Linear regulator LM1084 5.0V capacitors choice

Question

I'm trying to make power supply for RaspberryPi and another one for usb hub and another for 9 Ping sensors(20mA)+ few servos + few microcontrollers like Atmega328P. I have only LM1084 or LM7805. So my choice is not big and I choose LM1084 as it allows more current. So it makes 3 "supplies". It all going to be powered by one LiPo 2S 2200mAh(~8V).

So far my choice of capacitors as follows: on input side it's 0.47uF electrolytic capacitor and on output 0.1uF ceramic cap and 220uF electrolytic. Datasheet says put 10uF tantalum on both sides. I don't have these. Only ceramic. Should I follow datasheet with ceramic caps? Now I'm using values from 7805, my bad. And without load it's 4.99-5.00 and with changing load of 2 servos between 0.320A and 0.630A in second voltage changing like 4.89-4.93 and like this. What will be better choice of capacitors here? Maybe I should put 10uF ceramic on input not electrolytic?

Or better I just go and buy car cigarette usb adapter and not make any fancy and large things?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

For a detailed explanation on this and other related topics, though probably much more than you may be interested in, you can check the excellent "Input and output capacitor selection" Application Report, from Texas Instruments,

http://www.ti.com/lit/an/slta055/slta055.pdf

You can also check some general advice for troubleshooting circuits with linear regulators of the 78xx family,

Linear regulator (L7805CV) outputting 5.8V

Answering specifically your concerns:

• You MUST NOT use BIG ceramic capacitors on input or output. It will do more harm than good due to the very low ESR (Equivalent Series Resistance) of these capacitors. On the contrary, small ceramic capacitors (<1uF) can be added to increase the transient response at high frequencies (>1MHz). This can be of huge importance in high speed designs. However, you should better use those 100nF (0.1uF) ceramic capacitors for decoupling directly the power pins of the high speed IC devices.

  Keep the 100nF ceramic output capacitor in your prototype. However, it will probably make no difference, as your Raspberry Pi will have some its own high speed decoupling capacitors.

• The LM1084 is available in 3.3V, 5.0V, 12V and adjustable versions. Which one are you using? I am assuming you have the 5.0V fixed version. If so, the recommended capacitors for most of the applications, according to the datasheet (pages 8-9), are:

  1) Bulk output (load) capacitor: recommended 10uF (Tantalum) or 47uF (Aluminum electrolytic). You are not allowed to use BIG ceramic capacitors at the output of the LM1084. However, the datasheet explicitly states that "Output capacitance can be increased indefinitely to improve transient response and stability."

  So, you are perfectly OK with a bulk 220uF electrolytic capacitor at the output. In fact, the added capacitance will improve your circuit under "large" and/or fast changes in the load current (like those generated by a USB hub or a Raspberry Pi board).

  2) Bulk input (decoupling) capacitor: recommended 10uF (Tantalum) or 47uF (Aluminum electrolytic). Apparently, you are not allowed to use BIG ceramic capacitors at the input of the LM1084. And I say apparently because the input capacitor ESR has very litte or none impact in the stability of a regulator.

  My advice here is twofold:

  Keep the 470nF ceramic input capacitor in your prototype. It will help remove input transients and noise. It is low valued (<1uF) and at the input, so it won't create stability issues.

  Add a 47uF or bigger Aluminum electrolytic capacitor. If you have a spare one, another 220uF like the one at the output will serve good. If you don't add this bulk capacitor, you regulator may oscillate under heavy loads and/or get damaged.

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My personal, ethical choice: I try to avoid Tantalum capacitors, whenever possible, due to the enviromental concerns and conflicts it is creating worldwide, and specially in Congo and other African countries,

http://en.wikipedia.org/wiki/Coltan

Answer 2

Why don't you read the data sheet regarding capacitance on the LM1084? It clearly says that a minimum ESR corresponding to a 500kHz pole is required. If the capacitor has very low ESR such as a ceramic type, you can just add an appropriate series resistor. Ignoring the stability requirement can lead to oscillation- LDO regulators are not as forgiving as regular 3-terminal regulators.