Li-ion IC -- LTC-4054-4.2 Questions

[jasonck08]

So I've been experimenting with the a Linear Technologies Li-ion charging IC LTC 4054-4.2. I replaced a generic Chinese TP4054 IC on another charger with the LTC-4054-4.2. I was under the impression that the TP4054 was a cheap Chinese clone to the LTC4054-4.2.

So far I've seen some strange things happen. Maybe its because of the other resistors / capacitors onboard that is causing this to happen. So far I've observed things that are not suppose to happen with this IC.

1) During most of the CC stage voltage jumps up and down .1v constantly. So say its charging at 3.6v. My DMM will record 3.5v for a split second, then back up to 3.6v then back down etc. It does this for most of the CC stage, and seems to stop jumping around as the current drops and the IC enters the CV stage. (This behavior reminds me of Xtars Wp2).

2) Current also jumps around during the CC stage from 600mA down to 100mA. Probably averaging around 400mA or so.

3) The LED goes off, but the charging never really terminates. I can observe 7mA being dumped into the cell, when in reality the datasheets say C/10 termination, which should be around 60mA.

Could the first two problems be related to overheating? I think the IC should just reduce current, rather than making the current / voltage jump around.

Maybe later I'll post graphs of whats going on...

Any thoughts?

EDIT, Pictures Added:

 

[malow]

weird.

i got some ready-made chargers with TP4054 (cheap clone) and my measurements where very good.

they have real termination and in my test it have a constant current without this "dips" from time to time.

the only not-so-ideal feature is the amp charging curve its not "perfect", it start to reduce amps a bit earlier, like 4.15. but they end correctly at 4.2v, and good cells don't even drop from 4.2 if charged about 0.3c

 

[Mr Happy]

Looks like the circuit might be going unstable and oscillating. Is the overall circuit consistent with the application note and application guidelines for the IC? For example, is the bypass capacitor present on the input and is the input power supply suitable?

 

[jasonck08]

weird.

i got some ready-made chargers with TP4054 (cheap clone) and my measurements where very good.

they have real termination and in my test it have a constant current without this "dips" from time to time.

the only not-so-ideal feature is the amp charging curve its not "perfect", it start to reduce amps a bit earlier, like 4.15. but they end correctly at 4.2v, and good cells don't even drop from 4.2 if charged about 0.3c

Here is my testing for the TP4054 (Chinese IC clone of LTC-4054-4.2):

As you can see the CC stage is almost non-existent, and there is no CV stage at all! You are right through, it terminates very well. The testing I did with the TP4054 was on the same board as the one I installed the LTC-4054-4.2 on. In the Voltage test graph, you can see the voltage dip a little maybe .03v after the charging process terminated. This is what I wanted to see with the LTC-4054-4.2, but I did not because it never terminates.

Looks like the circuit might be going unstable and oscillating. Is the overall circuit consistent with the application note and application guidelines for the IC? For example, is the bypass capacitor present on the input and is the input power supply suitable?

I'm far from an electronics expert, and know very little about this circuit, but I'll do some more testing and check the datasheet to see if I can determine what resistors / diodes and what not may be out of spec for the IC.

Also the power supply should be sufficient, its a Samsung 5V, 700mA cell phone charger type power supply. Maybe I'll try hooking it up to my BK Precision Bench power supply for some more "precision"

 

[Lion251]

Could the first two problems be related to overheating? I think the IC should just reduce current, rather than making the current / voltage jump around.

Voltage and current are related by the characteristics of the battery.
If you charge with less current, voltage will be lower.
So, if the IC overheats, and reduces the current, voltage will drop also.
There is no way to reduce the current without voltage dropping as well.

So, it may very well be that the IC overheating is the cause of what you see.
How hot does it become? Can you decrease the power it dissipates, or increase thermal conductivity?

 

[jasonck08]

Voltage and current are related by the characteristics of the battery.
If you charge with less current, voltage will be lower.
So, if the IC overheats, and reduces the current, voltage will drop also.
There is no way to reduce the current without voltage dropping as well.

So, it may very well be that the IC overheating is the cause of what you see.
How hot does it become? Can you decrease the power it dissipates, or increase thermal conductivity?

The behavior of the IC is not even close to what the datasheets say it should be doing in the event of overheating. Just for grins I may try to heatsink the IC. It gets quite hot, hot enough to burn you if you put your finger on it for more than a fraction of a second. The other IC onboard I removed also got very hot, but the current was less <500mA.

What I'll probably end up doing though is just laying out a new charger design on a breadboard, then go from there.

I ordered a couple SOT-23-6 to DIP PCB boards that should work nicely with the breadboard.

