Running a CL0116 solar light controller with two NiMHs instead of one

[5U4GB]

The ChipLink CL0116 solar light controller is always shown as using a single 1.2V NiMH cell, which isn't surprising given its one-page data sheet containing next to no information which means you're pretty much stuck copying their design for lack of any further information.  However I recently came across a circuit that used two 1.2V cells in series, so it would be charging to 2.4V instead of 1.2V.  Does anyone know how this would work?

[SeanB]

Well Vdd, which is Vbatt, is 3V, and this is within the voltage range of 2 AA NimH cells. Solar panel will need to have a voltage output of around 3V to charge it, so you will probably be using a 6V panel, and relying on the battery to absorb excess charge. As to the LED you want 2 in series, and size the inductor to have around 100mA of current through the LED's, as the main reason to go to a series connection is driving multiple LED's, so a 2S arrangement of the LED's is used, and you need a higher power output in the same package. 3S might work, even though it means the inductor pin will see around 9V peak voltage, which is a little above the 5V rating of the part, and some might fail long term.

[5U4GB]

I was more wondering how the charge controller works when it's being asked to handle charging at a very different voltage than what it's apparently meant for.  So using voltage-based termination wouldn't work... unless it uses something fancy like -dv/dt-based termination, but I can't imagine that from a 30-cent IC.

Or maybe there's barely any "controller" actually present and it just continuously trickles the solar cell output into the battery/batteries on the assumption that you won't get more than 8-10 hours of usable sunlight so it'll cease charging by itself at some point.

[edavid]

Or maybe there's barely any "controller" actually present and it just continuously trickles the solar cell output into the battery/batteries on the assumption that you won't get more than 8-10 hours of usable sunlight so it'll cease charging by itself at some point.

The QX5252 datasheet just shows a diode between SBAT and BAT, so I think this idea is correct.  If you want overcharge protection, you have to add it yourself.

[5U4GB]

Ah, well spotted! 

So the over-discharge protection shown in the QX5252 data sheet, if present in the CL0116, probably won't work because by the time both cells in series drop to the never-exceed limit for a single cell it's already too late.

[5U4GB]

Googling around a bit more, it looks like there's an entire class of these devices, almost all TO-94 and with identical circuits so possibly all cloned/copied from each other.  There's the CL0116 I ran into, the QX5252, YX805, YX8018, and no doubt many others.  For operating at a higher voltage the very common QX5252's less-common cousin QX5253, for which I've only found Chinese data sheets so I can't get more than the voltage and circuit diagram, would seem to be the way to go since it's specced for two cells and 1.8-3.0V operation so you'll get proper discharge protection when using two cells.

Apart from the use of the cheapest possible parts, this may also explain why solar garden lights are so failure-prone, you're mostly relying on cell chemistry to manage the batteries.

I also found this page which describes the circuit's operation:

The YX8018 is more or less just a gated oscillator which runs at approximately 200 KHz driving an open drain NMOS switch ( output on pin1 ). The circuit pulses the small inductor to step up the voltage to drive the LED in a similar way as a Joule Thief circuit. The use of a single AAA NiCd rechargeable cell keeps the cost down. Which also means the solar panel can be a cheaper low voltage version. The 3 cm x 3 cm one from this example generated 2.7 volts in full sunlight with a short circuit current of 17 mA.

To reduce the component count the application of the YX8108 chip is rather ingenious. They use the internal ESD diode between the CE ( chip enable ) input and ground for charging the NiCd cell from the solar panel, but also use the voltage ( or lack thereof ) from the solar panel to detect when it is dark enough to turn the LED on. The CE input includes a small pull-up current. I measured about 30 uA with 1.25 volts on VDD . This small current will pull pin 3 high gating on the oscillator if the solar panel is not generating more than 30 uA of current.