Dear team,
I use TPS54218 to generate 2.5 V supply for FPGA (see fig. 5). The capacitors right sides from power supply (C60-C66) is cappacitor near FPGA. When the TPS54218 is in temperature between (+100 Celsius and -20 DegC) it working normally and ouput voltage of this power supply is constant 2,5 V. But when the theperature gets under -20 DegC the ouput voltage of power supply with TPS54218 is not constant, but there is ramps between 2,2 and 2,9 V. I tryed add 3x 100 uF capacitors on output and changed inductor to 4,7 uH and too I changed compensation according to Texas Webbench (fig. 6) but result is the same.
I also tried to put more load on the power supply output but result is the same.
When I meassuring current and switched output voltage on TPS54218 I found that probably regulation of TPS54218 is wrong, because there is sometimes negative current from inductor and output capacitors. Propably sometimes the TPS54218 does not turn off "bottom switch". I meassured this by small meassuring resisstors (RM, RN on fig. 6 and fig. 5), see figure 1 - 4. I am sorry for the noise on this figure .
I using this IO TPS54218 for another power supplies such as 3.3 V, 1.8 V etc. but in this case it working well.
Fig. 1: Output voltage from power supply with TPS54218 (temperature under -20degC):
Fig. 2: Current "from" power supply meassured by 1R Ohm ressisto (temperature under -20degC)r:
Fig. 3: Current from TPS54218 (meassured by 0R22 Ohm resistor), (temperature under -20degC):
Fig. 4: Current from TPS54218 (meassured by 0R22 Ohm resistor),(temperature under -20degC) - detail:
Fig. 5: Schematic of power supply with TPS54218 - (without changes)
Fig. 6: Schematic of power supply with TPS54218 - (added capacitors, changed inductor and compensation):
Best Regards,
Radek
