The Texas LP2950 oscillates as an adjustable regulator

Nowhere in the datasheet does Texas tell you “hey use this fixed regulator as an adjustable”. However, I’m used to being being able to do that with the venerable 78XX series – indeed Texas tell you that you can do that with the 78L05 datasheet in Fig 14.

Adjustable 78l05. bear in mind the shocking Iq of 3mA that'll stand you up an extra 3V if resistor R2 is 1k, keep 'em low...

Adjustable 78l05. Bear in mind the shocking Iq of 3mA that’ll stand you up an extra 3V if resistor R2 is 1k, keep ’em low…

Given that there’s an adjustable variant of the LP2950 that appears on the same datasheet (the LP2951) I laid out a PCB and being the lazy sort I am I assumed that since I was using a load of these parts in their 3.3V KY5033 variant, where I wanted an 8V stabilised voltage for an audio mic amp sourced off a 12V supply I can simply do the LM317 trick, drop in a couple of resistors from the output to ground and the ground pin to real ground, job done.

what I planned...

what I planned…

For this I made R1 6k8 and R2 10k.I expected an output voltage of 3.3+3.3/6800*10,000=8.2V or near enough. I screwed up labelling the o/p 10V, mistakes happen…

What does that look like then?

Oy vey, about 4V of massive oscillation (I’m using 10x probes). At least it’s centred on the right value-ish. Let’s take that output capacitor out

1503_lp2950_openLooking good, only 1V of oscillation, now at 370kHz or thereabouts.

So if you come here from Google wanting to know why the LP2950 doesn’t work as an adjustable reg, now you know. There is a tiny clue in the datasheet in the ground current variation

LP2950 variation in ground current with output current

which varies by two orders of magnitude with a load current variation of 1000. This will be impressed upon R2, varying the target voltage – as more current charges the capacitor the target voltage will rise, then ease off as it is charged, making a handy relaxation oscillator.

There’s another clue that the output cap can give interesting results in this line

1503_2950_cap

which actually specifies a ESR range, rather than less is better

No criticism of Texas’ product implied – these are great little fixed voltage regs with a low quiescent current and are my goto device for running 3.3V devices off a 5V rail because of that superb dropout voltage of 600mV max, across the entire range of load current and -40 to 125°C which is easily in spec off a 4.75V min 78L05. It’s just one less thing to worry about. Im future I won’t be a doofus and try and use one where a LM317L is called for 😉

 

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