Programmable Boost LED Driver
Single-Cell Programmable Constant Current Boost LED Driver
This project was born from the desire to create a simple, compact,
low-cost, but very bright LED torchlight with high-quality light. The
result of this was the creation of a custom, extremely small yet
powerful and full-featured LED flashlight driver which I've named the
GXB20. This page documents the development and engineering choices of
My only real experience in the flashlight world was introduced to me by
my friend Tim back in secondary school (grade 7 onwards eqv.), when he
introduced the Princeton Tec 40 (PT40) to me, which was the first
real flashlight I bought. It featured a 28 lumen halogen bulb
powered by 4 AA batteries in a compact,181g waterproof plastic body,
featured a twist bezel, an orange-peel reflector. At that time, I was
amazed by its bright output, rugged nature and compact size, and used it
for several outdoor trips. A few years later, a crack appeared on the
case and was the end of the life of this light.
A couple of years ago, I found a nice deal online for a single AA LED
flashlight for just a few dollars. I
liked how compact it was, that it used only a single AA
battery, and had a zoom lens. It was actually pretty bright compared to
regular incandescent flashlights I had previously used, and had a light
output of probably something around 100+ lumens, which was much more
than that 28-lumen PT40! I liked it so much that I carried it with me a
lot and it became my default flashlight, and it's accompanied me to many
places, from the temples of Angkor Wat, surviving power outages in
Boston, to lighting up the back of my TV cabinet.
Several years passed and LED technology
has progressed significantly - it's time for a brighter flashlight!
So in 2016 (time of writing), I did some research about flashlights and found that manufacturers like Zebra Light, Nitecore and Fenix
etc were making some really bright, >1000 lumen lights using very
good quality LEDs (and by good quality, I mean those which are not only
efficient, but also having a high Colour Rendering Index). That's an
order of magnitude or more brighter than my little AA flashlight! However, they
were very expensive, typically around the ballpark of $100. At the same
time, I also found some nice, cheap single-18650 torchlights on Amazon
for just a few dollars each. This made me wonder - is it possible to
create a cheap yet powerful flashlight?
This was the motivation for the creation of an LED driver which I have named the GXB20
- G for my name, X denoting the fact that Cree's line of 6V/12V
XHP50/70 LEDs were the motivation for creating this driver, B denoting
the fact that this is a boost driver, and 20 denoting that the PCB
diameter is 20mm.
[Update] The V1 driver went through a minor revision in early 2017, and the
newest revision as of writing is GXB V2. This page describes the development
of both of these drivers.
[Update - Jul 2017] A 17mm version
of the GXB20 has been created, the GXB17. Please check it out here!
Both sides of an assembled GXB20 V2 LED Driver less the spring. Note the
mistake in the silkscreen - it should read 6V / 3A instead!
GXB20 (v2) Feature-List
- Single-cell (e.g. 18650 lithium rechargeable
battery) 2.7 to 4.3VDC input (6V
- True Constant Current Limiting (no PWM or flicker)
- Synchronous Boost topology
- Nominal 18W out (6VDC at 3000mA), (9VDC at 2000mA) or (12VDC at
- Designed primarily for XHP50/XHP70 CREE or Nichia 144-series LEDs running in 6V configuration
- 20mm Driver Board diameter
- Low-Battery-Voltage management and cut-off
- Programmable with ATtiny84A MCU and 6-pin SlimStack header
> Configurable modes with EEPROM memory for modes
> Real-time temperature sensing and cut-off
> Two extra GPIOs for Mode Select
> E-Switch capable with firmware
- Over 90% efficiency throughout most output ranges
- 256 levels of true constant current brightness levels
- Off-time Capacitor and mode-memory
In this page, I'll outline the motivations of this project, how I
designed this driver, and how I put them all together. Hopefully this
will also be a useful resource for many like-minded hobbyists around the
world who are also fascinated by flashlights and power electronics :-).
Designing the GBX20
The core challenge of this project is to create a suitable boost
driver, and we need to do some electrical engineering to get us there.
So let's begin! But first, we need some constraints to frame our
engineering decisions. We will start by determining where we want to fit the driver
into, and what LED it will drive.
One of the main motivations of this project was to be
able to modify low-cost of-the-shelf flashlights to make them better. I found that the 'SSK98 UltraFire
XML-T6', which retails for just a few dollars each on eBay or
Amazon, was an excellent
cheap flashlight to base this project upon.
The SK98 has a reasonably compact size, houses a single 18650
Lithium cell, has a zoomable lens, and best of all, costs only $12 for two!
I bought a pair of them on
Amazon and proceeded to take them apart to see how we can modify
them. Note that this particular flashlight was just one of many with the
same generic housing, perhaps all being made cheaply by the same Chinese
supplier, and despite the 'XML-T6' moniker, it actually features a
completely different, non-Cree LED. Regardless, it was a good,
commonly-available host and a good place to start.
As a side note for those who are not familiar with
Cree, Cree is a semiconductor company and one of their main branches is
in the LED industry. Other big players include Nichia and Osram. The XML
is one particular line of LEDs made by Cree and are known for their good
quality light, and consequently is an industry benchmark. T6 refers to a
particular bin of LED brightness. Typically LED brightness (efficiency)
comes at a cost of LED colour quality (Color Rendering Index).
XML and XML-2 LEDs from Cree - notice the difference in substrate, bond
wires, and phosphor texture/p>
NNow these flashlights supposedly come with a 'Cree XML LED'
in them. If genuine, the XML range is one of the best performing 5x5mm
single-die LEDs on the market at time of writing! After taken them apart
(see below), I found that they indeed turned out to be non-genuine XML
LEDs, and are instead LatticeBright LEDs.
Don't get me wrong
LatticeBright by itself is a completely legitimate company producing
quality LEDs, just typically at a lower price point and consequently
lower performance. I only say non-genuine since they are often marketed
as Cree LEDs, which are typically more expensive and slightly higher in
efficiency and/or colour rendering quality.
Out of the box I was pleasantly surprised how bright
these cheap flashlights were compared to my existing AA LED light! I
turned on the SK98 flashlights and measured that they were drawing
around 6.7W during operation. These flashlights were bright, but not
bright enough, considering that they were running at much less than the
~10W maximum power that
XML LEDs can be driven
(datasheet spec - in real life with better cooling, they can be driven
much more!). There a
few things we can do about this:
1) Replace the LatticeBright LED with a
better one (e.g. XML-2) with better CRI and at desired colour
2) Modify the driver to run the LEDs at higher current to make it
3) Create a new driver to use even better LEDs to make the flashlight
The first 2 are easy to achieve, and in fact I did
modify one of them and replaced it with a genuine XML-2 LED with high
CRI and warm colour temperature (see below). The second is easy to
achieve by simply changing out the current limiting resistor(s). Next of
course, we will try to see if we can do (3) - make a whole new driver
Disassembly of the SK98 UltraFire Flashlight
Disassembly of the flashlight was actually quite
straightforward. Doing this will allow us to take a look at the internal
construction as well as the electronic components.
The construction of this flashlight is extremely simple
and everything came apart very easily.
In about a minute, we were in! The LED is soldered to a
metal-backed aluminium PCB and is held down to the pill via a retaining
threaded ring, squeezing it down to the aluminium pill for good thermal contact. The pill also houses
the press-fit 20mm diameter driver PCB, where the battery spring is also
soldered on to. After some desoldering, the entire assembly comes apart
Replacing the LatticeBright LED with a genuine Cree XML
Now let's take a closer look at the LED. We can easilly see that the LED is not in fact a true Cree LED, but an
alternative part by LatticeBright. They do make decent LEDs, with
different qualities than a genuine Cree, to hit a lower price point. We
can upgrade our light quality by swapping this out. Again I want to
point out that I'm not bashing on the LB LED, and in fact the LB most
definitely produces more light per dollar!
We can see the difference in LED construction by
comparing the green LatticeBright LED with a new, genuine Cree XML-2 LED
I bought. Notice the slightly different die size, phosphor application
and bond wires.
Next, I replaced the LED. This was done
easily using a nice fat-tip on my soldering iron heating up a copper
sheet, where the aluminium MCPCB sits on, and desoldering the LB LED. The new
Cree LED was then reflowed on
the same way. We now have an upgrade light! - not any brighter, but of a
higher light quality!/p>
Next, let's take a closer look at the driver board. It's
labeled YN-20-3, and essentially only has one 1uF capacitor, one SOT23-3
IC, and 4 resistors! Very efficient in components!
CClearly all the control is done entirely inside the
SOT23 IC, which has part numbers GY02-45. I could not find anything
about this IC, but it seems nothing more than a very basic PWM-based LED
driver with likely a small integrated FET inside, including some smarts
with off-time memory using C1. I've drawn the schematic above for
reference. It's really built down to a price.
Clearly the LED is driven directly from the lithium
battery with R1 to R4 acting as series resistors. Brightness control is
done by Pulse Width Modulation. It's easy to increase the brightness of
the LED by decreasing the series resistance, but if we do it too much,
we run the risk of blowing the internal FET in U1 from overcurrent.
Now we have seen the SK98 disassembled and did one
simple mode, we can design a
new driver for it./p>
Cree XHP50 and Nichia 144 LEDs
XML-2 LEDs are already some of the highest performing single-die
5x5mm LEDs on the market today. Yet we are limited to 'only' 10W drive
(datasheet maximum). If we want to to get more light, we need to go one
step further. This is where the Cree XHP50 and Nichia 144 series LEDs
One of the goals of this project was to create a very
bright light, so the logical step was to utilize the extremely high performance
CCree XLamp XHP50 or
XHP50.2 LEDs (XHP50.2 LEDs were
only released to the market shortly after the writing this page). The 144-series LEDs from Nichia are
also impressive candidates and have even better colour rendering, but
with marginally lower power handling capabilities. So unlike the XML2
which can handle a nominal 10W of power, the XHP50 handles twice the
power at 19W!
To achieve such high power output. these LEDs pack
4-dies into one substrate, and come in either 6V or 12V configurations
(depending on PCB layout for the Cree, or part number for the 144). For the
rest of this project, I decided to focus on
the CREE LEDs since they were easily available on consumer electronics
sites like Digikey and Mouser, unlike Nichia who do not distribute their
LEDs in low quantities to normal consumers (at least at time of writing
From left to right, the XHP50.2, XHP50 and XML-2 LEDs (photos by Cree)/p>
Let's do a quick comparison of the XHP50 and the XML2
(left-most two in the image above). The XHP50 packs 4 dies on
a same-size 5x5 mm substrate and has a widened dome. With 19W drive,
this allows a maximum luminous
flux (at datasheet maximum) of just about 2400 lumens (with the highest
efficiency 70 CRI LEDs) - this is twice as bright as the brightest XML-2
single-die LED. In
addition, the XHP50 also come in some very nice high CRI (>90) of
different color temperatures, but at a cost of reduced luminous
Finally the XHP50.2 LEDs which were just
announced at time of writing in end 2016, pushes the maximum brightness up
to 2654 lumens, efficiency up to
165lm/W, and also comes in 90+CRI flavours (these may be a little difficult to
buy until they start to become easily available on the market).
The 4 dies of the XHP50 are arranged as two parallel
strings of 2 dies. Therefore instead of a V_fwd of
~3V, we have an option of running it at 6V or 12V depending on the PCB
layout. So one cannot simply
replace a XML LED with a XHP one without changing the LED driver, since
the drive voltage required is different.
As an example, let's take a look at the XHP50A-00-0000-0D0UH250G, which
is a >90CRI 5000K XHP50 LED with 1032lm at 25C (900lm at 85) with a 12V 700mA
/ 6V 1.4A drive current, and available on Mouser Electronics for just about $10 per
piece in 1x quantities. This should give us just around 1900lm with a
3A drive current. /p>
Chart from Cree - XHP50A Datasheet
There are 3 main brightness settings I'd like to figure
out for this project - full brightness, medium, and a dim setting for extended run
time or thermal cut-off. I picked these 3 arbitrarily:
1900lm - Full brightness - 3000mA Drive, 6.125Vfwd
770lm - Half brightness - 1000mA Drive, 5.725Vfwd
200lm - Low brightness - 250mA Drive, 5.3Vfwd
Assuming a drive efficiency of 90%, that we use 90% of
battery capacity, and that we use a 3.7V 2500mAh 18650 battery, we can
conservatively say we will get roughly 25mins of run time at the highest
power, 1hr 20min at 'half brightness' and up to 6 hours of run time at
We will use these values later when designing our power
Boost Converter Design
TThere exist many LED drivers on the market today which you can
buy to fit into a flashlight host. These are very popular among hobby
flashlight builders or modders. These drivers range from simple constant current LED drivers,
to more complex ones with programmable capabilities and direct drive FET
ones.. These drivers work well
and are often based on the AMC7135 which is a 350mA low-dropout constant current
regulator. To increase current handling capability, several of
them are usually used in parallel. In simple single-die LEDs driven by a
single cell, this is a good
and easy solution to achieve Constant Current (CC) operation.
However, none of the drivers I've come across address the main challenge we face -
to be able to drive a 6V LED from a single cell (3.7 to 4.2V), at least
at time of writing./p>
[Update Jun 2017
- After the completion of this project, I was alerted that
there finally exists a commercial driver called the
H1-A which achieves what we want! Unfortunately it is 20mm in size,
has an off-board inductor (a little less robust), and doesn't fit into
17mm flashlights. Putting the functionality of the GXB20 into a 17mm
package was the motivation for the successor to the GXB20, the GXB17,
which you can read about here!]
To solve this, we need a
boost converter capable of boosting the voltage up to 6V
(or 12V) to drive the LEDs of our choice (e.g. XHP50) from a single
18650 battery. And at the same time, it needs to be able to handle the high currents required
(3A at 6V). Fitting an 18W DC-DC converter in a small space is
challenging! Let's create a list of requirements.
Boost Converter Requirements:
- Input voltage - single Li-ion cell
- Be as efficient as possible
- Be as small as possible to fit on a 20mm PCB to fit inside the host
- Output voltage boosted to at least 6V
- Some sort of constant current limit and/or capable of controllable limit
- Capable of at least 3A output (i.e. something like ~7A on the input)
Image from TI's TPS61088 Website
After a little bit of searching, I found that Texas Instruments makes an integrated PMIC
(power management IC) which looks as if it was specifically designed for
this purpose! This is the TPS61088
Synchronous Boost Converter! I won't go through all the details here, but it not
only checks all the boxes in our requirements, but also adds several
cool features such as thermal shut-down and programmable soft-start.
Best of all, it comes in a fully integrated package with
in-built high and low side FETs for high-efficiency synchronous operation, and comes in
a very small 4.5 x 3.5 x 0.9mm package!
Boost Converter Design
With our choice of IC done, we need to figure out how we
should configure it, and this depends on what our desired input and
output requirements are. So let's determine the
constraints and design around them. To determine the values, I used TI's
excellent WEBENCH designer to do a quick reference design and modified
the suggested values:
Soft Start and Enable
In order to prevent high-inrush current during startup (which will
be enabled via the EN pin connected to our microcontroller), the
TPS61088 has a soft start function controllable by a capacitor on pin
SS. Lets arbitrarily choose a soft-start time of around 10ms - in this
case we can use a 47nF capacitor for a ~11.3ms start-up time.
Switching frequency can be varied between 200kHz to 2MHz, and is
set via a resistor between SW and FSW. In this case, lets just use the
recommended value of 267kR, which gives us a switching frequency of
around 600 to 595kHz.
Peak Current Limit
In order to protect both the battery and the TPS61088 from a fault
condition, we can set a peak current limit. Assuming a poor 90%
efficiency driving the LED at 3Apk (6.125V Vfwd) and a battery voltage
of 3V, we can see something around 7A from the battery. Due to device
variation, we can add ~1.3A as recommended from the datasheet to give us 8.3A.
Adding another ~10% margin gives us a value of 9.2A so we will set
a current limit of 9.2A using a 110kR resistor at ILIM.
The TPS61088 can operate in both forced PWM and PFM mode. Considering that we will likely not be driving the LED
often below something
like 200mA, the efficiency shouldn't drop below 90% for most of the
time. Forced PWM also has
slightly higher typical efficiency across the range, so this will be the mode we will be
using. The MODE pin will be connected to GND to run the device in FPWM.
I bolded the title for this section because I think this is a
critical part of LED control. LEDs have an unusual characteristic
whereby changing the applied voltage across the LED corresponds to an
exponential-ish change in current draw (and thus brightness). In
addition, manufacturing differences and thermal effects result in
Constant Voltage LED control to be quite finicky, usually leading to
potentially dangerous situations with thermal runaway, over-current
drive and large LED brightness variation. Therefore we cannot run
this boost converter in constant-voltage mode, and we certainly
don't want to run it in constant voltage mode + a series resistor,
because this will burn a bunch of power in the series resistor (though
it will probably work!).
There is a much more linear relationship
between the current draw of the LED and its brightness. It seems
advantageous to approach LED drive with a Constant Current driver, which
varies the applied voltage automatically such that the current is at a desired
But how to achieve this, given that the feedback for the TPS61088
is based on a fixed output voltage? - and how to
make this adjustable?
Here's the approach I came up with - Since all the feedback
pin cares about is a certain voltage, all we need to do is to create a
system to convert the drive current to an appropriately scaled voltage
To achieve this, the LED is driven with a series 30milliOhm current sense resistor. Running a full 3A
through it will give us 90mV. This is too small to feed into the
feedback pin (which for the TPS61088, has a comparator reference of
1.2V), so all we need to do is to amplify this.
I used a
simple non-inverting op-amp topology which its output connected to the
FB pin, and the gain controlled via a digital SPI-based potentiometer. By hooking this up to a microcontroller, we
can adjust the gain of the amplifier on the fly, and therefore use this
method to create true constant-current regulation.
With a microcontroller on board, we can control the
brightness of the LED by changing the gain of the amplifier, and we can
also use to to create some interesting effects like a 'candle-flicker'
or 'fireplace' mode. This can be coupled together with
the EN pin of the TPS61088 to also create flashing effects, all without
requiring an additional FET for PWM brightness control, saving us BOM
cost and PCB space.
The main driver board needs to
contain the boost converter, associated passives, and a microcontroller. PCB space is at a premium
and we need to be able to fit everything in a really small area.
