Review my power supply circuit - Page 1
Forum section: electronics / projects

author: DTJ
author_rank: Super Contributor
timestamp: April 18, 2016, 10:30:03 am
content: Anyone interested in a quick review of my power supply circuit?The application is a hand held battery operated device. I need soft power on / off via a button.I could use a switcher to get the most out of the battery but this thing is only low power so I'm happy to keep it stick with a LD linear regulator.I'm not using a 78L05, I just stuck that in there for the purpose of the review.D2 will also probably be deleted.I'm not sure that I need D3.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 18, 2016, 11:23:37 am
content: Looks good.What's the purpose of D3?

author: Kalvin
author_rank: Super Contributor
timestamp: April 18, 2016, 11:56:27 am
content: You may want to flip Q1.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2016, 12:24:34 pm
content: You're not making very efficient use of your Alkaline battery discharge curve. The fully discharged point is 4.8V. Having D1 in series with the supply immediately wastes quite a bit.An LP2950-50L makes a good LDO substitute for a 78L05 (needs a bit more output capacitance for stability). I'm not sure if it's actually worth going to a switcher or not. With the substituted LDO and D1 removed you'd get down to 5.something quite small, which is about 95% of the battery capacity.

author: Rick60
author_rank: Contributor
timestamp: April 18, 2016, 12:36:28 pm
content: yes swap D<->S on Q1 otherwise the body diode will be in forward conduction ,and the circuit will never switch offBe wary of internal pull up resistors on micro ports ( some micro's have them enabled by default ),so when you power down if the micro has brown out , it will reset ,set the ports back to input , the pull up could now provide just enough voltage to turn Q2 back on ,so your circuit re-powers itself, simple fix put a 1-10K to ground on pwron

author: DTJ
author_rank: Super Contributor
timestamp: April 18, 2016, 12:41:47 pm
content: Quote from: Gyro on April 18, 2016, 12:24:34 pmYes, I've actually used an LP2951 in the test circuit I've lashed up. I was trying to utilise the shutdown pin to control the power on/off but the battery drain when off was a bit too high.D1 was initially placed to protect against reverse battery connection, now that I've added the high side FET I can probably delete it.The micro needs 5V so I guess with an LDO I could run the battery down to 5.5 to 6V without the series diode.Quote from: Kalvin on April 18, 2016, 11:56:27 amYep, thanks D<=>S are arse about.Quote from: Richard Head on April 18, 2016, 11:23:37 amD3? I'm not sure! I think it can be deleted. The circuit has evolved a bit and it now serves no purpose.Changes so far!Flip the BS250 S/D reversed.Delete series diode D1.Delete protection Zener D2.Use an LDO - run battery down to ~6V.Put a 1-10K to ground on pwronI might see if I can find a SOT23 dual diode to replace the two 1n4148 diodes.

author: DTJ
author_rank: Super Contributor
timestamp: April 18, 2016, 01:35:49 pm
content: Quote from: Rick60 on April 18, 2016, 12:36:28 pmGood point on the pull down, good to start and finish in a known state. Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2016, 04:00:10 pm
content: If the µC can go to a very low power mode, one could use a low standby current regulator like MCP1702 and get away without turning the regulator off. The 78L05 needs quite some current and higher voltage than an LDO - so not a good choice for battery operation.It's also a question if you really need 5 V - many µCs also run on less and need less current this way.

author: DTJ
author_rank: Super Contributor
timestamp: April 19, 2016, 05:24:28 am
content: Quote from: Kleinstein on April 18, 2016, 04:00:10 pmI would have used the sleep mode but the other circuitry around the uC draws too much quiescent current and would need a supply that could be turned off.The Vreg has been changed to a LDO type form microchip.I'd normally run at 3 or 3V3 but the peripheral parts in this project all require 5V so I've just gone for 5V across the board.Cheers.

author: DTJ
author_rank: Super Contributor
timestamp: April 19, 2016, 11:27:10 am
content: Ok - here's the revised circuit. Parts are ordered, I'll see how it performs.