core which allows for a higher resonant frequency.

    The amount of energy stored is determined by the cycle frequency matching your inductors resonant frequency

    Please see http://en.wikipedia.org/wiki/Inductor

    http://en.wikipedia.org/wiki/Resonance

    http://www.g3ynh.info/zdocs/magnetics/appendix/self_res/gallery.html

     

    Also see joule thief circuit

    http://en.wikipedia.org/wiki/Joule_thief     

     

    2. The input cap is for two reasons

    1 it stores energy the higher uf the more it can store, to low and your circuit will suffer

    2. it acts as a filter,, against voltage ripple or noise. to low and you will get fluctuations in your output which will make your circuit unstable and it will make your Regulator work harder.

    I hope this helps.

    Jeff / FrostyTheDadMan         

  • Hi Gehan,

    LM2678 probably isn't the best choice because it has internal compensation. Your application requires 100W of output power therefore you should chose a controller IC such as LM3481 or LM5020 or LM5022. We recommend SEPIC topology and a DAC controlled fixed resistor summing into the FB input.

    1000uF should be no issue for a properly designed power supply in terms of stability.

    Thanks,

    Anston