# Input Current?

Discussion in 'The Projects Forum' started by ihaveaquestion, Jun 22, 2010.

1. ### ihaveaquestion Thread Starter Active Member

May 1, 2009
314
0
Hi guys,

Please keep in mind I'm by no means an expert in circuitry or reading data sheets.

I'm a little confused in how to determine how much current one could expect to be consumed by a DC-DC converter, or any electronic device for that matter. If we know how much voltage we're inputting, and how much voltage the DC-DC converter is outputting, how do we know what the input/output currents are?

For example:

I've attached an example datasheet. On page 6, if we want Vo=3.3 V and input Vi=5V, then I believe we should be operating at ~92% efficiency.

1) The x-axis, or load current. Are both of these statements correct?
a: We can represent the load current as the amount of current necessary to operate a device that we are hooking up to the DC-DC converter.
b: We can choose an appropriate valued resistor and use Ohm's law to choose the load current we want.

2) How do we determine the current the DC-DC converter needs to operate? Is it correct to use the efficiency relationship as follows:

Pout/Pin * 100% = efficiency percentage
therefore, ( (Vout*Iout) / (Vin*Iin) ) * 100% = efficiency percentage

We know the efficiency percentage from the graph, so we know everything in the above equation except Iin, therefore we can solve for it and this is the amount of current that the DC-DC converter requires? Isn't there an easier way? Is this type of info not normally provided in the spec sheet?

Thanks guys...

2. ### GetDeviceInfo AAC Fanatic!

Jun 7, 2009
1,608
237
The current demands for the device are typically as spec'd, but a real time circuit operating at load is really the only way. Remember that the spec sheet is for the device only. Elements that you tack on to complete the circuit, there relational positioning, operating environment, will affect the end results.

3. ### nanovate Distinguished Member

May 7, 2007
665
1
If you want to simulate a constant load then "Yes"

See Figs 5 & 6 in the datasheet for quiescent current. The instantaneous current is not constant but you can use the efficiency to get an estimate of the average input current assuming you are running in continuous mode.