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I have put together a circuit with TPS2115A, it's feeding a camera from two 5V 20Ah (USB) power banks. The IC (TPS2115A) is used in automatic mode, i.e. whichever supply is higher is sourcing current, hysteresis is between 0.1-0.2V. I added two LEDs on the inputs so I would know if the batteries are alive but also to keep them alive, since I knew the power banks shut down after 30min if not used. Resistors used in series are 220Ohm and the current through each LED is about 13-14mA. I set everything up yesterday and today the camera was still on and both LEDs are on, indicating both power banks are still on. Was pleased to see it worked and I left everything on, letting the power banks drain themselves and see how it behaves.

Meanwhile I was thinking that maybe I could use larger resistors to the LEDs and minimize current consumption. I have a third power bank that is identical, so I started to experiment what current draw is needed to keep it alive. Started with 470Ohm and the current was appr. 7mA, after 30min. the power bank shut down. Ok, so I tried a 220Ohm which I was sure would keep it alive since it works in circuit feeding my camera, but no, to my surprise the power bank shut down after 30min. I have tried several times.

Since my camera setup with power banks is in a garage about 20min. drive from home, I currently do not have access to it, but I have started to try to figure out why the power banks are alive, or that in fact only one is alive and the LEDs are fooling me. I checked the LEDs and they are bidirectional, green when connected as normal and red when polarity is reversed, however, they are giving off green light, I'm wondering if putting these LEDs on the inputs can mess with ICs intended operation. What do you think, please look at the schematic below: circuit

Thank you.

Edit: I tried to figure out what was going on yesterday. Turned out that the both power banks were feeding current to the camera. Total current appr. 400mA, one power bank sourced 330mA, and the other 70mA (LEDs were disconnected). It (the IC) used IN1 (input 1) when camera was off, but as soon the camera was switched on and thus a larger current needed, the IC switched to using IN2. I had a small LED connected to the STAT input to see which input was connected to the output.

There was some horrible ripple and noise on both input and output when both power banks where connected and OUT was connected to IN2, if I forced the IC to use IN1, through the logic inputs, the ripple disappeared. The same happened when using only one power bank, no ripple.

See pictures below. I tried every combination but I couldn't get it to function properly so I gave up on it. The IC is in a TSSOP-8 package which I hand-soldered to a DIP-adapter, it's possible it got damaged, although I'm careful. I'm now going to use an evaluation board of the LTC4370, which shares and balances the current/load between two sources, which conveniently also keeps the power banks alive. This one.

Oscilloscope inputs are AC coupled in pictures. The first one shows one input (yellow) and output (blue) when IN1 (forcing it through logic) connected to OUT or whenever only one power bank is connected, camera is on in both pictures. The other picture shows the default mode when IC is in automatic mode; it selects IN2 as soon the camera is turned on. Any idea what could cause these ripples? I used heavy filtering at the end of the evening.

I should add that a small current was being fed to a small peripheral device through a 1W DC/DC-converter, which was the primary suspect, it only draws about 30mA and uses an input LC-filter. Naturally, I had it disconnected in the beginning, but it made absolutely no difference.

No noise

Noise

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  • \$\begingroup\$ How long are you expecting the power banks to last and on what basis do you make that estimation? \$\endgroup\$ – Andy aka Sep 1 '18 at 17:50
  • \$\begingroup\$ @Andyaka The camera draws just under 0.4A when not taking pictures. So 40Ah/0.4A = 100h, but realistically I expect 60-70h depending on amount of pictures taken and that each power bank are rated at 74Wh (and not 100Wh). \$\endgroup\$ – Ma Sa Sep 1 '18 at 17:54
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The circuit is fine, the LEDs wont affect the IC and the LEDs are probably telling you the right thing.

It sounds like its actually the camera which is keeping the power banks on, not the LEDs. The TPS2115A will repeatedly switch between the two power banks as the load will cause the voltage of the power bank to sag.

So the power banks are alternatively supplying the load and therefore staying on.

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  • \$\begingroup\$ Please see updated post. \$\endgroup\$ – Ma Sa Sep 3 '18 at 7:03
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Both LEDs could be on nearly all the time unless I am missing something.

Once the inputs switch the "dead" battery will likely self charge enough to light the LED.

You should use the largest resistor (no LEDs) on the inputs that will keep them alive and use the STAT to sink an LED. If the LED is on then IN1 is being used. You may not need to keep the banks alive depending on exactly what you need to know. Stat will always tell you which is active.

enter image description here

Stat gives you the state of the mux.

enter image description here


I do not like this TI chip, I'd use an Ideal Diode like one from Analog Devices.
LINK: PowerPath, Ideal Diodes & Load Switches

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