Hi all,
I've built a regulator circuit using the LM2679 Simple Switcher....my first time using switching circuitry. It was designed to deliver 2.7A into a resistive load at either 8 or 12.2V (controlled by a switch). The circuit works fine at low loads, but as soon I get the load above 1.5A, the voltage will very slowly increase and eventually will stop rising far short of the desired voltage, usually in the 5-6V range. Oddly, throwing the switch from 8 (startup) to 12V will not affect how high the voltage will rise, which seems to rule out problems in the feedback network.
Using a scope across the diode, I can see what looks to be a missing cycle every 2nd time the wave tries to go positive (switch turning on), with only a spike to indicate where it should. I found by accident that setting the scope probe to 1x, the voltages come up immediately (most of the time) and I see a nice square wave. I can also cure the problem just by touching a 100-330pF cap directly across the diode. When I scope this I can alternate seeing a nice square wave and full voltage output, to a missing cycle and slow rising voltage simply by touching or removing the cap.
My question is, what is causing this? Ground bounce? Poor parts choices? I do wonder about the diode that I chose...it has 800pF of capacitance and 4mA of reverse current. Given that the problem occurs on the positive going cycle (diode switching off, switch turning on) this seems to point to the diode but I don't know enough to be sure.
I'd prefer to get the circuit and layout right rather than to have to add a band-aid in the capacitor. Any help/teaching would be appreciated!
I've built a regulator circuit using the LM2679 Simple Switcher....my first time using switching circuitry. It was designed to deliver 2.7A into a resistive load at either 8 or 12.2V (controlled by a switch). The circuit works fine at low loads, but as soon I get the load above 1.5A, the voltage will very slowly increase and eventually will stop rising far short of the desired voltage, usually in the 5-6V range. Oddly, throwing the switch from 8 (startup) to 12V will not affect how high the voltage will rise, which seems to rule out problems in the feedback network.
Using a scope across the diode, I can see what looks to be a missing cycle every 2nd time the wave tries to go positive (switch turning on), with only a spike to indicate where it should. I found by accident that setting the scope probe to 1x, the voltages come up immediately (most of the time) and I see a nice square wave. I can also cure the problem just by touching a 100-330pF cap directly across the diode. When I scope this I can alternate seeing a nice square wave and full voltage output, to a missing cycle and slow rising voltage simply by touching or removing the cap.
My question is, what is causing this? Ground bounce? Poor parts choices? I do wonder about the diode that I chose...it has 800pF of capacitance and 4mA of reverse current. Given that the problem occurs on the positive going cycle (diode switching off, switch turning on) this seems to point to the diode but I don't know enough to be sure.
I'd prefer to get the circuit and layout right rather than to have to add a band-aid in the capacitor. Any help/teaching would be appreciated!