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Board bring up & evaluation

A project log for Ultra low power LED

<1.5µA current consumption when continuously on, optional flashing mode

Christoph TackChristoph Tack 06/24/2020 at 08:240 Comments

Revision 1

In first instance, we'll try to compare the performance with the #TritiLED.  In order to do that, we'll assemble the following variant:

  1. Placing R8 instead of the TPL5110 circuit so that the LED doesn't flash.  This will make it easier to measure power supply current.
  2. Placing R11 and R4, effectively using only one LED.

First of all: add a little bit of solder to the GND pad of the CR2032.  If you don't do it, you risk having an unreliable ground connection to your cell because the GND pad lies deeper than the surrounding solder mask.

After soldering CN1 up to U2 by hand, we could start programming U2.  This went alright and now U2 outputs a 64Hz square wave on its CLKO-pin.

I measured the power supply current by feeding the circuitry through a 100K-resistor to CN1.  In parallel with CN1 I put some elco's with a total value of around 10µF.  At power up, the 100K resistor needs to be short circuited.  After that, the short circuit can be removed to measure current : 56nA with a 3V3 power supply.

Now U5 and its peripheral components could be soldered as well.  The package of U5 is too wide, but soldering the component is still possible.  C7 had to be increased to 22pF to get an output pulse of about 3.2µs.   The current consumption now is 73nA using a 3V3 power supply.

The LED driver circuitry, the only circuit that hadn't been fully bread boarded beforehand doesn't perform well enough.  See the LED driver log for more info.

The CR2032 clip needs a bigger outline.  If you insert the cell too deep, you could create short circuits with other circuitry.

The RTC needs to be constantly on, because it takes 250ms for it to fully initialize.  During the initialization phase it likely draws much more current.  The TPL5110 will not be used to power the RTC and other circuitry on.  The TPL5110 will only enable the one-shot timer at regular intervals.

Revision 2

The 100nF for C5 and C7 were not enough to avoid the bright LED flash at power up.  C5 has been increased to 1µF.  C7 has been put to not-placed.

The placement of L2 limits the viewing angle of the LED.  I placed L1 instead and shortcircuited L2 with the 0ohm resistor.

Evaluation

  1. The LED is too bright according to my wife.  This led her to covering the LED with paper tape.
  2. The device often lingers on the floor.  It happened twice that someone stepped on it, leading to a crushed housing.  A sturdier housing would be welcome.
  3. The big inductor is more fragile than expected.  In a few months time, I replaced three broken ones.  So I covered the inductor with hot glue.  2-component epoxy might work as well.

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