Fatman driver's lowest input voltage?

[EatSleepRunBike]

The Shark converter found at the Shoppe says that the input voltage should not start below 1/2 the output voltage/Vf for proper regulation, and should not fall below 1/3 of the output. Is the same case for the Fatman at Taskled.com, or is the Fatman more efficient than the Shark at lower input voltages?

 

[.308]

From Taskled website:

Input voltage range is 2.7V to 12V



Examples on how to calculate battery/LED combinations:


Step 1: Determine output power

Power_output = Number_of_LEDS * Vf * Output_current

Step 2: Determine input power

Power_input = Power_output/efficiency

For calculation purposes we can assume efficiency will be around 90%

Step 3: Determine input current

Input_current = Power_input / Battery_voltage

Do this calculation for the lowest Battery_voltage you plan to run

Now, Input_current should be around 2A or less for optimal performance of Fatman. Fatman can run up to around 2.2A max. When running at high input power it is recommended to thermally epoxy the back of the Fatman PCB to a heatsink or the body of the flashlight.

Good Luck,
Chris
.308

 

[chris_m]

I've run my triple Cree at 500mA through the discharge of a single cell LiIon with a Fatman - ie down to 3V, which is less than 1/3 of the Vf at that current. It works, but isn't too efficient with that sort of use (<80% at a guess). I actually programmed a decreasing power curve in my uC, starting at 700mA when the cell was fully charged, but decreasing the power to limit the Fatman input current as the cell discharged - otherwise I would go outside spec - but dumped the whole idea when I saw how poor the efficiency was (with a double cell LiIon it is >90% efficient with this light).