vrijdag 2 november 2012
A 555 based simple low power boost converter
boost converters to increase the voltage delivered by NiMH or solar cells. I played around with the TL499A which is a moderately priced IC, about 3€. It can convert from 1.1V upwards. A pricier modern alternative is the LT1073 for about 8€. If you just need to convert from 3V upwards then you're lucky, the MC34063A is available for 50 cents! Finally, an interesting 1.0V to 3.3V (or 5V) ready to use breakout board. When you just need to double the voltage and have modest current requirements (e.g. less than 10mA) then a charge pump voltage doubler is an option. The Intersil ICL7660S is a modern and efficient solution for 1.75€. You can also build one yourself, e.g. with a 555 timer IC, there exist plenty of references on the internet. Note that you loose about 0.8 volt, so 2.4V fed to the doubler, will result in about 4 volts. I used these doublers to trickle charge super capacitors or NiMHs from solar cells at dark locations. The cells deliver e.g. 0.5 mA at 2V. That is insufficient voltage to light a led, but by using a doubler you can charge a (super)capacitor/battery with 0.25mA to 3V, and use it to power your (AtTiny) micro controller, so it can flash the led. Note that if you build one yourself that you should use the CMOS 555 versions. The normal 555s will not run at these low voltages and use far too much power to be useful in low power applications. Ok, so you decided to build your own boost converter. The schema shown below is quite versatile. It will accept an input in the 2V to 12V range and the output can be in the 2V to 24V range. Note that you NEED a CMOS 555 for input voltages below 4.5V! Output currents in the 20-100 mA range should be no problem with the schema as shown. How does it work? The concept is simple, the 555 generates an on/off signal in the 100kHz range which is used to switch the Darlington transistor pair. As soon as the required output voltage is reached the 555 stops oscillating for a while because the BC547 pulls the 555 reset pin to the ground. FDN339AN which should be fine at 2.0V or even a bit lower! Note that the output Zener should not be used as a regulator when you want high efficiency. You should calculate the duty factor of the 555 and the Zener should only become active when the output voltage rises over the desired upper output voltage limit because of a very low output load. You can replace the Zener diode with a TL431 programmable voltage regulator as shown in the next schema if you want to experiment: calculations for converters. You really should study this if you want to adapt my example converter for optimal efficiency. If you want more power, an interesting schema for a 6 to 12V converter. You don't need a precise output voltage and have a square wave signal from eg a micro-controller? Look here. The MC34063A is your preferred solution? A handy calculator aid. Another simple converter circuit.