I searched for some of the smallest possible micro-controller units
(MCUs) I could find on the market, which where hobbyist-friendly and still possible to
hand-solder without reflow (i.e. not BGA, which eliminates a lot of very
nice options). Regardless, the Atmel ATtiny13 Micro-Controller Unit
(MCU) comes to mind, and comes in a 3x3mm QFN package.
The ATtiny13 comes in a diminutive
3x3mm 10 pin 'QFN/MLF' package, and not only works down to 2.7V (1.8V
for the V version), but also has a small amount of EEPROM for memory
functions, and has a 4 channel ADC. It's also compatible with the
hugely popular Arduino
environment. There is in fact another smaller ATtiny version - the
ATtiny4/5/9/10 coming in with a very small SOT23-6 or 2x2mm UDFN
package, but unfortunately lacks EEPROM. (The ATtiny45/85 is another IC
I'm quite fond of but unfortunately it's smallest package is a 4x4mm QFN).
But considering that the smallest ATtiny13 comes in a 3x3mm
I thought I'd do another quick search to see if Atmel offered any other
controllers in a QFN package of the same size, and I found that the more
had exactly what I needed! (Note the A version is an identical drop-in
replacement of the non-A version, but the A comes in a few power
improvements with new manufacturing processes, and importantly comes in
a small 3x3mm package which the non-A versions don't).
Using the 20M2 VQFN package gives us the same 3x3mm
footprint as the ATtiny13, but has an additional 5 GPIO pins to use, as well as several more
features over the ATtiny13 (though we probably won't need to be using
them). I arbitrarily decided on some pin functions and wrote them down
to help in schematic capture:
PA6 - ISP Programming MOSI & Master Out for SPI
PA5 - ISP Programming MISO
PB3 - ISP Programming /Reset
PA4 - ISP Programming SCK & CLK for SPI
VCC - 3V from LDO Voltage Regulator
GND - Ground
PA7 - CS for SPI
PA3 - Boost Converter Enable
PA2 / ADC2 - RC Network for momentary mode change
PA1 / ADC1 - Thermistor / Temperature Monitor
PA0 / ADC0 - Battery Voltage Monitor
For PA3, we'll use this for toggling the enable pin on
the TPS61088 high or low. The TPS61088 has a 1.2V threshold so it will
work well with the 3V logic level we will be using for the ATtiny84. CS,
MOSI and SCK will be connected to a SPI digital potentiometer adjusting
the gain of our amplifier. ADC0 can be wired to a resistor divider to
monitor battery voltage to make sure it does not fall dangerously low.
Finally, ADC2 can be used during start-up to sense the voltage at a pin
connected to a large RC network. This node will be charged up to logic
high when the light is running. When power is turned off, a capacitor
will store charge for a while, discharging slowly through a resistor. If
the microcontroller is started up within say ~1s of turning off, it will
see a high voltage here during start-up, and then change the flashlight
to the next mode. The modes of the LED can then be stored in EEPROM for
memory. This feature is known as OTC (Off-Time Capacitor).
PCB Design (v1)
With the main design and schematic completed, I proceeded with
PCB layout. (Note - Version 1 has been replaced with V2, scroll
down to see that in detail).
I started off by doing some careful measurements of how the
driver would need to fit into the SK98 flashlight host and settled on
the main dimensions. The SK98 requires a 20mm-diameter driver,
so I decided on a 19.9mm PCB diameter to allow an easy fit with
minimal filing required. Heat-sinking and ground connection is accomplished
via a threaded retaining ring, so I saw no need for a tight
Next I created a 1mm border ring around the board as the main
ground connection to the case of the flashlight, and placed a
~8.5mm diameter truncated pad in the middle for the main spring
connection to the battery. With the main constraints in place, I
could then drop in all the components and begin PCB routing.
Ideally I'd like to do a 4 layer board with ~2oz copper
thickness, but I
tried to do the best I could to fit everything in 2 layers, and
wound up with the above result with a reasonable minimum 6mil
trace and 5mil spacing.
To increase current carrying capacity,
some of the large traces also have solder mask removed to allow
for additional solder. Finally I was able to use no smaller than
0402 components, which should make DIY soldering a little easier
(the tiny QFN packages are more difficult to solder
but should still doable by hand under magnification).
After a day of work I was mostly done. The result for the top
and bottom of the boards looks a bit like this:
A little bit of a squeeze, but I there's actually still quite a
bit of space left on the board!
