We have this really strange problem with a regulator on a circuit we have.
Usually it outputs a good 3.3V , from an input source of 5V.
When we consume more current from the 5v source (not the REG output) , we get a voltage drop from 3.3V to 2.8V on the regulator output itself.
This happens only when we pass the 1.5A from the source itself.
The 5v source is capable of 4A , and it happens when we consume about 1.6A from the source. The source stay at 5V when the REG drops.
Please check the REG schematics here (c14, and c15 are in place) :
EDIT:
- the regulator output is consuming less then 100mA.
- I found out that the transformer source is indeed 4A output, but it says 0.68A maximum input – is that changes things ? how is that possible ? we replaced a few of them, non worked.
Best Answer
When the LM1117 is providing 100 mA load current, the maximum drop-out voltage can be as high as 1.2 volts. This means that the input voltage to the regulator needs to be at least 4.5 volts (all the time) or you will see a lowering of the output voltage.
If the 5 volts is in fact (say) 4.7 volts and drops just a little bit under load this could explain the problem. Alternatively, the 5 volts may look like a solid voltage but whatever external current it is providing may occur in regular bursts and there may be an instantaneous voltage that drops below 4.5 volts that your meter cannot see.
You may need to use an oscilloscope to see this.
Check the input voltage to the LM1117 rather than the output voltage from whatever generates the 5 volts because there could be additional volt drop you haven't factored-in.