5 ms·

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

jacquesm 3y ago
1.6V is definitely not 'low drop'.
ejiblabahaba 3y ago
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 https://www.ti.com/lit/ml/slup239a/slup239a.pdf
- muffles 3y ago
  Damn, I stand corrected. Apologies.
- ChancyChance 3y ago
  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%.
  - ejiblabahaba 3y ago
    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 https://www.ti.com/lit/an/snva020b/snva020b.pdf
    - ChancyChance 3y ago
      That’s a great apnote (app report?) and I agree with you I just think LDO is a confusing term.
kevin_thibedeau 3y ago
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.
- deleted 3y ago
  [deleted]

1.6V is definitely not 'low drop'.

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 https://www.ti.com/lit/ml/slup239a/slup239a.pdf

muffles 3y ago
Damn, I stand corrected. Apologies.
ChancyChance 3y ago
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%.
- ejiblabahaba 3y ago
  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 https://www.ti.com/lit/an/snva020b/snva020b.pdf
  - ChancyChance 3y ago
    That’s a great apnote (app report?) and I agree with you I just think LDO is a confusing term.

kevin_thibedeau 3y ago

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.

deleted 3y ago
[deleted]