Can someone help me in answering questions I have involving voltage regs.
1) If you have a vcc of 18v and you need two different voltages in your circuit of 5v and 3.3v, can you have the vcc feed the 5v reg and then the 5v reg feed the 3.3v reg OR is it better to run EACH reg from the VCC of 18v? I was thinking the first option would be better as going from 5-3.3v would create less heat and allow a smaller package.
2) Can someone describe to me the proper way to choose a volt reg as the last time I did I did not take into consideration the junction temperature, ambient temp, theta ja, etc but only the input voltage, output voltage and the output current. Needless to say my regulator package heated up pretty quickly( lessons learned as my motto is becoming " fail and fail often" in my learning process.)and went into thermal shut down.
3) Do I need to derate the voltage regulator any? In my circuit on the 5v side I used a 250mA reg and had a current useage of about 220mA, I thought this would be ok but I am not sure. As is on the 3.3v side I used a 3.3v regulator rated at 30Ma and needed only average of 17mA draw.
4) I was using linear regulators for my 5v and 3.3v regulators. I am looking for the smallest package I can as the pcb is less than one inch in square and I put my voltage regs on the bottom layer to keep them away from my microcontroller and other ics that are on the top layer.
Any direction would be appreciated. Thanks for your help on my learning as it is a vast topic to digest.
Val
1) If you have a vcc of 18v and you need two different voltages in your circuit of 5v and 3.3v, can you have the vcc feed the 5v reg and then the 5v reg feed the 3.3v reg OR is it better to run EACH reg from the VCC of 18v? I was thinking the first option would be better as going from 5-3.3v would create less heat and allow a smaller package.
2) Can someone describe to me the proper way to choose a volt reg as the last time I did I did not take into consideration the junction temperature, ambient temp, theta ja, etc but only the input voltage, output voltage and the output current. Needless to say my regulator package heated up pretty quickly( lessons learned as my motto is becoming " fail and fail often" in my learning process.)and went into thermal shut down.
3) Do I need to derate the voltage regulator any? In my circuit on the 5v side I used a 250mA reg and had a current useage of about 220mA, I thought this would be ok but I am not sure. As is on the 3.3v side I used a 3.3v regulator rated at 30Ma and needed only average of 17mA draw.
4) I was using linear regulators for my 5v and 3.3v regulators. I am looking for the smallest package I can as the pcb is less than one inch in square and I put my voltage regs on the bottom layer to keep them away from my microcontroller and other ics that are on the top layer.
Any direction would be appreciated. Thanks for your help on my learning as it is a vast topic to digest.
Val