TPS23756EVM: High Frequency Audible Noise

Sincere Tang

Part Number: TPS23756EVM
Other Parts Discussed in Thread: TPS23756, , TLV431

Hi TI experts,

I have an access point (AP) module, which requires 5.5V to operate. Hence, TPS23756 was implemented to be part of the power circuit.

No matter when it is powered up by PoE 56V or DC 12V, 24V or 48V, with low current drawing from the load, there is an annoying high frequency (4kHz - 6kHz) audible noise which I have no idea where does it come from.

I have basically followed to use most of the suggested components, except some diodes which I picked any other vender only because I have those in hand.

Mainly I was measuring the voltage delta between C1 (refer to TPS23756EVM pdf figure 1). The delta between the two ends is as follows:

Capture has taken when powered up by DC24V. The purple trace is the delta of yellow and green, hence the delta voltage across C1.

This is just one of my measurements, which I also probed at other places, even the 5.5V output, contains the 4k - 6kHz oscillation.

What have I done wrong? How should it be fixed?

Thanks.

over 5 years ago

0 Darwin Fernandez over 5 years ago

TI__Mastermind 41760 points

Hi Sincere,

Yes I do not expect this kind of waveform for the clamping cap. It also makes me question the switching waveforms. Typically I hear audible noise if there is some transient load operating in the audible range, a saturated transformer, or some instability of the converter. Can you send your application schematic for review? Thanks!

0 Sincere Tang over 5 years ago in reply to Darwin Fernandez

Prodigy 30 points

Darwin Fernandez said:

Hi Sincere,

Yes I do not expect this kind of waveform for the clamping cap. It also makes me question the switching waveforms. Typically I hear audible noise if there is some transient load operating in the audible range, a saturated transformer, or some instability of the converter. Can you send your application schematic for review? Thanks!

I have compared the EVM design and mine, and I found out that the PPD pin of mine is tied to the RTN. Would this cause the GATE and GAT2 having an unstable on-off sequence?

GATE & GAT2 signal measurements (Yellow = GATE; Green = GAT2; Violet = Drain of Q5; Pink = Drain of Q1):