Voltage regulator unstable when attach input cap - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: June 13, 2014, 02:46:15 pm
content: I have a battery device drawing ~300 mA with an on-board ADP3338 (3.3 V) regulator. I found that when I attach a 1000 uF tantalum cap to input while the device is running, the regulator then gets very hot and outputs a voltage higher than 3.3 V. This state persists even after I have removed the cap and will only disappear if I remove and reconnect the battery. The problem does not occur if I power up the circuit with the cap attached.I know that input/output caps and output load can be critical for closed-loop stability but I'd like to understand more about what is going on. I'd be very grateful for any comments/links you can share.

author: Marco
author_rank: Super Contributor
timestamp: June 13, 2014, 03:46:17 pm
content: Some type of latchup if the input voltage drops below the output voltage?

author: dannyf
author_rank: Super Contributor
timestamp: June 13, 2014, 03:57:42 pm
content: Not sure whose "input" you are talking about. High capacitance on the regulator's input is generally not an issue - read the datasheet to be sure. High capacitance, coupled with low ESR on the regulator's output (load's input) can be deadly.Low ESR is generally evil and should be avoided unless you cannot.

author: Marco
author_rank: Super Contributor
timestamp: June 13, 2014, 04:12:04 pm
content: Input and output of the regulator.My guess is that when he plugs in the cap it pulls down the input voltage of the regulator below it's output, with quite a high slew rate at that, causing it to latchup (which explains why it needs to power down before resuming normal operation).

author: dannyf
author_rank: Super Contributor
timestamp: June 13, 2014, 04:30:41 pm
content: I doubt that's the case.maybe the polarity is inverted, or a wiring or soldering issue. Or some silly issues if that caliber.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: June 13, 2014, 05:05:49 pm
content: There are no soldering/wiring issues and the capacitor polarity is correct. Furthermore, I have 4 identical arrangements (battery-powered device + removable-cap) and the problem can be replicated on each.

author: SirNick
author_rank: Frequent Contributor
timestamp: June 13, 2014, 06:45:28 pm
content: Try adding a series resistor between the cap and the Vin to avoid that temporary short to ground when connecting. Might also try a reverse diode between Vout and Vin to test that latchup theory as well. It doesn't sound unreasonable to me.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 13, 2014, 06:51:22 pm
content: Connecting the cap momentarily causes the input to fall to 0V, and a very large current spike flows the wrong way back through the device. I'd bet on it being latch-up, the clue being that the condition persists until power is removed, which is absolutely characteristic SCR behaviour.

author: wraper
author_rank: Unknown
timestamp: June 13, 2014, 07:12:49 pm
content: Quote from: AndyC_772 on June 13, 2014, 06:51:22 pmOr starts to oscillate. That would describe why it becomes hot. I would attach oscilloscope to the output and watch what happens there.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: June 14, 2014, 02:20:19 am
content: Having read a bit more, I'm wondering if the problem could belatchup. I am working on location and will not have access to a scope until next week; though I would be very interested to plot the transient voltages and currents.If the problem is latchup, what voltage difference should I expect to see between output and input and why? Is the behavior when current flows the wrong way back through the device defined? If not, then what could be speculated?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2014, 02:39:22 am
content: Quote from: Marco on June 13, 2014, 03:46:17 pmSeconded...or, fourth'd?Try a schottky diode from output to input and see if it still happens.Lesson learned: plan for transient conditions and add protective diodes and such where useful.Tim