LFlex Technical

TASKLED - Smart LED drivers

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Operating Manuals (PDF) for LFlex V1,V2 and V3 hardware:

Drivers shipped after Jan 20, 2014:

lFlex.pdf (manual for lFlex UI-UNI4 firmware, V1.00)
lFlex.pdf (French, manual for lFlex UI-UNI4 firmware, V1.00) Many thanks to Dominique for the translation.

Drivers shipped prior to Jan 20, 2014:

LFlex_V3 (English, V1.00, manual for LFlex V3 hardware - UI-UNI3 firmware - drivers shipped on and after March 1, 2012)
LFlex_V3 (French, V1.00, manual for LFlex V3 hardware - UI-UNI3 firmware - drivers shipped on and after March 1, 2012)
LFlex_V2 (V3.00, manual for LFlex V3 hardware - UI-UNI2 firmware - drivers shipped prior to Feb 28, 2012)
LFlex_V2 (V1.01, manual for LFlex V1 and V2 hardware - UI-UNI2 firmware)

Release Notes:

There are 3 versions of the LFlex drive and all are 0.780" (nominal) in diameter:

V3 is a totally new design that utilises a hardware based linear current regulation core. It also adds the STAT pin for optional battery voltage warning via a 3mm or 5mm low power LED.
V1 and V2 are essentially the exact same driver with V2 only having some minor thermal path refinements in the PCB layout. They can be functionally treated as the same driver and both use a software algorithm to control the current regulation hardware.

The reason for the V3 redesign was to add full hardware current regulation to the LFlex driver to eliminate any slight flicker artifacts that could occur (and have been observed) in the software based V1/V2 drivers. The human eye can be very sensitive to small levels of flicker at low light intensities.

Technical Information:

LED+ wired to anode of the LED load (electrically the same as IN+).
LED- wired to the cathode of the LED load.
IN+ wired to battery positive.
IN- wired to battery negative. Do NOT wire battery negative to SWB or you will compromise the current regulation path.
SWA wired to momentary action switch.
SWB wired to momentary action switch (electrically the same as IN-).
STAT wired to 3mm/5mm voltage status LED (max 2.5V drive with 100 ohm series resistor on board). LFlex V3 only.

V3 top side of the driver .

V2 top side of the driver:

V1 top side of driver:

The following block diagram shows how to connect the battery and LED(s) to the LFlex driver. V3 uses a 0.030 ohm sense resistor versus 0.015 ohms for the V1/V2 boards.

SWA and SWB (SWB is electrically the same as IN-) are provided to enable a momentary action (normally open) switch to be connected to LFlex to control its operation. The switch only switches a control signal and carries at most 300 microamps. Even though SWB and IN- are electrically the same, SWB should be used only for the momentary action switch and IN- should only be used to connect to the negative of the battery.

Determining Power Dissipation:

The LFlex driver is a linear regulated driver. It regulates the current by shunting excess voltage across the power FET. At higher currents, higher power is dissipated in the FET requiring the user to mount the PCB via the supplied thermal adhesive tape to the heatsink/body of the light.

The user should measure the Vf of each LED at the rated current. For example, if using two XM-L LEDs, measure each Vf at the target current (say 3A) and sum the total Vf (lets assume they are 3.2, 3.3 for this example, so the total is 6.5V). Then take the fully charged battery voltage (say 2 li-ion cells) of 4.2 x 2 = 8.4V. Calculate the worst case power dissipation (difference between battery voltage and total Vf multiplied by the drive current). This power dissipation must be within the capability of the heatsink/body of the light to dissipate the heat.

First let's look at theoretical fresh off the charger li-ion cells that can output 4.2V even with a 3A load applied.

(8.4V - Total_Vf) x 3A = (8.4V - 6.5V) x 3A = 5.7 Watts. At this power dissipation level the FET requires substantial cooling efforts. In addition to mounting the bottom of the LFlex to a heatsink with the supplied thermal pad material the user would need to provide a thermal path from the top of the FET package to a heatsink.

Next let's look at a real world 2 x 18650 fresh off the charger example with a 3A load applied.

