Hacker News Reader

Comment by ejiblabahaba | Hacker News Reader

A few points:

1) LM317 and LM7805 are not LDOs. LM317 is a series regulator, which is way more flexible and has some infrequent but difficult-to-emulate use cases. LM7805 is a linear regulator but has substantial voltage drop relative to a modern LDO. Enthusiast/hobbyist space would do well to consider alternatives to the LM317/LM7805 if they're designing PCBs with surface mount components and very simple power rail needs, but for quick and dirty through hole designs there's few well-known alternatives that are truly LDOs - maybe MCP1700?

2) Enthusiast/hobbyist market isn't too sensitive to bulk pricing. Case in point: LM317 is about a dollar in hobbyist/enthusiast quantities, and LM7805 is comparable. TI lists 1ku TO-220 pricing at $0.65. You're definitely right for real products with mass manufacturing, but hobbyists don't generally haggle over pennies.

View on HN

They are better than they used to be but LDO's are finickier than traditional NPN pass element based regulators. Unless you need a low drop out regulator there isn't much advantage to them.

Tip for power supply stuff, derate the heck out of everything. Voltage current and power derate by 2X and you usually won't have issues.

LDO's are linear regulators. Both the LM317 and LM7805 are considered LDOs.

TI pretty clearly distinguishes between LDOs and other categories of linear regulators on the basis of using a FET pass element (which is true of neither the LM317 or the LM7800 series, both are Darlington-based).

See section 2: https://www.ti.com/lit/ml/slup239a/slup239a.pdf

Damn, I stand corrected. Apologies.

It's not really clear from this paper, it is more handwavy based on today's circuits.

They claim that the darlington voltage drop is ~2V which is suitable for 5V to 2.5V regulation, and then introduce 100mV with an NFET as low-dropout for cases where LiPO cells are 3.6 V, or 300 mV above industry standard 3.3V (or the new embedded expectation of 1.8V or 1.2V off a 2x 700mV cells).

However, what if I need to regulate 3.35 V to 3.3 V, do I need an Even-lower LDO (ELLDO)? That meants we have HDO (2V), LDO (.1V), ELLDO (0.05V), which becomes an absurdly semantic situation.

I think the confusion is that engineers picked the words and it is based on the technology of a current point in time. In their mind, the breakpoint is NFET dropout of 100mV, which is purely subjective. Although I could also argue that my example is silly because it is within the range of tolerance of most datasheets spec for Vin of 3.3V +/- 1%.

I'll agree that it's largely a semantic difference and that I overstated the significance of the MOSFET involvement as the pass element - you can design single-transistor PNP elements with only a few hundred millivolts dropout, and at one point TI referred to these as LDOs.[0] Indeed, even the Sziklai pair gets called "quasi-LDO". Individual engineers likely have different personal thresholds for what constitutes an LDO. But it's worth pointing out that in practice there are only a handful of plausible linear regulator pass elements, and the physics of the BJT-based pass elements sort them into a distinctly higher minimum dropout voltage than what is achievable with MOSFETs.

I still stand by the characterization of LM317 and LM7800 family as "not LDOs". Both devices are Darlingtons with at least two Vbe drops across the series pass element. On the continuum of LDO------Not_LDO, both LM317 and LM7800 are firmly on the Not_LDO side.

[0] https://www.ti.com/lit/an/snva020b/snva020b.pdf

That’s a great apnote (app report?) and I agree with you I just think LDO is a confusing term.

They are not. They are inherently high dropout devices which is why LDOs were created in the first place. LDOs are a refinement on those early generation of linear regulators.

The rise of 3.3V devices created a demand to derive regulated power from a 5V rail. Traditional linear regulators are unsuitable for this because the dropout voltage is in the range of 2V. LM317, the most likely old school variable output candidate, is 3V drop and can't do the job either.

1.6V is definitely not 'low drop'.

Reply on news.ycombinator.com