Hi Ryan, The role of these low value resistors is to sense the peak inductor current within each operating cycle of the circuit. If the inductor, and hence the switching trransistor current Q1, current exceeds some value determined by these resistors, then the current cycle is quickly terminated.
The voltage threshold the controller IC is using is 300mV on pin 7 below the supply voltage on pin 6 (V+). Therefore, the current trip point is simply 0.3Volts divided by the effective resistane value (if it is a network of resistors). This also appears in the datasheet. For some reason on the linked schematic the tank/filter capacitor C1 is connected after the current sensing resistors. As a result the resistors are no longer measuing the current peak but some mean value close to the power input devided by the input voltage. I am not sure if this was done on purpose. Regarding your question about R3/R4, you can trim any value you wish to alter the mean output voltage. I would go for R4 as R3 can be a standard fixed resistor and you dont risk shorting the feedback pin to the output which could be at a high voltage. The equations posted are simplistic and might not hold true if you exceed the range you can cover of mean output voltages you can cover with the rest of the components in the circuit; a complete redesign might be needed. Having said this, you should be able to get away with the current components as you are still within the intended application. Regards, Alex On Sep 1, 5:48 am, Ryan McDonald <[email protected]> wrote: > Ok some questions > abouthttp://threeneurons.files.wordpress.com/2011/08/mc34063_mk15cm1.gif > > I plan to run the supply from 9v and need to get 180V at approx 13mA. > > How do I go about choosing how many of the 1ohm R's to use? Do the . > 9A, 1.2A, and 1.5A labels shown designate the required input current > from the supply? > > I completely understand the equations surrounding R3 and R4 for > determining Vout. Lets say I use the default values shown, but end up > with 185V requiring me to adjust R3/R4 to get my desired 180V. How > would I decide which R to adjust "first"? I don't want to just > coarsely adjust R3 then compensate by fine tuning via R4. It seems to > me this undereducated approach would lead to the circuit getting "out > of balance" between R3 and R4. Can someone help me understand how I > would go about adjusting R3 and R4 *together* to get the desired > Vout? Again, I do understand how to calculate Vout given a R3 and > R4. What I want is to compute R3 and R4 to attain a given Vout. > > Thanks! > -Ryan -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
