Linear regulator needs 1R resistor in series with its output capacitor?

[zenerbjt]

Dear Engineers,
We are using the LP2981A linear regulator (5V input, 3V3 output)

The load is a STM32F042K6T6 micro and a TCAN332 CAN transceiver.

The LP2981A datasheet on page 8 says that a ceramic output cap must have a 1R resistor in series with it….

….On our board, which just got sent off for manufacture, we don’t have this. We believe that since our load is only some 10mA, (nowhere near the 100mA full load), we should be OK with the 1uF Ceramic cap that we have on its output? (we didn’t place a series resistor)

We’ve tried to change to a SOT23-5 linear regulator that is able to drive ceramic output caps (without needing a series resistor) but there’s none with the same footprint.

Do you think we’ll be OK due to our light-ish load?

LP2981A-33DBVR datasheet
https://www.ti.com/lit/ds/symlink/l...=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLP2981A

 

[audioguru]

The series resistance of the output capacitor is very important with low-dropout voltage regulators to avoid oscillation.

 

[gophert]

It is a snubber to prevent oscillation.

Dear Engineers,
We are using the LP2981A linear regulator (5V input, 3V3 output)

The load is a STM32F042K6T6 micro and a TCAN332 CAN transceiver.

The LP2981A datasheet on page 8 says that a ceramic output cap must have a 1R resistor in series with it….

….On our board, which just got sent off for manufacture, we don’t have this. We believe that since our load is only some 10mA, (nowhere near the 100mA full load), we should be OK with the 1uF Ceramic cap that we have on its output? (we didn’t place a series resistor)

We’ve tried to change to a SOT23-5 linear regulator that is able to drive ceramic output caps (without needing a series resistor) but there’s none with the same footprint.

Do you think we’ll be OK due to our light-ish load?

LP2981A-33DBVR datasheet
https://www.ti.com/lit/ds/symlink/lp2981a.pdf?ts=1598302002553&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLP2981A

If you only have a limited number of boards, you can try hand-soldering them by "tombstoning" the cap on one solder pad and then adding the 1 ohm resistor at a diagonal back to the other solder pad.
Or, you can suck it up and pay the change fee if it is a local board fabricator
Or, just suck it up and submit a new board file to jlcpcb.com and you'll have the right design in about 7 to 10 days.

 

[zenerbjt]

Would it solve things if we solder a 1R resistor in series with the output of the LP2981A?

...otherwise...

....there are multiple 0603 output caps........so can we say have a 1r in series with the 2u2 one...then the four 100n caps can do without this resistor?

 

[zenerbjt]

Thanks, there are multiple 0603 output caps. (five).......so can we say have a 1r in series with the 2u2 one...then the four 100n caps can do without this resistor?

 

[Diver300]

The 2.2 μF capacitor will be the most important one to have the resistor on.

It's not stated in the data sheet, but many of the applications of this regulator will be supplying ICs that each have a 10 nF or 100 nF capacitor near them, and there is no mention of those other capacitors. The data sheet says to put the resistor in series with the output capacitor.

The other capacitors would be decoupling capacitors. I guess that they will be further away from the regulator.

You could also reduce their value to 10 nF. They are there to reduce the impact of very fast switching spikes in the ICs, which the 10 nF caps will do fine, and anything slower is covered by the 2.2 μF capacitor,

 

[zenerbjt]

Thankyou
MIC5233-3.3YM5 seems to be ok with any Cout >2u2......i believe we will change to this.

MIC5233-3.3YM5 datasheet:
https://www.mouser.co.uk/datasheet/2/268/MIC5233-Data-Sheet-DS20006033D-1627563.pdf

 

[Diver300]

Thankyou
MIC5233-3.3YM5 seems to be ok with any Cout >2u2......i believe we will change to this.

MIC5233-3.3YM5 datasheet:
https://www.mouser.co.uk/datasheet/2/268/MIC5233-Data-Sheet-DS20006033D-1627563.pdf

That seems fine, but be aware of the second half of section 4.3. Ceramic capacitors, especially when you get small ones of high value, have reduced capacitances when the voltage goes up. Make sure you know what the capacitance is at the operating voltage, over the entire temperature range you need.

Just because it's a 2.2 μF, 10 V capacitor, doesn't mean that it will be that large at all voltages up to 10 V.