how much ripple in a mcu power supply ? - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 07:40:05 am
content: I'm putting a small SMPS in a system with an MCU, what sort of ripple figure should i calculate for ?

author: nctnico
author_rank: Super Contributor
timestamp: July 18, 2014, 09:49:58 am
content: I usually go for 100mV p-p. But do check the datasheet. Some microcontrollers (Atmel) don't allow to go one milliVolt below the lowest specified voltage. Others have some margin (usually 150mV).

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 09:51:17 am
content: Well I'm working at 5V anyway so I won't be supplying too little, I've calculated for 4mVpp by the sounds of this that is going to be great

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2014, 10:24:45 am
content: Depends what your microcontroller is going to work with. I would say below 50mV should be fine.For example, on PICs, if you want the voltage reference to work at 4.096v they recommend a minimum of 4.75v. Some LCD displays will reset if the input voltage drops below 4.5v and so on.If your microcontroller uses ADC (and input voltage as voltage reference), it may be worth configuring the smps psu for 5.1-5.2v and then using a LDO regulator likeMCP1700orMCP1824to smooth it out further.Since you're only going to power the mcu through them so a current of about 10-20mA probably, these would work perfectly because they have less than 50mV drop up to 50mA and go up about 175mV drop at 150mA. You can power the other stuff directly with the 5.2-5.3v your smps outputs (lcd controller, other chips connected to your mcu through i2c or something like that)Such a LDO saves board space (and potentially money), for example it can save you the space you would use for an inductor and a capacitor that's part of the output PI filter of the smps.

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 10:58:41 am
content: Well I'm only powering the MCU but I am using it's internal 1.1V reff so should not be a problem, I've designed for 25mA and 4mV or better ripple but should need much less current than that so should be pretty smooth and stable.

author: tom66
author_rank: Super Contributor
timestamp: July 18, 2014, 11:24:43 am
content: Why are you using a switcher for 25mA load current? What's your input voltage?

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 04:15:06 pm
content: 30V so plenty of waste heat and at 83C any linear regulator de-rates very fast to a couple of hundred mW

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 05:47:02 pm
content: I'm a bit surprised that I need 820uH for just 25mA but i suppose that is due to my minimum of 20V stepping down to 5V ? I've had a few hundred mA out of 280uH but then that was a free running lashup and I have no idea of how stable it is as it was just to run an LED, I'm going to scope it and maybe do a little video of it.

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2014, 06:09:32 pm
content: Use a switching regulator running at higher frequency.For example, see LMR14203X :http://www.digikey.com/product-detail/en/LMR14203XMK%2FNOPB/296-35194-1-ND/3738905(datasheet here:http://www.ti.com/lit/ds/symlink/lmr14203.pdf, see page 9)TI webbench software recommends 150uH though, I guess i chose a smaller voltage range and less current compared to the datasheet.If you want cheaper, see MCP16311 (though it's max 30v input, absolute max 32 and 38v) :http://www.digikey.com/product-detail/en/MCP16311-E%2FMS/MCP16311-E%2FMS-ND/4079914Or AOZ1282 ... under 1$:http://www.digikey.com/product-detail/en/AOZ1282CI/785-1613-1-ND/4147125

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 06:12:47 pm
content: I'm using around 100KHz

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2014, 06:20:02 pm
content: Yeah, at 100kHz you may as well use a MC34063. See my edits for a couple of good chips that are also cheap..It costs more for the chip, but you're saving on pcb space by being able to use ceramic capacitors and cheaper inductors.On the downside, you'll have to be more careful about the layout because it's much more important. As a suggestion, read the datasheets several times and see Linear Technology datasheets for switching regulators that run at similar frequencies, those usually have pictures or recommended layouts and explain why some traces need to be shorter or thicker and so on.

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 06:29:00 pm
content: Unfortunately I have my good old friend of extreme ambient temperatures to thank for making anything off the shelf virtually as useful as unobtanium, i need to work down to -46 or lower so very few chips available very expensive, i ended up making my own with a couple of comparators in MC34063 style using all parts that will work down to -55C and up to +125C, the voltage reference on it's own costs £4.90 because it will work at -55 but on the plus side it incorporates a rough linear temperature sensor that I can use to turn on the heater resistors around my microcontroller. i stuck to 100KHz because the datasheet for the LM193 states a 1.3uS rise time so it will just manage 100KHz but i tried building a relaxation oscillator with a chip from the same series covered on the same datasheet and I measured rise times of 100nS or less although I was on 15V and the datasheet said something about 5V. It will be interesting to see what frequency my free-running lash-up works at.My traces are as short as can be and 2mm wide, I think that has me covered. the inductors that I had to use two of in series are only 6.6x4.4mm

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2014, 06:45:46 pm
content: At -40c and lower, I would worry about 2mm traces cracking or something like that.How long would your device be disconnected from some power source? Wouldn't it be possible (and cheaper) to include some sort of rechargeable battery that would run some heaters all the time trying to keep the temperature above -20c or something like that (point where regular regulators work)?When the device is powered on, it will then recharge the battery AND run heaters separately, disconnecting the battery from load...Quoting from that MPC16311 datasheet (absolute maximum ratings) :Storage Temperature ....................................-65°C to +150°CAmbient Temperature with Power Applied ....-40°C to +125°COperating Junction Temperature...................-40°C to +150°CBeing so small and as you require so little inductors and ceramics, couldn't you potentially bundle the whole regulator inside a can with a heater element and keep it 24/7 above -30c..-20c (i'm thinking a to-3) ?

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 06:49:01 pm
content: I can't guarantee that the device will have power 24/7, so far no boards have suffered during cold tests. They are also fully potted.

author: mariush
author_rank: Super Contributor
timestamp: July 18, 2014, 06:59:33 pm
content: I don't mean 24/7... If the device will be powered at least every 8-10 hours, you could in theory (depending on how big the device is and so on) put your regulator in the center of the device with a battery and some heater elements and run just a few heaters from that battery for those 8 hours of downtime. Enough to keep the center of your device to a reasonable -35..-20c .In the center, the potting compound should somewhat insulate the core from the cold outside and it shouldn't take so much energy to warm up by a few degrees and keep the regulator working.If the datasheet says it can run from -40c and upwards, why wouldn't it run normally from -35c or something like that (you basically just want to keep it within operating range for the moment when it's plugged in to work) ?

author: Simon
author_rank: Unknown
timestamp: July 18, 2014, 10:19:43 pm
content: I have no idea what sort of power arrangement there will be and it's only a small board so easier to know it will all work at the right temperature