Hi Team,
We would like to seek your inputs on this.
Trying to tie in the RC pin to sync with another controller but is having an issue. without the sync connected, the converter runs at 260kHz and the RC signal looks like a ramp where it goes from 0V to about 300mV. when it connect the sync via an n-channel mosfet that is running at 485khz the RC pin will actually go negative about 150mV and it looks like the ramp is almost inverted.
Thank you very much in advance.
-Mark
Hello Brigitte,
The signal coming in on the sync signal to Q8 is running at ~485kHz at about 45-50% duty cycle. There is a note on the datasheet about the duty cycle but I have even hooked up an arb to vary the duty cycle from 5% all the way up to 50% with the same result.
The controller is configured as a sepic converter.
Here is the waveform of the RC pin without the sync signal hooked up (Q8 gate is tied to gnd).
Using the arb, I set the frequency to 485kHz with a duty cycle of 10% and I feed that into the gate-source of Q8 and to confirm is what I am reading on the gate (note the voltage amplitude is higher on the scope because I didn't have a 50ohm termination on the output of the arb. I did notice the vgs of the signal mosfet is ±6V so I had the mosfet replaced and I even tried reducing the arb voltage and I got the same result)
With the arb signal running or even the default sync signal hooked up we are getting a waveform that looks like the image below on the RC pin. When Q8 turns on it returns to ~0mV but when Q8 is off it falls to -300mV and then starts to ramp back up.
Thank,
Brian Long
Hello Brigitte,
Thank you for looking into the issue. I have tried using a BJT (PN: MMBT3904T-7-F) and the waveform was identical to using the N-Channel MOSFET. The diode solution works but it doesn't seem like a great idea because I am loading down the sync signal that goes to other devices. I had to isolate the sync signal to the TPS40211 with a 1K resistor so when it resets the RC capacitor it wasn't a dead short on the sync pin. Now the waveform goes up to about 2V then the RC capacitor reset occurs and pulls the signal down to about 300mV (while the sync signal is still high) then after the sync pulse goes low the RC pin also goes low.
Just to clarify it is sync'd but it doesn't seem to be a great solution. I changed the 1K in series to 2K and the amplitude of the peak went down to about 1.2V. I then changed the value to 4.99K and it no longer works. Instead it just looks like it keeps trying to reset the RC pin but there is not enough hysteresis to keep it reset and it just oscillates while the sync pulse is high. The 1K seems like a viable solution but to draw 5mA on the sync pulse means I should probably buffer it. It is just unfortunate that it didn't work the way the datasheet said using a MOSFET or BJT.
1K in series with diode: 
2K in series with diode: 
4.99K in series with diode: 
Thanks,
Brian Long
