500mA, is a lot. You would be drawing over 4A, at the 3v rail. Driving a transistor efficiently with only a 3v supply, from a NE555, will be hard, and if not done efficiently, your consumption will shoot up... If the 'about 500mA', is on the generous side, then look at the National LM2623A. This will do about 400mA, and will be far more efficient than the discrete solution. This will deliver 300mA, with something around 85% efficiency, with the right inductor/capacitor/diode.
Best Wishes
There are lots of MOSFET's that could do this. One example is the Zetex ZXMN2A02N8: 40 milliohms at 2.5 volt gate drive with Id = 8.4 amps. STMicro also has some suitable parts that may be more readily available in Europe and the UK, don't know about Oz.
Hi All,
I am looking for a way to boost 3v to 12V unregulated at about 500mA
I was thinking pf a NE555 time, switching transistor and inductor and diode
Any ideas would be appreciated.
JG
Yes. However you have the problem of charging and discharging the gate efficiently, and quickly. If the system is going to be a reasonable size, the inductor/capacitor, needs to be a fairly low value. This implies a fairly high switching frequency, and charging/discharging a gate capacitance of perhaps 500 to 1000pF, at perhaps 500KHz, is not trivial. The circuit complexity starts to 'creep up'. Given the cost of the 2623, provided it'll do what is needed, it is going to be much the easiest solution.
Best Wishes
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