On 4/22/06, Jinx wrote: > I'm trying out this MC34063 circuit, taken from > > http://www.onsemi.com/pub/Collateral/AN920-D.PDF > > Fig18, Step Up/Down (which works as advertised) > > using the calculations below (alternative values for 100mA are in > parentheses). The question I have is about Ipksw. In the 34063 > datasheet that's stated as 1.5A. Now, if I increase L(min) from > 127uH to at least (2.95/1.5)*127 = 250uH, that would lower > Ipksw to 1.5A > > BUT - what I'm not sure about what Ipksw applies to. Would it > be Q1, which is internal to the 34063 ? If so, then using 127uH > as the inductor will take out Q1.......???? and using > 250uH > would be the sensible thing to do The peak current will apply to both the external transistor and the internal Q1. Here is a short analysis of the circuit. I will call the external transistor TIP42C as Q3. I will also call the diodes D1 and D2. Read page 17 of the application notes as well. Q3 and Q1 are on at the same time. When they are on, the two diodes are off. The voltage across the inductor is Vin. Here we assume that all the components are idea. The inductor current will ramp up with the rate of Vin/L and reaches the peak curret at Ton. When the switches are off, D1 and D2 conduct. The voltage across the inductor is -Vout. The inductor current will ramp down at the rate of -Vout/L. In steady state, the voltage second balance needs to hold. Assume that the inductor will have continuous current (so called continuous conduction mode or CCM), we will have the following equation. (Vin/L) *Ton = (Vout/L) * Toff ==> Vout/vin =Ton/Toff =d/(1-d) d= duty cycle = Ton/T So you can see that the current through Q1 and Q3 are the same since they conduct at the same time. You can do some further analysis under CCM for the relations of the current once you draw the voltage and current waveform. The steady-state analysis of the basic converter topologies is relatively easy. The following URL is the book slide for the book "Fundamentals of Power Electronics". http://ece-www.colorado.edu/~pwrelect/book/slides/slidedir.html Regards, Xiaofan -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist