Why did they tweak the voltage?

[larrymoencurly]

I have several USB 3.0 to SATA adapters based on the ASMedia ASM1051 bridge chip, and on some of the boards the LM1117 ADJ type voltage regulator is tweaked with the addition of a couple of resistors to raise its output from its normal 1.25V to 1.30-1.33V. The resistors were tacked on to the pins; the boards weren't laid out for them:



There doesn't seem to be any relationship between the dates of the boards and chips and whether the voltage was tweaked, but why would a digital circuit like this need its supply voltage tweaked?

[ratdude747]

Re: Why did they tweak the voltage?

Ouch. I've seen bad but not quite that bad of "last minute tweaking."

[budm]

Re: Why did they tweak the voltage?

It sounds like bad circuits design that it cannot handle 5% voltage tolerance.

[mariushm]

Re: Why did they tweak the voltage?

The description says the IC has a 3.3v to 1.2v regulator.

Maybe they accidentally wired 1.25v to that pin instead of 3.3v... which is obviously not enough, but 1.3v could be just enough to cover the ldo's voltage drop (100mV sounds reasonable for 100-200mA consumption)

I can't really think of something else... yeah, it doesn't look nice.

[larrymoencurly]

Re: Why did they tweak the voltage?

The description says the IC has a 3.3v to 1.2v regulator.

Maybe they accidentally wired 1.25v to that pin instead of 3.3v... which is obviously not enough, but 1.3v could be just enough to cover the ldo's voltage drop (100mV sounds reasonable for 100-200mA consumption)

I can't really think of something else... yeah, it doesn't look nice.

Each of these boards also has an "AMS1117 3.3" regulator, next to the regulator marked AMS1117 ADJ. I tried shorting the 1200 ohm resistor of the latter to ground, and the voltage dropped to 1.25V, and the hard drive worked fine with it that way.

At least this construction doesn't look as bad as what I saw on a board where somebody blobbed solder on one copper pad and somehow pulled hot solder up and over to a pad about 3-4mm away -- solder snot jumper wire!