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Regulators..??

How do i determine the input current requirement of a buck regulatror?

consider the regulator to be 100% efficient,

Vout= 5v(fixed).

Iout=400mA(min)

Vin=3v-25v.

so how do i find the required Iin?

please reply with a formula

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Iout*Vout=efficiency*Iin*Vin so Iin=(Iout*Vout)/(Vin*efficiency)

You don't exactly have to worry about that, since batteries only provide as much current as the load requires. What matters is the load. If the load is, let's say, 6 microservos (drawing around 500mA each when stalled), and your buck regulator can only handle 1A, you'll fry the buck regulator. So there is no fixed current input value. There is only a maximum current output (and thus input) value. To find that you can look at the datasheets. If the datasheets are crappy and don't even say that, then try and find a component in the regulator (i.e.:an IC, some sorta chip, whatever) and look at the maximum current ratings. Hope that helps.

actually I am trying to use solar cells to power small components in the device, so will it be possible to obtain the outcome?

 

All DC-DC converters pulse the current so yo can only calculate the average. As the peak current requirements can be quite high it will help if you have a reasonable sized capacitor (at least 470uF) in parallel with the solar cells. The solar cells will charge the capacitor which will then supply the high current needed by the regulator.