At 3A current draw a li-ion cell (e.g. 18650) even freshly off the charger will drop to <4.0V within seconds of having the load applied. So, let's re-work the numbers assuming the li-ion cells drop to 3.9V per cell with a 3A load. (3.9 x 2 - Total Vf) x 3A = (7.8 - 6.5) x 3A = 3.9W which is well within the ability of the LFlex dissipate by mounting the bottom to a heatsink with the supplied thermal pad material.

Thermal Imaging results:

The following images were taken with a thermal imaging camera with the lflex V3 board driving an XML LED with a Vf of 3.3V at 3A drive. The first picture is to help provide a visible reference for the thermal images that follow. The lFlex driver is mounted to a substantial heatsink (block of Aluminium) with a piece of Bergquist BP100 thermal tape. FET Q1 has an absolute maximum Junction Temperature rating of 150°C

FET Q1 is the device that will dissipate heat while the lFlex driver regulates the output current.

The first thermal image shows the driver running with 4.2V input and 3.3V output @ 3A. This means the driver is dissipating (4.2V - 3.3V) x 3A = 2.7W. FET Q1 reaches 86.2°C on the top surface (which will be close to junction temperature) as shown by point Sp1. The heatsink is nominally at 26.4°C (Sp2).

The next thermal image shows the driver running from 4.0V input and 3.3V output @ 3A. This means (4.0V - 3.3V) x 3A = 2.1W dissipation. FET Q1 reaches 65.8°C in this case.

The final thermal image shows the driver running from 3.7V input and 3.3V output @ 3A. This means (3.7V - 3.3V) x 3A = 1.2W dissiption. FET Q1 reaches 51°C on the top surface.

Efficiency:

Some customers ask about the efficiency of the Lflex. Well, being a linear regulated driver it works by dissipating excess voltage (x current) as heat. So, lets assume 8.4V in (2 series freshly charged li-ion cells) and 6.6V out (2 XML LEDs in series) at 3A (actually the current doesn't matter as will be seen in the calculation.

Efficiency = power_out/power_in x 100%

Efficiency = (6.6V x 3A) / (8.4V x 3A) x 100% = 6.6V / 8.4V x 100% = 78.6%

Now, let's assume the batteries are somewhat discharged and at 3.7V each. So, now we have 7.4V input.

Efficiency = (6.6V x 3A) / (7.4V x 3A) x 100% = 6.6V / 7.4V x 100% = 89.2%

Thermal Pad information:

The supplied thermal pad material is nominally 0.25mm thick, white in colour and will conform to the heatsink and PCB surfaces to provide an excellent thermal path. Please ensure the heatsink and bottom of the LFlex are cleaned of any grease or contaminants that would prevent the adhesive from properly bonding. Pressure should also be applied to help set the adhesive, refer to the datasheet for more information.

The thermal pad material (Li98 100 0.25mm) specifications can be found in the datasheet a copy of which is available here. The Li98 material provides a good compromise between the cost of the material and its thermal performance. The material is white in colour and has a protective cover on both sides that must be removed to expose the adhesive. Note, it is recommended to remove the Red protective material first and fix the pad to the heatsink or the Lflex prior to removing the white protective material. The adhesive is an acrylic base and takes up to 24 hours for fully cure/set. After a few thermal cycles and 24 - 48 hours the bond will become stronger and the thermal conductivity will improve.

As an example test case: 4.2V input, XML load, 3.2V Vf at 3A (nominally 1V x 3A = 3W being dissipated in the FET):

The FET operating temperature was measured at 72C when an LFlex was first mounted with the thermal pad to a large heatsink. After two days of allowing the adhesive to set and cycling the driver a few times, the temperature was measured again and the FET temperature stabilized at 55C.

Further information;

Battery input voltage can be from 2.7V to 17V (max), but power dissipation in the FET must be kept below 5W maximum if only the bottom side of the PCB is thermally mounted to a heatsink. Input voltage must be higher than the LED Vf at the require drive current to ensure the LFlex remains in current regulation.

Below is a picture of the back sideof the board (MOSI, MISO and RST pads/holes must be left unconnected, they are used during board manufacture to program/configure the driver board.

V3 bottom side of driver:

V2 bottom side of driver:

V1 bottom side of driver:

The red outlined area in the pictures above are the area behind the power FET that at a minimum must be thermally mounted to a heatsink or the body of the light. It is preferable to mount the entire board to a heatsink if possible.