NVergunst
Member level 1
Hi,
I am using an LTC3853 which is a triple buck controller in my circuit. I have x2 5v outputs and 1 3.3v output. Vin is about 12v nominal, +/-3v on a bad day.
The one 5v output works perfectly. The second 5v output and the 3.3v output not so much. They make an audible click sound, and the inductor gets hot. The output waveform is a choppy wave slowly getting higher and higher then eventually just resetting my entire board. The layout is pretty decent and on a 4 layer PCB with 2oz copper. The main difference I see between my first 5v output and the other 2 outputs are the max current designed for. I designed the 5v that works around 1.5A maximum output. The 3.3v rail is in the 8A output range and the second 5v rail is in the 15A range. The FETs are all the same but should be able to handle that (did for ease of BOM selection).
I have changed the switching frequency, and the audible noise changes frequency as well. But it still exhibits the same problem from 200KHz to 700KHz (loose ranges).
I have changed the inductor from a big beefy inductor to a little one. The little one just makes one big "click" sound, overcurrents my powersupply, then resets the board. I changed it to another big inductor in a different inductor family that was a powdered core and stays clicking longer but gets burning hot and resets.
I should mention when I say "reset" I mean the stable 5v rail dies which then resets my microcontroller which controls the enable pins for the other outputs so it turns on waits a few seconds and tries to start back up again in a viscous cycle.
I am not sure where to troubleshoot from here.
I feel the PCB layout is adequate since I have 0 load. I would expect a crazy non-regulated waveform for a poor layout. But my layout is pretty decent with all feedback connections being kelvin connections and running together and away from switching paths.
It is hard to pinpoint where the noise is coming from, but I feel it is coming from the inductor. The fact that it gets really hot when in this sawtooth of death mode tells me that it is continuously trying to pump in the full amount of current and then nothing (or a very small amount) is being stored in the FET-off stage. Perhaps why it is slowly climbing, then the overcurrent protection of the LTC3853 activates and shuts it off?
I have desoldered all output capacitors and measured between the output and ground and got an open circuit (no load, no connection, as I would expect). I put on various known good capacitors (thinking the ones I had on there were insufficient to store the energy during the off-bits) and there was no marked effect.
Does anyone have any clues or suggestions on how to proceed from here? I have put the schematic of the 5v rail below, which is almost identical to the working rail with the only difference being the inductor itself.
**broken link removed**
Thankyou
I am using an LTC3853 which is a triple buck controller in my circuit. I have x2 5v outputs and 1 3.3v output. Vin is about 12v nominal, +/-3v on a bad day.
The one 5v output works perfectly. The second 5v output and the 3.3v output not so much. They make an audible click sound, and the inductor gets hot. The output waveform is a choppy wave slowly getting higher and higher then eventually just resetting my entire board. The layout is pretty decent and on a 4 layer PCB with 2oz copper. The main difference I see between my first 5v output and the other 2 outputs are the max current designed for. I designed the 5v that works around 1.5A maximum output. The 3.3v rail is in the 8A output range and the second 5v rail is in the 15A range. The FETs are all the same but should be able to handle that (did for ease of BOM selection).
I have changed the switching frequency, and the audible noise changes frequency as well. But it still exhibits the same problem from 200KHz to 700KHz (loose ranges).
I have changed the inductor from a big beefy inductor to a little one. The little one just makes one big "click" sound, overcurrents my powersupply, then resets the board. I changed it to another big inductor in a different inductor family that was a powdered core and stays clicking longer but gets burning hot and resets.
I should mention when I say "reset" I mean the stable 5v rail dies which then resets my microcontroller which controls the enable pins for the other outputs so it turns on waits a few seconds and tries to start back up again in a viscous cycle.
I am not sure where to troubleshoot from here.
I feel the PCB layout is adequate since I have 0 load. I would expect a crazy non-regulated waveform for a poor layout. But my layout is pretty decent with all feedback connections being kelvin connections and running together and away from switching paths.
It is hard to pinpoint where the noise is coming from, but I feel it is coming from the inductor. The fact that it gets really hot when in this sawtooth of death mode tells me that it is continuously trying to pump in the full amount of current and then nothing (or a very small amount) is being stored in the FET-off stage. Perhaps why it is slowly climbing, then the overcurrent protection of the LTC3853 activates and shuts it off?
I have desoldered all output capacitors and measured between the output and ground and got an open circuit (no load, no connection, as I would expect). I put on various known good capacitors (thinking the ones I had on there were insufficient to store the energy during the off-bits) and there was no marked effect.
Does anyone have any clues or suggestions on how to proceed from here? I have put the schematic of the 5v rail below, which is almost identical to the working rail with the only difference being the inductor itself.
**broken link removed**
Thankyou