I could probably do a bit better but I was quite happy with the
result. Some layout considerations include placing the NTC
thermistor as close as possible
to the edge ground ring for best thermal contact with the host
(since I expect the LED to be the main heat source, not the
driver), as well adding a
50mil 6-pin header-pad for programming the ATtiny84A
microcontroller (this is in fact a regular 6-pin AVR ISP header
but half sized). The main power electronics are on one side and
the logic is on the other, with care taken to avoid high
current loops being near the controller and peripherals (but it
was difficult to completely avoid this without going
It's always nerve wrecking to check through the PCB to make sure
there are no errors but after looking through the board for a
while, I sent it off for fabrication and hoped I didn't make any
mistakes too big to fix!
PCB Design (v2)
After putting together V1 of the GXB20, I plugged it in and
fortunately everything worked exactly as intended! However there
were a few things that needed to be tweaked and improvements to
be made. Some of these were:
- I wasn't too happy with the layout and felt that it
could be improved a fair bit
- The variable gain amplifier suffered from high wiper resistance when
the battery voltage dropped, something I overlooked during design
- The variable gain amplifier had a fairly small range due to the way I
had it configured (less than 256
- Current measurement was low-side instead of high (though I suppose
nothing really wrong with this in this application)
- Programming via the 6-pin header was a little tedious
- Wanted to add more features including E-switch operation and mode
As a result, I made a few modifications.
The power side stayed mostly the same, but I added a new
high-side current monitor, tweaked the current amplifier
feedback system, replaced with digital potentiometer to a
better, smaller device with an internal boost, added a new 0.4mm SlimStack programming header, and completely re-did the layout
for even better performance.
The result is GXB20 V2.
With the new PCB completed, I sent it off for fabrication! I was
quite happy with the layout and was happy with how much tidier
V2 was compared to the first version.
Scroll to the next section to see how it turned out. :-)
PCB Assembly and Testing
Assembling the PCB (v1)
With the PCB layout done, I sent the boards for
fabrication and they all came in good order. The smallest component on
the board here are 0402 passives, which are fortunately still solderable
by hand with very small soldering irons tips and needle tweezers.
The first GXB20 driver came together without too much
difficulty. Ideally I would be reflowing the board but the
component count was low enough that I decided to do this by
hand with a soldering iron The only component which I soldered with hot air was the
TPS61088 which required a good bond to the bottom thermal pad.
With the driver assembled, I programmed it with my initial firmware, and assembled
it into the SK98 host.
The good thing about the SK98 flashlight is that the LED shelf
is just about large enough to accept the commonly
available 20mm 'star' Metal-core PCBs. I purchased a few excellent copper SinkPAD-II brand MCPCBs and reflowed some CREE XHP50 LEDs I
bought on Mouser. These MCPCBs are particularly high performance
with a direct copper bond from the center heat-tab to the MCPCB
core, but requires a lot of heat to solder. Soldering was done
using the technique I described above.
And... it works! The GXB20 driver is working!
Assembling the PCB (v2)
After testing out the GBX20 V1, a few changes were made
as described above and the board was completely redone for better layout and with an
updated design. New PCBs were ordered and I assembled a few of them for
Having had more experience, the GXB20 V2 drivers came together
more quickly and I was very happy with the result! Now it's time
to put it to the bench and to see if it actually performs.
Fortunately there were no electrical errors on the PCB but I did
mess up the silkscreen and the driver mistakenly says "3V 6A"
when it should say "6V 3A". Besides the layout changes, note the
addition of a new 6-pin MicroStax header for programming (this
turned out to be a non-ideal choice!), a new high-side current
monitor, a smaller digital potentiometer and better layout.
Above is a side view of how the driver looks like.
The new driver requires a small tweak in the original firmware
since the digi-pot was changed, but I got it up and running in
no time, and everything worked perfectly!
Like before, the GXB20 V2 driver was assembled into a SK98 host. Note the use
of Arctic Silver Ceramique thermal paste between the MCPCB and the
aluminium pill. The result is a new GXB20 V2 flashlight with a
90CRI CREE XHP50 LED - almost 2000lm of light from a single
18650 in a compact form factor!
In order to test the GXB20 driver, I needed a way to program the
ATtiny84A on board. The Atmel AVR microcontrollers are easily
programmed via the AVR ISP 2 programmer, which comes as a
standard 6-pin 0.1" pitch header. There's no space on board to
mount a 6-pin header of that size. Instead, I populated the
board with a 0.05" 6-pin pad (for the GXB20 V1).
Next, I soldered up a
simple 6-pin interposer consisting of a 0.1" 6-pin male header
to a 0.05" 6-pin female header. I also soldered a 0.05" male header
to the GXB20 V1 PCB (temporarily). This then allows me to program the
board easily, especially for debugging. Once I've figured out
the firmware, subsequent programming of the boards can be done
really easily by just pressing the programming header to the
pads and programming it the usual way.
