Confused which components/values to choose for DC>DC step-down converter circuit

Discussion in 'The Projects Forum' started by Leo Powers, Nov 1, 2009.

  1. Leo Powers

    Thread Starter New Member

    Nov 1, 2009
    Hi all,

    I'm trying to choose components for a circuit using a PWM CONTROL 2.5A STEP-DOWN CONVERTER. Datasheet here:

    I intend to use the completed circuit to convert 12v DC from a lead-acid battery into 8.1V DC to power a number of cameras continuously over a long period of time. So one circuit each to be on the safe side, although max. current draw probably won't go over 1A per camera and would probably be nearer to 0.3A most of the time as far as I can tell.

    The data sheet has an example circuit, with the values of components, but the output of that circuit is 5V and I'm looking for closer to 8V. Anywhere from 7.4-8.1V would do it.

    I'm not sure how to calculate values for most of the components despite reading through the "Functional Description" description section in the datasheet many times.

    I did find this calculator, which I think might help me, but I have no idea how to get some of the values, like "Diode Voltage Drop", or "Transistor Voltage Drop":

    Can anyone point me in the right direction? I'm a bit hopeless at working out complex (for me anyway) stuff like this!

    The other problem, is that this step down converter was recommended to me by someone on another forum, but I'm not even sure if he can get a small amount of them for me. I will try and find out, but if anyone can recommend an alternative part/source, please feel free to do so!

  2. SgtWookie


    Jul 17, 2007
    On page 4 of the datasheet, under the "Typical Application Circuit" it states:
    Vout = Vfb x (1+R1/R2)
    Vfb is nominally 0.8v
    In the application circuit, R1 is 685 Ohms, R2 is 130 Ohms.
    Vout = 0.8v x (1+685/130)
    Vout = 0.8 x (1+ 5.269)
    Vout = 0.8 x 6.269
    Vout = 5.0152v.

    So, does that 0.8v give you any hints?
    How about if R1 = 910 Ohms and R2 = 100 Ohms?
    Vout = 0.8 x (1+910/100)
    Vout = 0.8 x (1+9.1)
    Vout = 0.8 x 10.1
    Vout = 8.08v
    Try it with R1=1100 and R2=110 ;)
    The rest of the values you'd need are already in that typical application schematic.
  3. Leo Powers

    Thread Starter New Member

    Nov 1, 2009
    Thank you for explaining that SgtWookie.

    I completely missed that equation when I needed it - typical!

    It seems so simple now I know how :)

    Thanks again