adjustable low drop out voltage resistance value

[andreisss]

hi all!

I am working on a project and I need a power supply of 5v and a 3.3v one on a board to supply my microcontroller. I want to use a adjustable ldo: tlv111c, however i do not understand how to calculate the value of the two resistances R1 and R2 to adjust the ldo to the desired value.

Here is a datasheet for the mentioned ldo: https://www.ti.com/lit/ds/symlink/tlv1117-25.pdf

I would really appreciate a little help! Thanks in advance, Andrei!

 

[alexan_e]

The calculation is the same as the LM317, you can use any of the web calculators or use Vout = 1.25v * (1 + (R2/R1))
LM317 Voltage Calculator - Electric Circuit

Alex

 

[andreisss]

referring to the question above: is it ok if I use a resistor if 110 ohm for R1 and 180 ohm for R2 to get a 3.3 v output and a resistor of 110 ohm for R1 and 330 ohm for R2 to get a 5V output or do i have to use bigger values?

what other stuff do i have to be aware of when I chose the values for these resistances?

thank you in advance, andrei!

 

[alexan_e]

Your resistors will work fine with a slightly higher current consumption, usually a value of 220-270 is used for R1.
The value of R1 shouldn't be high in order to maintain the min output current of the regulator output (~5mA).

Alex

 

[FvM]

Provided, the minimum output current is consumed by other loads in your circuit, the voltage error caused by the adjustment pin current is setting an upper limit for the resistance values. Power dissipation suggests not to choose too small values.

 

[andreisss]

still on the adjustable ldo subject: how do i have to calculate the value of the input/ output capacitor. the datasheet only specifies the kind and suggests a value. how do i know what capacity I should use for my board?

also when is is necessary to use the Cadj capacitor?

thank you in advance, Andrei.

 

[RetroTechie]

how do i have to calculate the value of the input/ output capacitor. the datasheet only specifies the kind and suggests a value.

Don't. Just read the datasheet carefully about what types of capacitors to choose, where to place them, and roughly what values you need. Then stick to that. If you want to be extra safe, double or triple a value. For 99,99% of applications that's fine & there's no point in attempting to do better than the engineers who designed the IC.

People that have problems with voltage regulators, are the folks who think they know better & leave out capacitor(s) that should have been there, or use ridiculously small or ridiculously large capacitors in the wrong place. Or wrong types. Just read carefully, follow datasheet example & you'll do fine.

Btw. large buffer capacitors on the input (not the 100nF-1uF ceramics you often see there) depend on current you're drawing, transformer properties, allowed ripple voltage, the room you have between input voltage & what regulator needs as minimum to maintain output voltage, etc. If in doubt there, post schematic + your exact requirements & people can tell you if you're on the right track.

 

[RCinFLA]

The resistors high end limits are set by the 5 uA's maximum pull down needed on the adjust pin. You want to be sure the resistor to ground pulls significantly more current then the 0.2 uA to 5 uA variable current from the Adj. pin. 500 uA or greater will swamp out most of the I adj pin current.

A 110 ohm resistor across the output to adjust pin will create 11.36 mA. The adjust pin to ground resistor current will be 11.36 mA + (0.2uA to 5 uA from adj. pin). With 11.36 mA from using 110 ohm between output and adj pin the current from adj pin is very very insignificant.

The absolute output voltage result will depending on particular IC 1.25 volt reference between output and adjust pin which can range between 1.200 to 1.290 vdc. Then there is the tolerance of the resistors used.