LTC3490 Boost IC Q's...

Would it be a good idea to parallel a 0.1ohm resistor with the internal sense resistor(by connecting the external one in the CAP and LED pins) to double the output current? If it works, it'll make for one hell of constant-current boost IC. A simple, dimmable, 350-700ma output current boost with 1-2 cells. :naughty:

There's no mention on the specs of the internal FET's so I'm hesistant on trying it. I already ruined an IC due to the inductor coil(the insulation got nicked) shorting out with the output pin.

The IC details can be found here.

legtu said:

Would it be a good idea to parallel a 0.1ohm resistor with the internal sense resistor(by connecting the external one in the CAP and LED pins) to double the output current? If it works, it'll make for one hell of constant-current boost IC. A simple, dimmable, 350-700ma output current boost with 1-2 cells. :naughty:

There's no mention on the specs of the internal FET's so I'm hesistant on trying it. I already ruined an IC due to the inductor coil(the insulation got nicked) shorting out with the output pin.

The IC details can be found here.

Click to expand...

Internal FETs, 0.1 ohms and 0.13 ohms. If you heatsink the thermal pad very well, you might be fine, but not in the SO-8 package. There is still a input current limit that is separate, you might run into it. A fella might consider an inductor with a larger than normal value, to keep the peak current down, which will help with the input current limit circuit.

I might try it one of these days if I can find a local shop that carries 0.1 - 0.4ohm sense resistors.

Good IC heatsinking and beefier inductor, check! Thanks for the help.

Something to keep in mind if you are planning on running these with the extra sense resistor on a single cell.

Recall that Power Out = Power In - Efficiency Losses

For a Luxeon with a Vf of 3.5V @ 700mA, Power Out = 2.45 Watts

Assuming a converter efficiency of 75%, that means 3.27 Watts In.

A single NiMH AA will see the battery voltage sag to around 1.1Volts at that power level. That equates to about 3 Amps required from the battery! The run-time will be very short. See SilverFox's Battery shootout for an idea of runtime.

I put one of these (without the extra sense resistor) into an ArcAAA head with a TW0H emitter. It gets pretty hot and runtime is around 15 minutes.

Paul

Has anyone tested this yet? Maybe just try to raise the output to 500ma. I'm curious if adding the resistor will increase the output limit.

Update:

Tried placing a 0.25ohm(4x1ohm) resistor in the CAP and LED pins and the output current increased to ~500ma with the efficiency @ ~60%. There's a slight prob though, the heat generated by the IC is too much even for only a few seconds of runtime. I was able to confirm the LTC3490's overtemp protection 'cause it shutdowns itself whenever it gets too hot. :laughing: The IC would turn itself back on when the temp goes down and off again when it overheats causing the light to blink faster and faster as it cycles on and off. It's a 'nice' feature that informs me to turn off the light and let it cool down.

I removed the external sense resistors and the IC is back, running normal. It's a tough 'lil bugger...