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[on:]

hi
ive got a requirement to add a clock to a 328, so i need a battery backup for it if the power goes out just wondering what is the most common surface mount 100ma regulator i could use / most popular one. or maby is there a more suitable one that i could use for such a small current?

i did measure a single 328 running think it was about 15ma if i remember  

my first choice would be a 78L05L just wondering if this is a common regulator

thank you

[Reply #1 on:]

Your question is unclear,  are you trying to power the microcontroller,  or the real time clock,  or charge up the battery ?

[Reply #2 on:]

hi
ive got a requirement to add a clock to a 328, so i need a battery backup for it if the power goes out just wondering what is the most common surface mount 100ma regulator i could use / most popular one. or maby is there a more suitable one that i could use for such a small current?

i did measure a single 328 running think it was about 15ma if i remember  

my first choice would be a 78L05L just wondering if this is a common regulator

thank you

What are your requirements for dropout voltage, etc.?

[Reply #3 on:]

mmm maby i have to think about this a bit more, ok drop off voltage not a problem as the battery backup will be a 12v.

im just trying to power the microcontroller, for when the power goes off.

i have 2 328's in the same circuit i would like to isolate 1 and run it using using a rechargeable battery

[Reply #4 on:]

What is powering the circuit the rest of the time? You need to design it so that there is an orderly handover of power from the PSU to the battery. The usual solution is to feed the voltage regulator from the PSU through a diode and from the battery via another diode.

[Reply #5 on:]

No need to have battery power for the 328p just to maintain time/date information. Do what most do and add a simple I2C RTC module that has it's own battery back-up protection.

https://www.adafruit.com/products/264

[Reply #6 on:]

The other thing is, the 78L05L v.regs have a 5mA or so quiescent [bias] current, so not great
for low-power apps. Better choice would be something like an LP2950.

[Reply #7 on:]

I've just been using some TI LM2936Qs, they seem to have the characteristics you're looking for.

[Reply #8 on:]

i did measure a single 328 running think it was about 15ma if i remember 

That's about right. I usually use 20ma as the practical upper limit.

As to regulators, if you are only power the mcu, you may think if a) you need a regulator; and b) you need a beefy one.

A 78L05 would be a good choice here, or the 1117s - they are much cheaper.

[Reply #9 on:]

thanks for the input, i think i will use a   ti  LM2936MP-5.0/NOPB

my only concern is the max regulation 50ma,  is a bit close to the currents ive measured of a running 328 ie 20ma not leaving much margin for error

my 328 will be idle as power to the circuit will be turned off

will this be ok?

[Reply #10 on:]

Have you tried entering your requirements on webench and checking out TI's suggestions?

http://www.ti.com/ww/en/analog/webench/power.shtml

Just tell 'em your voltage/current requirements and optimize for price/footprint/efficiency.  Requires registering at TI I think, but it's well worth it.

[Reply #11 on:]

thats amazing thanks

[Reply #12 on:]

not leaving much margin for error

That can be a huge plus and a life-saver sometimes.

[Reply #13 on:]

I just happened to tabulate 9 TO-92 v.regs today for a project I'm working on. Here is the list
versus current spec [not exhaustive - just the datasheets I had on file, BTW].

50 ma: LM2936
78 mA: Seiko-817
100 ma: 78Lxx, LP2950, LM2931, Seiko-812, Toko-TK711xx
250 mA: MCP1702, L4931

Note - some of these parts have "unusual" pinouts. There are also a lot of smt v.regs that I
wasn't interested in here.

Thermal resistance for TO-92 is 200 degC/W, so you have to be a little careful about over
heating at the higher currents. Eg, with Pd = Vdrop * Iload = 0.5W, and you limit Vdrop to
0.5W/.25A = 2V, it'll still burn your finger.

[Reply #14 on:]

ok
LM2931 wins, looks exactly what i need many thanks for the help.