Power supply design help - Page 1
Forum section: electronics / beginners

author: Thane of Cawdor
author_rank: Regular Contributor
timestamp: December 01, 2013, 04:11:13 am
content: HiI've been trying to model a digitally controlled power supply in LTSpice, the circuit is very similar to Dave's in #221. Why am I getting 33mV at the output of the op-amp?Help would be much appreciated

author: ThomasDK
author_rank: Regular Contributor
timestamp: December 01, 2013, 04:53:19 am
content: The output of the opamp won't go all the way to 0V when running on a single supply. You need to supply a negative voltage to get 0V out.

author: ThomasDK
author_rank: Regular Contributor
timestamp: December 01, 2013, 05:06:42 am
content: If you want 0.000V, you have to have a negative supply, regardless of the opamp you pick.LM324 will only go down to 5-20mV on a single supply, so changing for that won't do muchYou don't need much, -0.7V will be plenty

author: Dave
author_rank: Super Contributor
timestamp: December 01, 2013, 05:08:41 am
content: You don't need 3 opamps. Your whole circuit can be done with just 1.

author: trackman44
author_rank: Regular Contributor
timestamp: December 01, 2013, 05:18:52 am
content: Your model is a little flawed. The output of V2 has a period the same length as the pulse width, the period needs to be longer. I modelled the same circuit in LTSpice ( pulse width 0.1us, period 0.2us), i'm getting around 5.5V across R1. The output of U3 ramps up to 2.5V.Will

author: IanB
author_rank: Super Contributor
timestamp: December 01, 2013, 07:31:11 am
content: Well at first glance it does look as if one of the two amplifiers U1 or U2 is redundant. What happens if you eliminate one of them and build the same circuit with just one op amp?

author: uranoos1233211
author_rank: Newbie
timestamp: December 02, 2013, 10:01:23 am
content: I don't find lt3080 ic's.please help me.

author: Dave
author_rank: Super Contributor
timestamp: December 02, 2013, 08:07:43 pm
content: Quote from: Thane of Cawdor on December 01, 2013, 06:15:58 amIt's not that tricky.

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2013, 03:32:17 am
content: Quote from: Thane of Cawdor on December 03, 2013, 02:22:42 amWell, be careful, because you don't know exactly what the internal circuit of the LM317 looks like. What they show on the datasheet is a simplified picture just to give you a feel for the overall structure of the circuit. The real circuit on the chip is going to be a whole lot more complicated than that, with, in addition, the various design parameters carefully tuned and adjusted for good performance.If you construct a similar circuit from scratch it won't be as stable as the LM317 unless you do a lot of careful design calculations and experiments to make it so. Remember that someone didn't one day just say, "hey, lets make an LM117" and design it on the back of an envelope. It is a design that would have taken weeks or months of refining to get right before committing it to silicon.

author: trackman44
author_rank: Regular Contributor
timestamp: December 05, 2013, 07:21:04 pm
content: Here's a link to a SPICE model of the LM317 :http://www.electro-tech-online.com/threads/how-to-use-lm317-ltspice-model.95782/.Will