Russell McMahon wrote: > Challenge: > > Convert low voltage DC such as battery or 3v or 5v rail > to "somewhat higher" dc voltage at modest power levels > for e.g. processor operation, FET gate drive, LED drive, > Op-Amp supply, Programmer Vpp supply, and more. I have a requirement to take the output of a Ni-MH battery pack that has a terminal voltage of 3-4 volts and convert this with good efficiency to 5V at up to 100 mA. I came up with a 2-transistor circuit that I posted on 10/1 (reproduced below). It's remarkably similar to your circuit, with the addition of output voltage regulation. I achieved regulation by sourcing the emitter of the PNP from a fixed reference. DC feeback from the output node keeps the transistors turned off whenever the output voltage rises too high. In order to keep the reference current low, I use a "capacitor discharge" circuit to provide the base current for the NPN. The combination of falling base current and rising coil current initiates the switch-off, which is then sped up by positive feeback through the 1nF capacitor. Efficiency is good, but not great. With no load (i.e., just the feedback divider, ~0.3 mA), the circuit draws about 4 mA. About half of that is the reference circuit. With moderate loads (10-30 mA), I'm getting about 55-60% efficiency, which seems low. I'm using a cheap choke that has about 3 ohms resistance, and the peak coil current is 200 mA. I figure that accounts for at least half of my losses. diode 3-4 V o----+------------LLLLL---+---AK----+---+----o 5V @ 100 mA | 68 uH | | |+ R +------+ R --- 100uF 1000 R | .001| R --- R PNP | --- R | | 100 C --- | V +--+---E C---RRRRR--B NPN | | Gnd | | B E +--+ LM336 Z --- | | | | -2.5 Z --- | V | R Z |.047| 2200 Gnd | R | V +---RRRRR-----------+ R V Gnd | Gnd V Gnd I'm going to try the following circuit next. I've added an emitter follower to reduce the quiescent current of the reference supply, eliminated the capacitor that discharges into the base of the main switch, and added a diode that disconnects the output voltage feedback during the switching cycle. Also, I now have a much better coil: a 47 uH toroid with just 50 milliohms resistance. diode 3-4 V o----+---+-------LLLLL---+----AK------+---+----o 5V @ 100 mA | | 47 uH | | |+ R | +-----+ R --- 100uF 12K R | | 330p| R --- R |NPN | --- R | | C | --- | V +--B PNP 390 C | | Gnd | E--E C--RRRRR--B NPN +--AK--+ LM336 Z B E |diode | -2.5 Z | | | R Z | V | R | | 68K Gnd | R V +---RRRRR---------+ | Gnd V Gnd However, for really high efficiency and low parts count, it's hard to beat ICs like the Maxim MAX1678 ($1.89 in 100s from Digi-Key), which can maintain 80% efficiency all the way down to 1 mA of load current in this application. I got a couple of samples and will be testing it out soon as well. -- Dave Tweed -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.