I have a supply of 17V@890mA. I am using COTS DC-DC converter, which has efficiency upto 90% at Minimum input voltage and 83% at Max Input voltage. It has input range between 4.25-24V. It provides output at 3.3V@1A.

So what will be the current consumption/Input current requirement of this converter? On what factor it depends?

I couldn't find this info in the DC-DC converter specs.

  • \$\begingroup\$ Link to specs? What do you mean 17V @ 890 mA - where did that figure come from? This sounds like an assignment. Is it? \$\endgroup\$
    – Russell McMahon
    Dec 18, 2016 at 22:11
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    \$\begingroup\$ assume it's 83% efficient: to get 3.3W out you need to put 3.3/0.83 in - somewhere around 4W, from 17V , 4x4 is 16 so about 1/4A \$\endgroup\$ Dec 18, 2016 at 22:12
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    \$\begingroup\$ You can find the calculations in this answer: electronics.stackexchange.com/a/275566/4245 I'm blowed if I am typing them out again. \$\endgroup\$
    – Majenko
    Dec 18, 2016 at 22:14
  • \$\begingroup\$ @RussellMcMahon No, this is the supply for Led Driver. Not an assignment. It came from calculating Leds and their combinations. \$\endgroup\$
    – MICRO
    Dec 18, 2016 at 22:17
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    \$\begingroup\$ like 250mA but less precise, \$\endgroup\$ Dec 18, 2016 at 22:21

1 Answer 1


The factors that determine input current are output power, input voltage, and converter efficiency. Power = Voltage x Current. Efficiency = Power Out / Power In. If you know the converter's efficiency at the voltage and power it is operating at then calculating the input current is trivial.


...which has efficiency upto 90% at Minimum input voltage and 83% at Max Input voltage

That's a problem, because it only tells you the maximum efficiency the converter can achieve, not what it will be at any particular operating point. Efficiency typically peaks at less than rated output current. Here's an example:-

Pololu 3.3V, 1A Step-Down Voltage Regulator D24V10F3:-

enter image description here

At 5V this converter's efficiency peaks at ~93%, but is down to 83% at 1A out. At 24V it peaks at 83%, and is down to 79% at 1A. At 17V we would expect it to achieve about 81% at 1A. Now we can calculate the input current.

3.3V x 1A = 3.3W out. 3.3W / 81% = 4.08W in. 4.08W / 17V = 0.24A.


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