unboosty blue notes dim range ~cost
PWM-mode mod buck, 8-12V output
3 LED segments, 9.6V nominal
8:1 $12-17
  • Wide, customizable dimming range with continuous-drive mod
  • PCB layout more conducive to compact modding than original Unboosty
  • Requires modification to work with Thinkpad brightness controls
  • Narrow dimming range
  • Brightness can flicker with load

This appears to be another vendor's direct copy of the Unboosty... or is the Unboosty a copy of this kit? It's the same circuit (using the DF6113 boost controller IC [datasheet] as a low-side buck-down converter!), but on a different PCB using a smaller inductor and configured to put out lower current by default. It expects to drive a constant-current LED string at between 8 and 12 volts such as the usual 9.6V constant-current strips that can be trimmed in groups of three LEDs.

Like the Unboosty, this kit requires modification, such as the PWM mod below, to work with a Thinkpad. The modification process I've come up with for this board is slightly easier and more compact as compared to the original Unboosty. If you can solder SMD, this is currently my favorite kit.

Update and Warning: Like the original Unboosy, these boards can flicker noticably with the slightest shift in input voltage, eg with processor load, regardless of the mod used. There's no correcting the problem no matter how fancy the mod--- it's a consequence of lobotomizing a boost controller to use it in buck mode.

Above: Schematic of stock, unmodified "Unboosty-Blue" LED driver board

PWM-mode Mod

The PWM mod alters the driver board to use the Thinkpad's PWM brightness signal as a direct backlight switching control. The PWM frequency is 200Hz unless the OS changes it. If backlight flicker bothers you, definitely use one of my continuous-mode driver boards instead.

Above: Schematic of the Unboosty-Blue driver board modified for PWM-mode operation with a Thinkpad. Red marks new or altered components and connections, light gray shows removed connections and components.

parts needed

  1. 1N4148 diode (through-hole DO35 recommended)
  2. 1.2Ω resistor (SMD 1206 recommended)
  3. Low-profile 4.7uH power inductor, e.g., Bourns SRP5030T [if grafting onto a non-tablet inverter]

Specific package recommendations above are known to fit, but feel free to mix whatever surface mount or through-hole components are actually convenient. Resistor values should be 1% tolerance for best results.

modification process

  1. Remove C5 completely.
  2. Remove R3 completely.
  3. Replace current sense resistor R4 with a 1.2Ω value.
  4. Solder the cathode of a 1N4148 diode to the left hand pad of the former R3; solder the anode to either the lower exposed pad of the former C5 or to pin 4 of the controller IC.

Above: LED driver board after PWM mod. Changes from stock are circled in red.

After modding, the DIM input is now the board enable and the ENA input is now a PWM-compatible brightness input (they're mostly interchangeable really, but this arrangement is slightly preferable). The board will put out approximately 320mA at max brightness. Minimum brightness depends somewhat on PWM frequency; the PWM signal switches the chip enable and the converter has some turn-on lag. At the BIOS default of 60Hz, min brightness is about 40mA average for an adjustment range of 8:1.


If there's no space to stash the LED driver PCB somewhere in the lid (or if you simply find adding the extra board inelegant), the driver PCB as modified above can be trimmed as indicated below to fit directly onto a stock Thinkpad inverter board. We only need to move the output filtering capacitor first.

Above: Trimming the Unboosty-Blue board down to a size appropriate for grafting onto a Thinkpad inverter board requires relocating only the output capacitor (red arrow), then parting the board at the dotted red lines.

  1. Remove the output capacitor from the board and set it aside now; this makes later work a little easier.
  2. Scrape away enough blue conformal coating to make two new solder pads for relocating the output capacitor to just above the inductor L1. The pad on the right side should be long enough to extend all the way to the edge of the trimmed board as it will also serve as the LED- solder point. Consult the photo below for the correct location; note that it's not necessary to remove the inductor, I did so in the photo to make the traces and pad locations more visible.
  3. Also scrape clear a new pad just below and right of the inductor; this will serve as the LED+ solder point. Again, consult the photo below for location and size.

Above: The locations to clear solder pads when preparing the Unboosty-Blue board for trimming are circled in red. Inductor L1 has been removed for clarity; it's not necessary to remove the inductor to make the new pads.

  1. Solder the output capacitor into its new location above L1.
  2. Using diagonal cutters, part the left side of the PCB between the input connector and input capacitor.
  3. Part the right side of the PCB to the right of the inductor along white line belonging to the white silkscreened box for L1.

Above:Unboosty-Blue LED driver board with completed PWM mod, trimmed so that it can be soldered onto a Thinkpad inverter. This board has also been fitted with a low-profile inductor. Mouse over the image to highlight and label the connection points.

That's all the trimming necessary to fit onto a tablet-model inverter, but if we're adding this board to a non-tablet inverter, we'll need to file down the top and bottom edges by about a millimeter. It might also be necessary to swap for a lower-profile inductor. See the section about dimensional restrictions on the inverter page. Otherwise, the trimmed driver PCB is ready to be grafted onto an existing Thinkpad backlight inverter.


—Monty (monty@xiph.org) September 2, 2014