LED Flashlight Again

I wrote about some LED flashlight that I built or purchased many years ago.   Recently the incandescent bulb on my Garrity flashlight burnt out.  I tried to find an LED replacement bulb.  At the price $3.5 a piece, it was disappointing: 0.3W@3V, 30 Lumens.  The LED bulb went from drawing 100mA at 3V to 7mA at 2.6V and is completely off at 2.4V.  That means it cannot be used with rechargeable NiMH batteries and the Li-Ion battery voltage is too high.  I decided to build my own LED replacement bulb (P13.5S type) again.  I want to run on both two NiMH cells or one Li-Ion cell (1.8-4.2V).  LTC3429 used previously can run on a Lithium-Ion battery but as a linear regulator with the efficiency in the 60s%.  

Here is a simpler solution.  We configure the circuit to be always boost.  We can take advantage of the built-in current limit of converters.  In LT1615, the switch current limit is 350mA.  We eliminate the Schottky diode; only an inductor and a capacitor are needed.  It works down to 1V (single alkaline cell).  Because LT1615 operates with a fixed off-time control, the switching frequency changes with the input voltage, from 200-300KHz. The average current through the LED varies from 85-140mA. The efficiency is around 80%.  It has as few components as it can possibly get.  The deficiency is that LT1615 is relatively expensive ($3 a piece at volume quantity) and its current limit is a little too low.

                                

Here is the actual implementation.