Above you can see how programming the GXB20 V1 works. The
problem though was that the 0.05" header needed to be soldered
on and off each time I wanted to do any sort of development work
on the driver, and this got annoying real quickly!
For the GXB20 V2, I used a 0.4mm SlimStack header instead! This
allowed me to program the board much more easily. However I
needed to spin another small PCB which would allow breakout from
the corresponding 0.4mm female header, to the standard 0.1"
6-pin header of the AVR programmer.
To help ease programming, notice I also modified by AVRISP
header to output 5V or 3.3V on the VCC pin, powered via the USB
bus. However, care must be taken when powering the board as
such, because there is reverse diode protection across the
TLV700 LDO regulator, which then feeds directly to the boost
circuit. Therefore some sort of current limiting may be required
when programming in this method. The usual method of powering on
the driver via battery is of course the recommended method in
The first thing I did was to verify basic functionality - can
the driver turn the LED on? Can the driver control the LED
brightness via adjustable current limiting? If these two basic
requirements are met, we can then test for low-battery cut-out,
over-temperature, as well as off-time mode memory. I'm
definitely not the best firmware writer but I quite clobbered
together a quick firmware and achieved my desired operating
functionality! The GXB20 is complete!
How does the driver do in real life? To find out, I set
up the driver on my bench and conducted some tests.
One test was conducted as follows. The GXB20 driver was hooked up to a
Agilent power supply. In this case I
conducted the test at 3.7V input across the range of output levels. The driver was also tested
at a variety of input voltage from 4.5V to <3V but less
Input currents and voltages was measured via Kelvin
terminals to avoid errors due to lead resistances. Output current and
voltages were also measured to determine total driver efficiency across
a range of power levels.
Above shows a photo of the test in action. The XHP50 LED
was mounted on a 20mm star heatsink, which was in turn mounted on a big
heatsink to keep it cool during tests. As a side result, the data may
have some slight inaccuracies as the LED die temp rises, but I really don't have
so much time to do a full datasheet-style test, and the measurements
should give us generally decent real-life results.
Here's an example scope shot showing the driver
output at 5 different and arbitrary power levels. This was achieved by configuring the
on-board MCU to run a 'test program' during startup at different power
levels for 1 second each. Cyan shows output voltage, and yellow shows
voltage across the 30mR current sense resistor. Regulation is fairly clean,
as expected with the very high switching frequency.
Test across 20 current
Test of LED current (blue) and datasheet light output (orange) against
20 different current levels, as well as overall driver efficiency
I also spent some time and conducted a little more
reasonable test at 20 arbitrary but equally-spaced constant current
In this test, the maximum power tested was a massive
22.4W into the driver, driving the XHP50 LED at a measured 3053mA. At
this power level, just putting my finger near the LED light feels very
hot! Based on the specific bin of 90CRI XHP50 I had on hand, this translates
to a datasheet lumen output of around ~1700 lumens. Lower CRI bins less
than this 90+CRI one used should yield close to 2000 lumens.
mentioned, heat-sinking becomes very critical at these levels. Maximum
efficiency was measured at around 98+% efficiency at lower currents of
around 750mA out. This drops gradually to 87% or so at 3A output, meaning
about 2W dissipated in the driver itself.
Most of the heatsinking of the
driver occurs through the ground ring around the driver so mounting it
in a host properly is also critical. Increased temperature after longer
runs will certainly affect the efficiency. In all, the driver seemed to
regulate current just fine all the way up to the maximum 3A load. Again,
note that these tests were done at a 3.7V constant voltage input.
Finally, keep in mind that this efficiency number should probably
be treated more as a ballpark figure. I expect the overall efficiency to
vary significantly in various host configurations since there probably is non-trivial ohmic losses in connection
points / spring. A a properly seated driver in a good host with a good
switch should do much better than my cheap SK98 host. In addition, the
efficiency measured above was the total
system efficiency - driver efficiency should be a little higher (esp on
the high currents), since some power is lost in the wires and
connections of the test set-up.
Overall, the efficiency graph shape looks roughly in-line with
simulation I did with the TI Webench tool, so I was at least reasonably
confident with my test results!
In conclusion, the almost 90% or greater efficiency
overall was a good result :-).
The proof of the pudding is in the tasting - did the
GXB20 live up to its intended designed purpose? Let's compare them to
some existing lights I have to find out.
