Other Parts Discussed in Thread: TPS543321
Tool/software:
Dear Team
TPS54338 is used as the main Buck power in my design. The design detail is as follow:
5V input, 3.3V output. There is an about 8KHz ripple on the output, whatever the load is open or 0.2A.
The BUCK configuration is as follow:
1, FCCM to get a low output ripple;
2, FSS enable to get a better EMC performance;
3, 0.47mS soft-start;
4, 1MHz switch frequency;
5, Output LC = 2uH + 45uF @ 16V
6, Feedback capacitor: 6.8pF.
7, Input Capacitor: 11uF @ 50V
The schematic is shown as bellow:
The main placement is as bellow:
The red area is the input capacitor. the yellow area is the output inductor, the blue area is the TPS54338 controller and the violet area is the output capactor.
The layer under the buck is the whole GND plane.
I want to know that:
1, According to your experience, what the main reason， the feedback loop? Which direction should I have a try?
2, Is this the fault the IC already have? How can I avoid this matter?
Thank you very much.
What I have done is as bellow:
1, The Output Ripple, AC coupling. 20MHz bandwidth enable。
When the load is about 0.15A, the output is：
And when I disconnect the load, the output voltage ripple.
Generally the same as the 0.15A load.
In this situation, I measure the switch voltage, it is pretty good. For the output load is open, so the figure means that the Buck is in FCCM mode.
The switch node voltage, DC coupling.
2, I buy a TPS543321 EVM, and I get the same result.
Whatever the load current is 0A, 0.2A, 1A or more, and whatever the buck is in PFM mode or FCCM mode, even I change the switch frequency from 1MHz to 500KHz, the output ripple is all the same.
