Using Current SOurce as Load Circuit

Hi all,

i have a problem during bench test. i am using current source as a load to my LDO. if i set my current source to about -50mA and turn on my supply for the circuit my meter will read an almost -0.7V which i thinks is the voltage of my forwarded ESD diode. however, if my SUpply is currently ON and i load a -50mA at my output, my meter reads a 3.3V which is my the ideal output of my regulator. what is the difference between loading a -50mA then turning the supply ON and loading a -50mA when supply is already ON?

thanks

arielski3 said:

Hi all,

i have a problem during bench test. i am using current source as a load to my LDO. if i set my current source to about -50mA and turn on my supply for the circuit my meter will read an almost -0.7V which i thinks is the voltage of my forwarded ESD diode. however, if my SUpply is currently ON and i load a -50mA at my output, my meter reads a 3.3V which is my the ideal output of my regulator. what is the difference between loading a -50mA then turning the supply ON and loading a -50mA when supply is already ON?

thanks

Click to expand...

Using a power supply as a load is a tricky proposition. The effectiveness of the test will be determined by the architecture of the power supply circuit. A true linear power supply (typically very heavy due to a larger transformer) can (generally) be used as current sinks. A switch-mode based power supply is unlikely to operate correctly as a current sink, due to the nature of the device.

That being said, all of the power supply designers that I work with in my company simply use banks of resistors to act as their loads... never an active device, especially during the initial design stages. Some of these engineers have been doing P/S design for 20+ years, so I tend to follow their methods.

Hi kateDaring52,
THanks for the reply.. that would explain it... right now i am using resistors to be my load.. but i came to notice one equipment which i think is somewhat better than a current source as a load. well, i am also trying to compare my result using a resistor bank with an electronic load. do you think an electronic load would be a good load during early stage of a design? or should i stick to resistors as my load? appreciate your help on this matter.

arielski3 said:

Hi kateDaring52,
THanks for the reply.. that would explain it... right now i am using resistors to be my load.. but i came to notice one equipment which i think is somewhat better than a current source as a load. well, i am also trying to compare my result using a resistor bank with an electronic load. do you think an electronic load would be a good load during early stage of a design? or should i stick to resistors as my load? appreciate your help on this matter.

Click to expand...

Stick with resistors. Once you get your power supply design stable, then you can start applying real reactive and transient loads. Using a power supply as a "load" is not really a useful measure, since you have two active devices working back and forth against each other. It only serves to disguise the reaction of the circuit you're trying to characterize.