The goal of the project was to modify a cheap flashlight
to accept a powerful and superior LED, and to develop a driver capable
enough to drive it. I wanted to
find a way to characterize and compare the results. Unfortunately I do
not own a light-meter nor can I quantitatively measure the quality (CRI)
of the light from a flashlight, so what I've done instead is to take a
series of photographs to determine the brightness of the new GXB20 SK98
Photos were taken with a digital camera on manual mode,
set at a 35mm focal length with an aperture of F3.6, ISO fixed at 400, white balance
set to Daylight (6500k), and an exposure time of 1/60 in dark
room. All zoom-able flashlights were zoomed all the way out. Batteries
were fully charged.
Let's take a look at how the GXB20-enabled flashlight performs:
Test above numbered from top left to bottom right
|E44320 Greenlite LED Bulb
|CREE 2016 15W LED Bulb
|SK68-3W AA LED Flashlight
|SK98 UltraFire Flashlight Un-modified
|Modified SK98 UltraFire modified with Cree XML2 LED,
|Modified SK98 UltraFire with
GXB20 V1 and XHP50A
|LatticeBright 'XML-T6'' LED ~>7000K?
|XMLBWT-00-0000-000HT30E7 80CRI 3000K
|XHP50A 90CRI 4000K LED (G2-Bin)
|Eneloop HR-3UTG AA
|LGDBHE21865 20A 18650 Lithium Ion
|LGDBHE21865 20A 18650 Lithium Ion
|LGDBHE21865 20A 18650 Lithium Ion
| Input Voltage
| Input Current
|800lm on paper
|1700lm on paper
I compared the two SK98 18650-battery flashlights with each
other - one with the original Lattice Bright LED - and another which I
had modified with a genuine 3000K 90CRI XML LED from Cree, along with my
old but trusty (and comparatively dim) SK68 single AA flashlight. These were
compared against regular 800lm and 1700lm LED bulbs, and of course the
All batteries were charged to full prior to the
photos being taken (e.g. 18650 charged to 4.2V open-circuit voltage, and
likewise ~1.4V for the NiMH Eneloop). All voltage and current
measurements were conducted using 4-wire terminal method for accurate
data, and mains bulbs were measured using a Kill-A-Watt meter.
You can judge the images for yourself; but what stood out
to me (and via looking at the histograms) was that the genuine XML2 LED
was slightly brighter than the Lattice Bright LED by a hair, but they
all paled in comparison to the modified SK98 with a GXB20 V1 driver and
a G2-bin CRI90 XHP50 LED! The project is indeed a success!
In addition, I did another test by zooming the
flashlights all the way in (since all of them were zoomy lights) and to compare how bright they are at maximum
throw. The flashlights were placed at the same distance from the wall.
was certainly quite dim, but it was able to make a tighter and still
fairly intense beam due to the smaller die size. It's interesting to see how the XML2 die is in fact quite a bit larger
than the Lattice-bright LED die, so the LB flashlight was in fact able
to create a beam of higher luminous intensity.
While the first two (left) LEDs have a fairly even
phosphor coating, it's clear that these two Cree LEDs (XML2 and XHP50A)
have a non-homogenous one evident by the speckle pattern. The 4-die
arrangement of the XHP50A is very obvious - it's an interesting pattern,
but perhaps not something desired by all. Finally, the latter 3 beam
shots give a very good size comparison of the die size, since they're
all fitted in the same host with the same optical set-up! This means
that while the XHP50 is the brightest by far, the beam intensity may not
be as bright as one would expect at max zoom, due to the much larger
Overall, the GXB20 driver project is a success!
Future thoughts - put in a XHP50.2 or Nichia 144 LED and
pair it with a GXB20 when I get my hands on those LEDs!
Firmware, Modes and Real Life Usage
Having completed the GXB20 in early 2017, I've since
brought it out in the field for outdoor use and am pleased to say that it has met my
expectations! I've also settled on an -ok- firmware which has
served me well over the past few months and seems to work w.r.t. the safety and
Above is the firmware modes and I hope the chart is
Overall I'm very happy with how this project turned out!
I certainly never made the GXB20 driver to be commercial in mind, and
really was a just-for-fun project and for me to try my hand at creating
something fairly tricky (i.e. a high power DC-DC converter in a very
small package). I've learned a lot from this project, and it's really a
bonus that I get a few really high-performance flashlights as a bonus!
BudgetLightForums - Thread of this project as it progressed
Jaxman E2 -
CPF) A flashlight with very high CRI 5700K Nichia
Fasttech - SK68 AA Flashlight for $3.85 and free shipping!
TEC40 - The
updated version of the TEC40 I originally had, updated with a
Digikey - A contender for an excellent linear regulator
Maxim - Reverse current protection
- Copper MCPCBs
Fonarevka - Russian Boost Driver
Operational Smoke - DIY Microscope
Gough Lui - Philips Bulb Teardown
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(c) Gao Guangyan 2024
Contact: loneoceans [at] gmail [dot] com