Current changes in regulator

[alphadog]

I have a regulator that outputs 5V, and its the Vcc of a switching circuit, that switches on and off 0.1A load (the regulator can push 0.25A).
I was told to connect an inductor between the regulator and the load to prevent discontinuous current changes in the regulator.
Why is it bad for the regulator to have discontinuous current changes?
Thanks!

 

[dknguyen]

Is this a linear regulator? Or switching regulator?

For a linear regulator:
1. the regulator can probably handle it just fine. It's the electronics on the output side of the regulator or the input side of the regulator that don't like the noise being generated by the other side. (Read part 2)

2. Regulators of all kinds (linear and switching regulators and things like op-amps too) have trouble filtering out high frequencies (it has to do with their gain-bandwidth product which is commonly seen to specify op-amps). So any high frequency noise or transients (due to source voltage dips/changes or current load changes) generated on either side of the regulator (output or input) can basically travel through the regulator with relatively little supression. THis means that noisy devices on either input or output side of the regulator can contaminate each other (it's a two way street). Additional filtering like inductors is needed if this lack of high frequency noise isolation between the two sides of the regulator becomes a problem.

Like if you have a motor on the input side of the regulator, it might send high frequency noise through the regulator and disturb your electronics. Or if you have two regulators in parallel powering analog and digital circuitry, the digital noise might pass through both regulators (output->input of the digital regulator, and then input->output of the other regulator) and contaminate the analog power supply. This happens with both linear and switching regulators, though switching regulators tend to be worse since they themselves introduce switching transients as a result of their operation.

FOr switching regulators:

1. A switching regulator's output is not constant DC at all and you need a regulator to smooth the current (and capacitor to smooth the voltage) to produce an output with lower noise, otherwise your load will misbehave due to a noisy power supply. Without it, you get a squarish waveform for current which is very noisy.

2. For a step-down switching regulator the inductor is needed to slow down the rate of current rise to a speed where the regulator's control loop can react fast enough to keep it under control. WIthout it, the current will basically short-circuit every time the converter switch closes producing excess current and overheating, as well as causing the output voltage to be equal to the input voltage (not a good thing for a step-down regulator since it will probably fry the load due to overvoltage).

Personally, if this was a linear regulator I would put the inductor on the input rather than the output. Inductors have lots of resistance, and placing it between the regulator and load introduces a resistive voltage drop that the regulator cannot see and therefore cannot compensate for. Putting it on the input eliminates this problem- you just have to make sure that this same current-iduced voltage drop doesn't cause the regulator input voltage to go into dropout. WIth switching regulators, you don't have a choice where the inductors go (though you can add more inductors to input and output to increase filtering).

*Last Edit of this message*

 

[alphadog]

Ma'an, i really thank for this answer (by the way, i think i said that i'll make you homework in some other post ).
I work with linear regulator.
So i understand that input noise causes output noise and output noise can cause input noise, which the former is something that i didnt know, i though it was one way street (thanks ).
I dont understand something, you said that high frequency current/voltage transient can go through output to input.
What does it mean? How does it expressed in input?
If regulator outputs 1uA, and suddenly outputs 0.1A, then how does this transient affect input, and whats wrong with it?

Thank you.

 

[dknguyen]

A transient is a sudden change in current draw or the voltage level. A common voltage transient is produced by a sudden change in current draw like in your example.

In your example, if the regulator's load suddenly changes from 1uA to 0.1A, this might cause a voltage dip due to line inductance producing an AC voltage drop due to the AC current running in it. THis voltage dip is a voltage transient and would be transmitted backwards through the ADC. In addition the increase in output current would also directly produce an increase in input current which would also produce a voltage dip due to line inductance on the input side of the regulator.

This is why we have decoupling capacitors close to ICs, as well as regulators and almost every other electronic component. THey provide a source of charge very close to the load which minimizes the inductance between the two. THe caps provide the intermediate charge to help smooth out the transient whenever the line inductance slows down the power supply's response to the changing load.