ldopower supplyswitch-mode-power-supply
I am performing a Load test on an LDO. I will be measuring:
In the oscilloscope measurement settings I can see MAX, MIN, MEAN, RMS, PK-PK etc. Which one is the most appropriate one for measuring the above parameters, and why?
The main criteria for placement of output capacitor when using LDOs is what it needs for stability of the LDO rather than what the load needs. Read the datasheet carefully. (LM1117 Datasheet)
The LM1117 requires a minimum output capacitance of 10uF with an ESR of 0.3 to 22 Ohms. LDOs can have bad transient response or even oscillate if you don't adhere to these recommendations.
The 200mm of trace will have a resistance of ~0.25 ohm if you are using 0.5 oz copper - less if the copper is thicker (PCB trace resistance calculator). This is not much and actually will ensure that you meet the minimum ESR.
You say you are using an electrolytic capacitor - what sort? What is its ESR? Normal aluminum electrolytics are not very good and probably has quite a few ohms ESR, it may not meet the 22 ohm requirement of the LM1117.
In general it doesn't matter where on a PCB the electrolytics are placed but it is important that the 0.1uF cap is close to the load. I think either A or B would work.
If I was doing the design I would use much larger than 10uF, maybe 47uF or 100uF placed close to the LDO. I would also use a tantalum capacitor. These days ceramic is also feasible but you have to then be careful about the ESR being too low as a ceramic cap may only have 0.01 ohm ESR. Some trace resistance can help then
Like @peufeu states, it is not clear what voltage regulator you really were planning to use, but if we should go by the schematics, it is the LM3480, and that is what the layout is suggesting (as far as I can see from the potato vision image attached) I can tell you right away, that if you as you state in the question - were trying to mount a XC6206 on the footprint for U4 from your picture, it will of course not work as expected. Have you even read the datasheets for the parts you use? The physical pinouts are literally on the first pages of the datasheets.
This is essentially what you have connected:
simulate this circuit – Schematic created using CircuitLab
This is the block diagram for the XC6206 (Taken from the Datasheet:
I don't think I will have to tell you why this is heating up, do I?
Best Answer
I would consider a load test on an LDO a DC test, meaning static voltages and currents.
So my test could for example be:
For that I would use current and voltage meters, an oscilloscope isn't really needed. Still, you could use an oscilloscope as well if that is all you have.
The min, max, mean, rms and peak-peak measurements on an oscilloscope are intended for AC signals. For example a 50 Hz sinewave could have:
All of these, except the mean value, are not very useful for a DC voltage or current. On the oscilloscope you could select mean but it is not really needed especially if the voltage isn't that noisy as then the standard value (no min, max etc.) is good enough already.
Things are different if you want to do a dynamic load test so for example applying a sinewave on top of the DC input voltage of the LDO. Then the scope's min, max etc. settings can become more important depending on what you want to measure.