Conflicting input/output capacitors of two components - Page 1
Forum section: electronics / beginners

author: buffoon
author_rank: Newbie
timestamp: September 09, 2016, 10:28:32 pm
content: My LDO (AZ1117) specifies a minimum 10uF output capacitor (with the typical application showing a 22uF); the MCU (MSP430FR2311) specifies 4.7uF to 10uF (max) plus a 0.1uF for its input cap.The LDO output cap and MCU input caps could be practically right next to each other. Do I have to be concerned about the input caps value adding up and causing incompatibility (since the MCU specifically mentions a max value)?How is this be done typically? Put some distance between the two chips and their caps? Use a single cap to satisfies both the output/input needs? In this case a single 10u would satisfy both requirements, but what if I am using other components that specify conflicting capacitor values?Thanks!

author: Ammar
author_rank: Regular Contributor
timestamp: September 10, 2016, 01:10:46 am
content: The 10uF for the MSP430 is a nominal value, not a maximum value. You could put two 10uF capacitors in parallel or use a single 22uF capacitor with 0.1uF in parallel with either choice.I cannot say I have ever seen a data sheet specify a maximum value for a capacitor on the supply rails for decoupling. Generally speaking, the higher the better, keeping in mind things like ESR, capacitor type etc. I hope that answers your question.

author: buffoon
author_rank: Newbie
timestamp: September 10, 2016, 02:52:11 am
content: Thanks! I misread the 10uF under 'NOM' as 'MAX'. I'll use the suggested 10uF for the MSP430 and 22uF for the LDO.

author: mariush
author_rank: Super Contributor
timestamp: September 10, 2016, 03:39:28 am
content: The 1117 regulators require an output capacitor with a minimum ESR that's somewhere above 0.1 ohm - some datasheets recommend capacitors with ESR between 0.1 ohm and 1 ohm.The capacitance values in datasheet are minimums, but if you go overboard like for example using a 330-470uF electrolytic or a 100uF or higher polymer capacitor, their ESR may be lower than 0.1 ohm and cause the regulator to act funny.So keep this in mind, especially if you plan to use ceramic capacitors for that capacitance. Maybe put a 0.2 ohm or higher in series with the ceramic capacitor if you do that just to be safe, or just choose another linear regulator, there's plenty out there that are more stable with all kinds of capacitors, including ceramic capacitors.

author: tatus1969
author_rank: Super Contributor
timestamp: September 10, 2016, 06:40:00 am
content: Quote from: mariush on September 10, 2016, 03:39:28 amWhen ceramics and low ESR are needed, then do not use a 1117 but one of thr many newer LDOs that are designed for stability with them.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: September 10, 2016, 07:35:03 am
content: The 1117 is a pretty inefficient regulator, and you would most likely end up with a quiescent current of the LDO higher than the current required for the MSP430 to work. In my (few) MSP430 projects I have personally used an XC6206. Much better quiescent current and works perfectly fine with ceramic capacitors. The main limitation is the 6V maximum input voltage, but for lithium-ion or USB powered projects it's great.For low current application you could use an HT7133-1 that works fine up to 24V input

author: buffoon
author_rank: Newbie
timestamp: September 12, 2016, 11:19:09 pm
content: Thanks everyone. I had some AZ1117 on hand because they're cheap and I had no idea what to look for when I was making an order.The datasheet on actually mentions compatibility with low ESR ceramics, so I am going to go ahead and use it for the prototype. I certainly look at other options in the future and pay more attention to quiescent current and other specs.