 

[jasonck08]

So I built a charger from scratch on a breadboard, using one of the sample circuit designs given in the datasheet... 2K Ohm resistor to program the current to ~500mA.

I cranked the voltage on my B&K Precision bench power supply to 5V, and noticed the IC was doing the same thing as it did when I transplanted it on another charging board.

The current was bouncing up and down! IC's voltage range is like 4.25-6.5v.

When I dropped the current down to 4.75v and slightly under, the current output was stable.

Seems odd, as the reference 500mA design should not require any heatsinking of the IC. But I guess I'll try heatsinking it to see if there is any difference.

I'm using the reference design on page 16 of the datasheet called "Full featured single cell Li-ion Charger".

Anyone else have any thoughts?

 

[HKJ]

Could it be a stability problem, i.e. try with a capacitor on the output, very close to the regulator.
Like they have done in the evaluation kit.

 

[SemiMan]

So I built a charger from scratch on a breadboard, using one of the sample circuit designs given in the datasheet... 2K Ohm resistor to program the current to ~500mA.

I cranked the voltage on my B&K Precision bench power supply to 5V, and noticed the IC was doing the same thing as it did when I transplanted it on another charging board.

The current was bouncing up and down! IC's voltage range is like 4.25-6.5v.

When I dropped the current down to 4.75v and slightly under, the current output was stable.

Seems odd, as the reference 500mA design should not require any heatsinking of the IC. But I guess I'll try heatsinking it to see if there is any difference.

I'm using the reference design on page 16 of the datasheet called "Full featured single cell Li-ion Charger".

Anyone else have any thoughts?

Part is in thermal overload. That is why the current is folding back. By changing even from 5V to 4.75 you are reducing dissipation in the IC by 250mW which is changes the temp of the IC by at least 32 celsius.

The part is intended to be heat sunk to a large area of copper on the board.

The TP54054 is a "clone" of the LT4054. Actually the part "May" be a clone, but the data sheet is certainly a clone. Does the part behave like the LT data sheet on which it is cloned ... I guess that is debatable. Perhaps it does not have a functioning thermal foldback circuit?

Semiman

 

[Mr Happy]

The part is intended to be heat sunk to a large area of copper on the board.

I wondered about that, but how do you heat sink a SOT-23 part? It appears to have a plastic body with sticky-out legs. Or do you heat sink it through its feet?

 

[HKJ]

Or do you heat sink it through its feet?

Exactly, you need large copper area connected to the pads.

 

[jasonck08]

I'll try heatsinking it, but first I'm going to try doubling the resistance from the program pin 2k ohms to 4k. This should cut my output current to the battery to 250mA, which should cut the wattage dissapated by the LTC4054 by 1/2. Then I'll see if it can handle more than 5v without being unstable. The thing that struck me funny is that the current would jump up and down. The datasheet doesn't mention the behavior of current output when the IC has overheated, but it does mention the current should be reduced. This pulse current thing just seems odd... Also, the other TP4054 (China IC Clone) is stable at the same current and voltages... But maybe it lacks thermal regulation which will lead to an early death for that IC?

Heatsinking the SOT-23 chip might be hard, but my idea is too arctic silver epoxy the IC to a small heatsink designed for cooling a single RAM chip.

 

[Lightfoot98]

Do you have another chip you could try??

Maybe you have a bad chip. (Overheated during soldering)?

Try a larger cap on input line, maybe a 2uf.

 

[jasonck08]

Do you have another chip you could try??

Maybe you have a bad chip. (Overheated during soldering)?

Try a larger cap on input line, maybe a 2uf.

Yea, I tried 2 chips. Same issue. I don't have a 2uf cap, but I could try 2x 1uf right?

 

[Lightfoot98]

Yea, I tried 2 chips. Same issue. I don't have a 2uf cap, but I could try 2x 1uf right?

Yes. Just parallel them.

 

[jasonck08]

Just wanted to mentioned that its fully stable at 5v with 4k ohms programmed, which yields ~250mA. I suppose the best thing that can be done is to add some resistance to the Vcc so it will reduce the heat dissipated through the IC's internal mosfet, which should provide better stability at higher currents.

 

[jasonck08]

Hmmmm... just logged the voltage and current from my breadboard charger. For the most part everything looks pretty good, but the problem comes near the end...

The charger terminates for a few seconds, then kicks in again and begins charging at about 50mA, then terminates, then charges again. I'm not sure why its doing this, because the datasheet says it should not try to charge the cell again, unless the cell voltage dips below 4.05v, then it will start a new charge cycle.

Please note that the voltage and current were logged in separate charge cycles, and the timing does not match up completely.

Any ideas? Did I do something wrong? I'm using the circuit diagram linked to in post #7.