Disclaimer: I have posted this question on Texas Instruments' E2E Community before (Here), but haven't found a solution to my problem yet. In the meantime, I haven't found any way to progress on this problem on my own either. I hope this cross posting between StackExchange and non-SE sites does not pose a problem.
I am working with a Texas Instruments TPS57160-Q1 step-down converter. The possible input voltage lies between 20V and 41V and the output voltage is supposed to be a (relatively) stable 5V. I chose all of the components according to the datasheet.
Thing is, now I am experiencing problems under load with high input voltages. As long as the input voltage stays below 25-30V, everything is fine. But when I increase the input voltage to values above 25-30V, the output seems to get unstable, especially under load. With 50mA load and an input voltage of 36V, the output voltage sometimes even switched between 4V and 6V, when it should actually be 5V.
Buck converter setup:
- Vin: 25V-41V
- Vout: 5V
- Iout (max): 900mA
- Switching frequency 950kHz
As to the components, all of the capacitors are (multilayer) ceramic capacitors, the input capacitor has a voltage rating of 100V, the output capacitor is rated at 10V.
The PCB layout:
Here the scopemeter plots, unfortunately I didn't have access to a regular oscilloscope that day: (From left to right: Vin, Vout (10mA load), Vout (250mA load), PH pin (switching pin))
Things I have tried so far:
- Changed the switching frequency to 400kHz
- Used a larger input capacitor (electrolytic in parallel)
- Used a larger output capacitor
- Changed the frequency compensation components connected to the COMP pin to a more "robust" combination (According to TI's webench designer: R3=130k, C3=470p, C2=4.3p)
Neither of these things made a big difference. The output voltage definitely changed, but it still was not stable enough.
Here are the plots in the modified configuration (As outlined above: larger input cap, 400kHz switching frequency, "robust" comp pin components). From left to right: Vin, Vout (10mA load), Vout (250mA load), PH pin
Now that I have tried varying almost all of the components, I am thinking the culprit may be the PCB layout and the placement of the input and output capacitors. Maybe someone of you with more experience in switching voltage regulators can shed light on this problem. If the problem is indeed the PCB layout, is there any way I can test/simulate if changing the distances between the components will make a drastic difference without getting another board manufactured?