From: "Dave Dilatush" said Actually, that circuit really isn't a bad starting point for a micropower switcher, even though it was designed for MUCH higher output currents than we're talking about. I set about revising it for low-power operation (optimized for 5 volts out at 20-50 mA) and low cost, and the result is shown below in ASCII-art form, followed by a parts list. I changed several things from the original design you showed: ............. _______________________________ I especially like the extra transistor Q3 which acts as a high side driver. Similar to my slightly more complex design. Use of a long tailed pair (as per NatSemi & your circuit) gives a more conventional and 'understandable" circuit. It would be worth building to see how it compares in practice. In terms of construction (number of parts, PCB area etc) its slightly worse than mine but may be worth doing. It's good to see the ideas that are coming out of this thread. Russell McMahon ________________________________________________________________ First, I ditched the Darlington-connected power switch; it's not necessary at low current, and it has high power losses both static and switching. Next, I replaced the LM336 precision voltage reference with an ordinary 4.7 volt Zener diode. This causes some degradation in output voltage regulation and accuracy, but for powering digital circuits it should still be adequate. I also scaled the circuit resistance values to reduce bias currents, which helps boost the efficiency at low output currents. And finally, I added a transistor (Q3 in the diagram below) to shunt base drive away from the main power switch (Q4) during turnoff to increase its switching speed. This reduces switching losses and improves efficiency significantly. The final design is shown below: 6 resistors, 1 inductor, one aluminum electrolytic cap, one Zener diode and one switching diode, and four "jelly-bean" transistors. Parts list and design notes follow the schematic. Q4 Vin o--+------+-------+---E C--+--L1--+---o Vout | | | B | | R1 | R4 | | | | E | | | | | Q3 B------+ | | | | C | | | | | | | | | | | +------------+ | | | | | | | +--R2-------------------+ | | | | | | | C C | | +-----B Q1 Q2 B--+-------R6--+ | E E | | | | | + | | | | +---+---+ | | | c | | c | D1 R3 R5 D2 C1 a | | a | | | | | | COM o--+----------+-------+----+------+---o COM C1 = 47uF 10WVDC Panasonic ECE-A1AN470U D1 = 1N4732A Zener 4.7V 5% D2 = 1N4148 L1 = 680uH TOKO 187LY-681J R1 = 10K 5% (for Vin = 15V) R2 = 22K 5% R3 = R4 = 2.2K 5% R5 = 10K 5% R6 = 1.0K 5% Q1 = Q2 = 2N3904, 2N2222, etc. Q3 = Q4 = 2N3906, 2N2905, etc. NOTES: R1 should be chosen to give approximately 1 milliamp of bias current for Zener diode D1: 7k ohms at Vin = 12V, 10k at 15V, 20K at 25V, and so forth. Insufficient Zener current will degrade regulation and increase noise. Excessive Zener current will just waste power. R6 and R5 set the output voltage (in this case, slightly above D1's Zener voltage). The values chosen should give about 5 volts out, and this could be trimmed if desired by making R6 a trimpot (ugh!). Output voltage will be roughly Vout = 4.7 * (R5 + R6) / R5. Changing catch diode D2 to a Schottky device (e.g., 1N5819 or similar) will increase efficiency at higher power levels, due to lower forward voltage drop. For lowest cost, stick with a 1N4148. Absolutely DO NOT substitute a 1N4001 or other power rectifier meant for mains-frequency operation: these have HUGE reverse-recovery times which drastically reduce efficiency. One short-cut I took in this circuit is that the hysteresis which governs the switching frequency depends on Zener diode D1 having a few hundred ohms of Zener impedance; the voltage divider formed by the Zener impedance and resistor R2, along with the input voltage, determines the hysteresis. If a precision reference (e.g., an LM236-5.0) is substituted for the 1N4732A for better regulation, a resistor will have to be inserted between D1 and the base of Q1, and R2 also connected to Q1's base. Otherwise, the circuit will operate at a very high frequency, and have poor efficiency. As shown, the regulator should give somewhere around 70% efficiency with 12 volts in and 30 mA output current, dropping to around 50% with 25 volts in. For the transistors chosen, maximum output current is around 50 mA. Few of my PIC projects take more than that. Enjoy... Dave -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.