Switchmode efficiency issue - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: August 18, 2021, 06:02:57 am
content: I'm using a AP62301 switchmode regulator to convert 6V down to 3.2V at 5-30mA but am not having much lucky with any efficiency.2.2uH inductor CIGT252008LM2R2MNElow current test - input 10mA @ 6V = output load 0.5mA @ 3VHigh current test - input 30mA at 6V = output load 22mA @ 3V6.6uH inductor (three 2.2uH in series)low current test - input 5mA @ 6V = output load 0.5mA @ 3VHigh current test - input 19mA at 6V = output load 22mA @ 3VMy current suspicion is either- The inductor is not suitable for some reason.The reg runs at 750 kHz and the inductor is rated for 2.2uH at 1mhz with enough current headroom so i'm not sure why it wouldnt be suitable. I did try a 1.8uH that I also had but it was even worse.The inductor is a little physically smaller than the datasheet shows on the PCB layout, but i only need 50mA max, not 3A so i picked a smaller one.- Maybe by bottom layer track for the reg input is forming a turn too close to the inductor and causing some sort of feedback issue.Anyone have other theories? Any help appreciatedThe PCB layout and SCH is pretty much just copied from the datasheet.Regulator datasheethttps://www.diodes.com/assets/Datasheets/AP62300_AP62301_AP62300T.pdfInductor datasheethttps://www.digikey.co.nz/product-detail/en/samsung-electro-mechanics/CIGT252008LM2R2MNE/1276-6925-1-ND/7041325

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: August 18, 2021, 06:37:45 am
content: Yea that look's about right with that regulator. Have a look at Fig 3 in datasheet, you don't see 70% till your load current is >100mA

author: Psi
author_rank: Super Contributor
timestamp: August 18, 2021, 07:28:33 am
content: Quote from: ANTALIFE on August 18, 2021, 06:37:45 amIt says "Up to 83% Efficiency at 5mA Light Load" on page 1So I was expecting something above 80% at my high current test of 22mA. but it took 19mA @ 6V to make 22mA at 3.2V which is 62%But if anyone knows of a better chip for efficiency at this sort of current please let me know. (needs to be under $0.80 in 100 qty)

author: Psi
author_rank: Super Contributor
timestamp: August 18, 2021, 07:44:56 am
content: hm.. i see the problemI have the AP62301which is the shit version. I need to get AP62300.The datasheet doesn't make that clear on page 1 statement about 83% at 5mA.At least it's an easy fix and cost looks the same.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: August 18, 2021, 08:10:21 am
content: Quote from: Psi on August 18, 2021, 07:44:56 amThat's the one ;^)Gotta read those finer details

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 18, 2021, 01:45:27 pm
content: Quote from: Psi on August 18, 2021, 07:44:56 amIt's not about better or worse, they are different versions of the same chip with different operating modes, with different tradeoffs. The 301 version operates in forced CCM, which generates much less output ripple below Fsw, at the cost of low-load efficiency. The 300 version goes to pulse-skipping mode at light loads, greatly improving efficiency but at the cost of subharmonic ripple on the output, which can be difficult to filter out due to its low frequency content and very wide spectrum. This can especially be problematic in audio applications, because the subharmonics change with loading and often end up in the audible frequency range.

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 18, 2021, 11:08:31 pm
content: Why are you not using a linear regulator at such low currents? They could be MORE efficient and have no "burst mode" operation at the operating points you want.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 20, 2021, 04:31:32 am
content: Agree with PhoenixAnd shouldn't have inductor some specifications about losses? Maybe I just don't see it, but there is nothing about it. At 750 kHz it can be huge compared to your load.

author: Psi
author_rank: Super Contributor
timestamp: August 21, 2021, 03:23:18 am
content: I'm trying to get maximum battery life out of two CR2032's running a 3.3V deviceAlso need to prevent 3.3V reg drop-out as the battery ESR starts to go up towards end of life.I can either put them in parallel and boost to 3.3V or put in series and buck down to 3.3V.My PCB does have a secondary footprint for a linear reg option, just in case switchmode doesn't work out.That was the main reason I choose to put the cells in series and do boost. To keep a fallback linear reg option

author: Someone
author_rank: Super Contributor
timestamp: August 21, 2021, 03:44:05 am
content: Quote from: Psi on August 21, 2021, 03:23:18 amThere are parts specifically designed for that:https://www.ti.com/product/TPS63900or even simpler:https://www.n-redc.co.jp/en/products/dc-dc-switching-regulator/spec/?product=rp604

author: Psi
author_rank: Super Contributor
timestamp: August 21, 2021, 04:24:16 am
content: yeah, but I don't really need low current standby.The circuit has a pfet which totally disconnects the battery until you hit a button to wake up the MCU which holds the pfet on until its done.

author: Someone
author_rank: Super Contributor
timestamp: August 21, 2021, 07:30:16 am
content: Low current standby often goes hand in hand with high efficiency at low power (Iq operating etc), look at the efficiency curves on those right down to the left. Buck-boost can be efficient.

author: EPAIII
author_rank: Super Contributor
timestamp: August 21, 2021, 08:27:55 am
content: With 6V in and 3.2V out, the best efficiency I would expect from a linear regulator would be 53%. And that does not include any quiescent current in the device itself which would only make it worse. He is already at 62% ("...but it took 19mA @ 6V to make 22mA at 3.2V which is 62%") so unless I am missing something, he can not gain anything by going to a linear.Care to elaborate on that?Quote from: Phoenix on August 18, 2021, 11:08:31 pm