TPS79933 LDO configuration question

paul miller44746

I am using a TPS79933 for a project. on the data sheet it appears that this should have a fixed output voltage of 3.3V. I have a couple of questions,

Is this the case that it is actually a fixed output voltage?

Is there a special configureation I need to apply to it to achieve this?

What are the packag markings that identify that it is actually a TPS79933?

I have it set up according to the discription below, and it is just tracking the input voltage, rather than supplying a constant 3.3V output. I.E. if I give a 3V input, it is giving a 3V output, if I give it a 4.5V input, it has a 4.5V output.

pin ->10 uF and .1uF capactior fromto ground (Vin)

pin 2 -> ground (GND pin)

Pin 3 con-> same node as pin 1 (ENABLE PIN)

Pin 4 ->.1uF cap to ground

pin 5 -> 2.2uF cap to ground with a 470k resistor to provide a load.

over 15 years ago

0 Jeff F over 15 years ago

TI__Guru**** 166626 points

Assuming you actually received a TPS79933 fixed output IC, then you have the pins connected correctly. Your test results make it appear that you have received an adjustable device. To quickly test this theory, tie Pin 4 to Pin 5 and see if the output regulates to 1.2x volts for various current levels. Use the website below to determine which IC you have based on the top side marking:

http://focus.ti.com/general/docs/partmarking/partmarkinglookup.tsp?startPosition=0&navigationId=&templateId=&siteFlag=&keyword=AXX&searchType=partmark

0 paul miller44746 over 15 years ago in reply to Jeff F

Intellectual 540 points

Jeff,

Thank you for the quick response. I just verified on the website that I did have the correct device (markings AXX). I had it on a bread board, and that section of the breadboard appears to have been bad, becaus after I moved it, it worked. However, from looking at the datasheet/website, it looks as though if I provide a voltage anywhere form 2.5V to 6.5V, this regulator should output 3.3V. I wanted to investigate this because it seems to me, with a "limited" knowledge of LDO's, that you would need at least 3.3V applied to attain 3.3V at the output. This seems to hold true as when I lower the voltage below 3.3V, the regulator begins tracking the voltage down again. Is this how the device should operate, and if it is, is there any chance of an update on the data sheet to reflect this?

Thanks,

Paul

0 Jeff F over 15 years ago in reply to paul miller44746

TI__Guru**** 166626 points

Yes, a linear regulator can only "buck" or step down a higher input voltage to a lower output voltage. Specifically, Vin >= Vo+Vdo where Vdo is the dropout voltage. The dropout voltage gets lower as Vin increases and/or Io decreases.

0 paul miller44746 over 15 years ago in reply to Jeff F

Intellectual 540 points

Jeff,

Thanks!

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TPS79933 LDO configuration question