BQ24278 charger output oscillating on battery insertion

Hello Jon,

I have not had time to look at this on the bench yet but looking through the datasheet the one operational feature that would operate at approximately is the OUT short detect which has a deglitch of 250us.

It looks like there is a corresponding voltage droop on the input voltage and the battery voltage when the current spikes occur.

Could you take a closer look by zooming in on just 1 or 2 spikes? Also what is the scale on the current since the scope shows it in mV?

Ryan,

My equipment is all out for calibration right now, but I previously captured this more zoomed in version, I hope it suffices. Current is 1A/div.

Hello Jon,

I have attempted to recreate these results in the lab using an EVM modified to the same values as your system. However I was not able to reproduce these waveforms.

We are are looking to understand what is happening within the system to narrow down the possibilities. Is all of you equipment out for calibration?

If you could obtain captures of the /CE and CD pins to determine if they are switching at that same rate as the oscillations that are being seen.

Also if you could measure the /CHG pin to determine if this is switching at the same rate.

Ryan,

My equipment just came back this afternoon.

Nothing to be seen on these signals save for a little switching noise being coupled into /CHG, but I grounded my scope pretty far away from the charger so I don't think this is of consequence, I gave you a zoomed in shot of that anyway. Also, I just had some trouble reproducing the problem, I tried a really dead battery, and an almost completely charged battery and no problem. I then discharged my completely charged battery for a while and the problem came back when I hot inserted that ~80% charged battery.

Thanks,

Jon

Yellow = /CE, Green = CD, blue = /CHG, red = input current 1A/div

Hello Jon,

I am still trying to narrow a few things down in my testing using the BQ24278 EVM.

One thing I did notice was that once I added the bleed resistor that you have place on the Vin pin, R1, that the charger was not as stable.

Have you tested the system without the bleed resistor R1?

I will continue to test to see if I can achieve the same operation that you have observed.

Ryan,

Thanks for going after this, we'd really like to know what is going on. I have not tried removing the bleed resistor, but I'm having trouble imagining how it could be a problem. What do you mean by the charger was not as stable?

Just some observations and thoughts I've had on this in the past:

-I was very critical of my design at first, reviewed datasheet and my layout (I think the layout is pretty good), and didn't see anything that should be causing oscillations or instability.

-Looking at the scope plots it doesn't even really look like an oscillation, could it be some strange state of logic the charger is getting into? I mean, I could understand blaming it on my layout or a component problem if the oscillations were consistent (perhaps if I had a crummy layout for example the chip might be "seeing" a current limit or voltage limit being hit, over and over). But the strange thing is if you look at the plot below, sometimes it will look normal/stable, for all of 2mS, and then its back to the "oscillation." It is really odd that this stable time of 2mS (stable and right on my target charge current) doesn't bring it out of the "oscillation," so I am led to believe its acting more like its stuck in some weird logical loop, not really an oscillation based on bad design or layout?? I have also never seen this happen when power is applied with an already installed battery, even with the same battery and input voltage conditions.