I designed three ADP151 LDOs (two 2,5V and one 1,8V) into my latest circuit.
Yesterday I got the PCBs and was suprised in a way.
All three are drawing too high currents without loads. If I would let them they would draw more than 100mA (I tried it).
After some experiments I removed the output capacitors. This had the effect, that the current was as specified at the datasheet.
Initially there had been a 100nF input capacitor and a 1uF at the output. The former because the supplying circuit allready provides a 10uF cap.
But for the tests there is just a benchtop power supply (Keithley).
I tried different configurations with 1uF, 10uF capacitors at input and output. Applied and removed load (about 3mA, which is the intended load). At last I reduced the input voltage from 5V to 2,9V resp. 2,2V. But in vain. The output was 0,2V too high without output capacitor or the current was too high if it was connected.
Temperature is about 23°C.
When I had 1uF each on input and output I did a measure with the oscilloscope. I measured a sawtooth oscillation with a 3,8ms period, with about 400mV Peak2Peak and about aV Average (but the supply limited to 1,4V then due to the very problem). Even without an output capacitor there is a slight sawtooth wich parameters I acutally not remember.
I, most likely, did something wrong with the selection of the capacitors. But had reason, I assume.
The datasheet requested 1uF at input and output, with ESR below 1Ohm. On page 4 it request a maximum ESR of 0,2Ohm. But both without specifiying the measuring frequency. But ESR is usually specified at a certain frequency.
However, as I have only a small set of capacitors available, I tried only two.
1. 1uF Samsung X5R
where ESR is below 1Ohm above 90kHz
2. 10uF Murata X5R
with ESR below 1Ohm above about 800Hz
3. 1uF Murata X5R
with ESR below 1Ohm abouve 400kHz
I read some application notes (f.e. AN-1072), even some works about LDOs and their quirks but can't figure out what's wrong.
The layout deviates from the one suggested in the datasheet in the way, that the capacitors are not placed along the long side, but along the short side. I saw that when the layouter showed its work but I thought it was OK anyway, as they are located near the IC.
Any hint or suggestion would be appreciated very much.