Help interpreting LDO datasheet  - Page 1
Forum section: electronics / general

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:19:35 pm
content: For the Texas Instruments TPS798xx-Q1 LDO, it is nominally rated to source 50mA. But in section 6.5 of the datasheet at the bottom it says it can source between 60mA to 200mA for ΔVOUT = VOUT(NOM) – 0.1 V (I'm using adjustable version). So is the datasheet saying that if you want to source between 60mA and 200mA, you can do it, but your Vout will droop by 100mV?

author: ledtester
author_rank: Super Contributor
timestamp: January 31, 2023, 11:39:51 pm
content: Link to the datasheet:https://www.ti.com/lit/ds/symlink/tps798-q1.pdfThe 50mA figure is what the device is guaranteed to give you continuously.I think the I_limit figure at the bottom of page 5 tells you that current limiting will be engaged somewhere between 60 mA and 200 mA.The test condition "\$\Delta V_{OUT} = V_{OUT(NOM)}-0.1 V\$" suggests that the output voltage during the test was 0.1 V -- i.e. a low enough resistance was put on the output so that current limiting reduced the output voltage to 0.1 V.I agree that footnote (9) is confusing in this regard.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:54:29 pm
content: So it's saying that somewhere between 60mA and 200mA, the LDO will refuse to source more current?

author: srb1954
author_rank: Super Contributor
timestamp: February 01, 2023, 12:35:41 am
content: Quote from: ledtester on January 31, 2023, 11:39:51 pmI would interpret that specification as saying the criteria for being in current limit is when the output voltage drops 0.1V from its nominal value, not when the output drops all the way to 0.1V. In effect, they are defining the onset of current limiting to be at that point where the output goes out to regulation by at least 100mV.When you reread the footnote I think it makes more sense using this definition although they have used a rather clumsy way of stating it. In particular, they have not clearly defined Vo(nom). Is it the design centre voltage of the regulator at its nominal 50mA output current, or is it the actual unloaded voltage taking into account production tolerances etc, or something in between?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 01, 2023, 07:06:47 am
content: Current limiting is a protection feature. 50mA means maximum current you can pull with the regulator working normally and maintaining regulated output voltage.Current limiting mode is not normal operation and by definition will drop the output voltage. You want to keep the highest load current below the minimum possible current limit onset value so that current limit never triggers.Additionally, you need to calculate the maximum power dissipation and the resulting die temperature to see if it flies. Usually, you can only pull the rated maximum current out of linear regulators when the difference Vin - Vout is small.P = (Vin-Vout)*IoutTj = Ta + RthJ-A * P

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 04, 2023, 04:05:52 am
content: Quote from: Jim from Chicago on January 31, 2023, 11:54:29 pmI've written data sheets like this before. What TI is doing is defining current limit of the part as being the point that the output has dropped by 100mV. The reason to do this is because an LDO is supposed to regulate to a tight output voltage--by the time you get 100mV below the regulation point, you're basically losing enough regulation that you're out of spec for where the part should be.Why write it like this? Because as a customer you want to know when you can't count on the output voltage to be where it is supposed to be. If it's 100mV or more away from where it is supposed to be, that's a problem.Yes, the regulator will refuse to source more current at some point between those two current levels. However, the regulator may start dropping off at 60mA but then continue sourcing a little bit more and more as you drag the output further down. Create a graph with Iout on the X-axis and Vout on the Y-axis. Imagine a 3.3V regulator--draw a line at 3.3V that is flat from 0 to 55mA. Then put a point at 3.2V and 60mA and draw a straight line from 3.3V, 55mA through the 3.2V, 60mA point straight down to zero voltage. This is the curve of output voltage versus current for the regulator.