Other Parts Discussed in Thread: TPS54308, TPS54202, TPS56339
Hi Team,
the issue we have met is a high output ripple voltage, about 150mV;
Here's a piece of our design:
Inductor pn: SRP5030T-3R3M;
Output capacitors pn: CC0603MRX5R5BB226
It's specification:
Uin=15V; Vout=1.2V; Iout_max = 1A; Ityp = 650mA; Imin=20mA; Uripple_max=25mVp-p.
Our conclusions:
1) TPS54302 is featured for low question current and max efficiency at light load by implementation of pulse skip mode(Advanced Eco-mode). In this mode the switching frequency falls to ~60kHz, that is leading to high ripple voltage ~150mVp-p at load 0-1A. When load >=1.2A the PSU operates in CCM with Fsw~400kHz and low ripple voltage ~25mVp-p, but it's out of the use case.
2)Modeling the buck converter on TPS54302 in WEBENCH® POWER DESIGNER , brought controversial results...
2.1)I_load=625mA
/cfs-file/__key/communityserver-discussions-components-files/196/WBDesign51_5F00_Steady-State_2D00_625mA_5F00_10mVpp.pdf
Uripple = 8.1mVpp, but in real design the ripple is higher. The report in clause "Operating values" has the value of ripple voltage
2.2)I_load=25mA
/cfs-file/__key/communityserver-discussions-components-files/196/WBDesign51_5F00_Steady-State_2D00_25mA_5F00_80mVpp.pdf
The report in clause "Operating values" does not have the value of ripple voltage, why so? (however it was got from the Vout waveform, below).
3) Changing PSU controller to TPS54308, which is featured to operate in force continuous conduction mode (FCCM) during light load conditions, have led to the issue relaxing. Ripple voltage <=25mVp-p within load range 0 - 1A. But with this solution we lose too much of input power at light load of 20mA (in so called STAND-BY mode)
4) We have also tried TPS54202, which has lower threshold of entering ECO pulse-skip mode - 0.3A. The ripple voltages measured at Ityp=650mA and Imin=20mA are 21mVp-p, 25mVp-p, respectively. It might be a solution, but we are not sure if at some Iload the stability of converter is lost and ripple will be out of range (continue the testing).
Our questions:
- Do TPS54302 and TPS54202 have known issues with high output ripple voltage or stability, when operating in described above conditions (Uin=15V;Uout=1.2V; Iload=0 - 1A)?
- How can we get the output ripple voltage not more than 25mVpp in load range 20mA - 1A?
- If not, could you recommend an alternative, preferably, pin2pin compatible IC, with high efficiency at light loads (so called ECO mode)?