output capacitor for MCP1702

Hello, According to the datasheet for MCP1702, 3.3V regulator, they list an output capacitor of .1uF as the minimum:

The output capacitor range for ceramic capacitors is 1 µF to 22 µF. Higher output capacitance values may be used for tantalum and electrolytic capacitors. Higher output capacitor values pull the pole of the LDO transfer function inward that results in higher phase shifts which in turn cause a lower crossover frequency. The circuit designer should verify the stability by applying line step and load step testing to their system when using capacitance values greater than 22 µF

Jeenode, which uses the same regulator, uses a 100uF output capacitor. Would it be OK to use a 220uF electrolytic capacitor? It's what I have. Does using a higher capacitor value effect efficiency for battery powered applications?

Did that quote come from the 1702 datasheet? I found this:

5.2 Output

The maximum rated continuous output current for the...

...A minimum output capacitance of 1.0 μF is required for small signal stability in applications that have up to 250 mA output current capability. The capacitor type can be ceramic, tantalum or aluminum electrolytic. The esr range on the output capacitor can range from 0Ω to 2.0Ω

Jeenode, which uses the same regulator, uses a 100uF output capacitor. Would it be OK to use a 220uF electrolytic capacitor?

Probably. Current flows through the regulator into the output-capacitor, charging it up, when you first turn-on the power. A bigger capacitor will "suck up" more current, and this could be a problem with some regulators. Most voltage regulator chips have all kinds of built-in protection and the worst that is likely to happen is that the regulator will fail to start-up... You are not going to permanently damage the thing with a big capacitor on the output.

Does using a higher capacitor value effect efficiency for battery powered applications?

The capacitor stores energy, but it doesn't use any energy so it won't affect efficiency. Once the capacitor is charged, it's just sitting there filtering-out a little noise and preventing oscillation before it starts.


Usually the datasheet is reliable and it's ALWAYS the best place to start, but sometimes you have to do something different... Where I work we are using a regulator that we haven't used before (MIC2937A). The board is designed exactly as recommended with 0.1uF on the input and 22uF on the output. It didn't work... It's a 5V regulator and we were getting 4 point something unregulated out of it (with anything between 6 and 18V applied). The solution was a 10uF capacitor on the input.

We put an oscilloscope on the output, and it was "unstable" but not really oscillating. When we looked at the input it was oscillating. I chose to try a 10uF capacitor just based on "engineering intuition and experience". When it worked, I tried a 1uF capacitor and that worked on the 5 "prototype" boards we have. Since 1uF worked, I'm confident that 10uF is not a borderline value and it will make a good permanent solution.

The funny thing is, it's a rather complicated board with a microprocessor, RAM, ROM, Ethernet, RS-232/422, USB, 5V, 3V and 1.8V regulators, and the ONLY problem we had was a "foolproof" linear regulator chip. (I didn't design the board.)

Thanks. The datasheet for the MCP1702-3.3V saids the output capacitor can be tantalum, ceramic, or electrolytic types. It doesn't seem like ceramic capacitors go up to 100uF. And tantalum capacitors in that range are pretty expensive, even a bit more expensive than some of the fancy OS-CON capacitors.

This is for an outdoor application, where temperature ranges from 100F to 0F. I was thinking of using OS-CON capacitors for the temperature stability and because they last longer. The low ESR of OS-CON caps shouldn't be a problem right? Ceramics have really low ESR also, and the datasheet specifically saids those are OK.

If this is a LDR ( low dropout regulator ) then you can have too much capacitance on the output which could cause it to oscillate.
If Jeenone ( who ever they are ) have such a large capacitor on the output dispite what is said in the data sheet then they are idiots.

arusr: Jeenode, which uses the same regulator, uses a 100uF output capacitor. Would it be OK to use a 220uF electrolytic capacitor? It's what I have. Does using a higher capacitor value effect efficiency for battery powered applications?

The Jeenode is using a .1uF ceramic cap on the output and a .1uF ceramic and 10uF electrolytic on the input. If you're referring to his MCP1702 page I think you're just misreading his chicken scratches; his BOM elsewhere on the site states it specifically as a BC1160CT-ND ceramic cap made by Vishay.

Grumpy_Mike: If Jeenone ( who ever they are ) have such a large capacitor on the output dispite what is said in the data sheet then they are idiots.

JCW, the guy behind JeeLabs, seems to be a pretty clever guy. I think you'd find it worth your time to check out his stuff, Mike.

J C Whippler is a good engineer. I have used his Jeenode stuff before and it works well. Input caps are required as well as output caps and while 220 uF might seem large it really isn't. The cap will work. Input caps are really required, the value should be a consideration of the power supply for the regulator... batteries always require a larger value because as they become depleted their internal resistance increases as there is less energy available and the capacitor stores energy... so for loads that are pulsed or intermittent the cap supplies the extra current. The other place where the input becomes critical is when the supply wiring to the regulator is long.... > than 3 - 4 cm. Further the input and output filter caps should be as physically closed to the regulator as possible to avoid any issues with inductance associated with the wiring to and from the device. All linear regulators are different. A 78Xxx device is a little different than a 78LXX Device, . Is different than an LM 2940.... Generally I use 100 uF on the input and 47 - 220 uF on the output side... Over the past 20 or30 years I've found that effective for 95+% of my designs.

Doc

So I guess you guys haven't used LDRs before. They are not the same as the normal 78xxx regulators.