faulty LDO or faulty circuit? - Page 1
Forum section: electronics / beginners

author: brindle
author_rank: Newbie
timestamp: September 11, 2021, 06:13:50 pm
content: A while ago I ordered some PCB's from JLCPCB to use in some projects. The board is basically a 18650 battery holder, an ESP-12e socket and some breakout headerpins to make it easy to connect things. This worked just fine so I decided to order some more. However, the HT7833 in a SOT89 package that I used in the first version was out of stock. They did have the HT7833 in a SOT23 package. I figured this would not really matter so I simply replaced the SOT89, updated my board and placed the order. I changed some other things like the position of the header pins and the battery holder forcing me to put the HT7833 in a more cramped space as well.Unfortunately the new version does not work properly. When I connect the battery, all seems fine. 3.3v on the output and off we go. After a a minute or so the voltage starts to fall. The amount of time before the voltage drop varies per board. I have had some work properly for days and some for less than 5 minutes.There is some weird behavior too, if you mess around with the HT7833 (pushing the pins down to the pcb or touching the top with your fingers) the voltage jumps back up to the specified 3.3v!first properly working version:second not properly working version:schematicThere are 2 parallel HT7833 circuits, only one (the left one) is used for the ESP8266, the right one is a separate circuit for sensors that require 3.3v.I'm trying to figure out if I received faulty HT7833's in my new version, or that my new setup is simply wrong for some reason. Maybe the pcb lines between the capacitors and the HT7833 are too long/thin? I hope you guys can point me in the right direction.

author: wraper
author_rank: Unknown
timestamp: September 11, 2021, 06:22:41 pm
content: You did not say input voltage and current that flows through them. I suspect they are overheating and thermal protection kicking in because a new PCB variant provides worse heat dissipation and SOT23-6 variant is worse at dissipating heat by itself.

author: brindle
author_rank: Newbie
timestamp: September 11, 2021, 06:24:22 pm
content: Quote from: wraper on September 11, 2021, 06:22:41 pmInputvoltage is max 4.2v from the 18650 battery. The ESP8266 operates on around 80mA. Another observation: The ESP8266 initially works fine (on some of the boards), then goes to deepsleep drawing about 30uA. During deepsleep and that 30uA draw the voltage still crashes. The same thing occurs even if there is no load attached and I cannot feel or measure any increase of surface temperature.

author: wraper
author_rank: Unknown
timestamp: September 11, 2021, 06:37:54 pm
content: Also HT7833 should not be used with ceramic output capacitor. It should be electrolytic or tantalum, if ceramic capacitor is used (very low ESR), it may become unstable and oscillate. When creating PCB layout for parts which dissipate significant heat, think about making copper fills which will help a lot with dissipating that heat. Li-ion + LDO works well only when battery charge is high. Once battery partially discharges, there will be not enough input voltage to keep output voltage properly regulated to 3.3V.

author: brindle
author_rank: Newbie
timestamp: September 11, 2021, 06:45:32 pm
content: I reckon it is the ceramic capacitors then, and the first working batch was sheer luck? I doubt battery voltage is the issue because the first version can use up the battery all the way down to 3.3v before I have the ESP put itself to an indefinite deepsleep. The same behavior occurs on a 5v usb supply.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: September 12, 2021, 06:44:00 am
content: I also vote faulty mlcc capacitors,https://www.holtek.com.tw/documents/10179/82844a36-1633-498c-a6e3-4d99f2d0f6d7this datasheet shows polarized capacitors which is typical for ldo's. I think you just got lucky the first time with the sot89 package.If you don't want to use tantalum capacitors just add a 1ohmn(anywhere between 0.5-10ohms is usually fine) resistor in series with the capacitors.

author: brindle
author_rank: Newbie
timestamp: September 12, 2021, 06:55:51 am
content: Quote from: bobbydazzler on September 12, 2021, 06:44:00 amahhh I did not notice the + symbol so I just added ceramics. I will get some tantalum ones and replace the ceramic capacitors. I also noticed that ZZZ between the gap is apparently a Japanese version for an electrolytic. You learn something every day.

author: thinkfat
author_rank: Unknown
timestamp: September 12, 2021, 07:36:51 am
content: You touching the circuit and pressing on it might result in dampening the oscillation, just by touching the capacitors for example. Could serve as an explanation why the circuit "recovers" on touch.