# LDO input below voltage range

I'm using LDO regulator MCP1700 @ 3.3V - LDO Low Quiescent Current

Dropout voltage is ~180mV. So below 3.3V + 180mV regulator should be in dropout zone with constant Rds at min value.

But if I test it, below 3.3V Vds ~ 0V, and there is no cutoff zone. I cannot explain it from functional diagram in datasheet p2. Can anyone explain it?

• What is your load? Often the dropout voltage is specified as a voltage drop under a specific load.
– W5VO
Commented Oct 15, 2015 at 18:01
• what is this cutoff zone? Vds should be very low, depending on a lot of factors. Commented Oct 15, 2015 at 18:09

@W5VO my load was too small < 25mA. According to Dropout Voltage vs. Load @25° on figure 12.2 in datasheet Rds = 0.7ohm x 25mA = max 17.mV... I increased load to 230mA. Now I can observe constant Ids in regulation zone and decreasing from ~3.4V input voltage (dropout zone). In this zone I can measure a Vdrop = 171mV increasing until 1.3V when input = 1.3V.

I found useful docs

From what i'm reading, to get the output regulated voltage, Vin must be equal to or greater than the regulated output (Vr in the datasheet) + 3% + Vdropout, which at 250mA is given as maximum 350mV, typical 178mV.

So for guaranteed output for all conditions, 250mA for 3.3V output, the input voltage should be at least 3.3 + 0.099 (3%) + 0.350 = 3.75V.

It is more likely to operate correctly from lower than this. This IC is supposed to work for batteries, especially Lithium Polymer ones,

EDIT: - the below is probably incorrect, but I'll leave it here anyway: ... it seems to have a built-in undervoltage lockout/enable feature requiring a minimum voltage to begin regulating on the output. This saves batteries from being discharged too much, and would sit in a shutdown/quiescent mode drawing only 1.6 microamps typically.

The minimum required input voltage feature is what is causing it to output 0V when you drop the input so close to the output. It is not a "dumb" linear regulator, which an older 7805 style IC might show the behavior you are expecting.

• Can you add a reference for the 'seems to have a built-in undervoltage lockout/enable feature' part? I can't find anything about it in the MCP1700 data-sheet. Also, when reading questions like LDO with low-voltage cutoff? it seems that LDO's with cut-off aren't available. Commented Mar 6, 2022 at 22:48
• @maxschlepzig I just had another read through the datasheet, and it's possible I have mis-understood the feature-set related to minimum input voltages of 2.3V. It's possible that it is just rated/designed to work with 2.3V or greater (often shown as > 2.5v in the datasheet as Vr). Perhaps if you give it less than 2.3V it just operates outside of the specifications. Commented Mar 7, 2022 at 10:44