TPS562209: Variable switching frequency

Bart Huijberts

Part Number: TPS562209

During qualification of TPS562209 the switching frequency appears to be variable. tONTIME = 560ns tOFFTIME varies between 288ns and 1.3us. Inductor current follows this behaviour and Vout=3V3 though. Component choices are as stated in datasheet. COUT increase and decrease does not help. Increasing load, only inductor currrent becomes higher (dc level), but behavious is the same. Does somebody know what is happening?

over 8 years ago

0 Yuchang Zhang over 8 years ago

TI__Mastermind 18510 points

Hi, Please upload you schematic/layout and waveform for review...

0 Bart Huijberts over 8 years ago in reply to Yuchang Zhang

Prodigy 30 points

Yellow is inductor current.

Magenta is switching node.

0 Bart Huijberts over 8 years ago in reply to Bart Huijberts

Prodigy 30 points

One more thing. After some more investigation it seems to be expected behaviour because of forced CCM. So in the picture above, the load current low enough to actually enter DCM, but is forced to CCM. But what I dont get is: when I increase the load current, the behaviour is still the same, but only the DC level of the inductor is higher. I expect there a pure CCM with fixed switch frequency.

0 Yuchang Zhang over 8 years ago in reply to Bart Huijberts

TI__Mastermind 18510 points

Hi,
Firstly, 562209 is a FCCM part,
Second, for frequency variation, please try below ways.
1, change inductor to 2.2uH to try
2, add a 10pf cap, parallel with R8017
3, If above two methods can't solve, please upload your layout for review.

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TPS562209: Variable switching frequency