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TPS732-Q1: Noise/Oscillation in the input of TPS732-Q1

Laud Laudencia
Prodigy 10 points
Part Number: TPS732-Q1
Other Parts Discussed in Thread: TPS62170-Q1, TPS62170,

Tool/software:

Hi TI experts,

I'd like to request your analysis on an observation we have on our circuit.

See below circuit using TPS62170-Q1 as primary 3.1V regulator, and TPS73201-Q1 as secondary 2.9V LDO. this 2.9V supply is intended for the analog supply of the image sensor. 

VIN regulated range = 7V to 12V

We have been observing some bad units and it looks like that the observation on the output image is related to this waveform below. 

this signal is measured on C508 (input of the 2v9 LDO).

The "bands" is roughly 25Hz (currently unknown source, but there's an expected 50Hz active power mode). 

The concentrated bands looks to be the active power mode and the other half maybe the low power mode.

The rms value is at 3.04V and the pk-pk is aound 80mV. The zoomed in shows that the signal has a 60khz frequency, also an unexpected frequency.

Now comparing these waveforms with a known good unit, see below diffference.

This is also captured from C508 of the known good unit.

The 25Hz band is still present, but the amplitude is significantly lower.

Zooming in, 104kHz was registered in the scope. Vrms registered is 3.04V as well.

Both units have the same circuit, and the measurement points are the same, C508 (input of TPS73201-Q1).

We are wondering, the why such difference in both units? what could have induced this unwanted higher amplitude noise/oscillation?

Additional information: C506 waveform was also measured with the unit where the higher pkpk is observed.

C506 and C508 is only separated by L503 ferrite bead.

see below C506 waveforms

Lastly, there was an experiment conducted where the C508 capacitor was removed. As per TPS73201-Q1, input, output, and Cff caps are optional. The expectation is when it was removed in the circuit, no effect on the circuit, but there was around 80khz oscillation observed. see below waveform

it would help our investigation if you could give insight as to how these observations is possible given the above circuit.

Regards,

Laud Laudencia

Electrical Hardware - Via Optronics

  • 0
    TI__Genius 17461 points

    Hi Laud,

    This is Colin who cover TPS62170 support. I would like to say the TPS62170-Q1 schematic is good.

    It is hard to say the unexpected  where noise/oscillation in the input of TPS732-Q1 came from  because i am not familiar with this LDO.

    But it looks like the problem is not caused by primarily regulator and more related to the bead you added that could cause some oscillate issue.

    I suggest you remove the bead and test again. You can ask the help from LDO team and they can give more suggestion.

    Thanks,

    Colin

  • 0 in reply to Colin Chen
    Prodigy 10 points

    Hello Colin,

    thanks for the reply, appreciate it!

    There were tests done regarding removal of the LDO input cap, but so far there afre different results:

    Remove L503 Returned same L503
    Unit A significantly improved did not returned to baseline measurement, improvement stayed
    Unit B slightly improved returned to baseline measurement
    Unit C slightly improved returned to baseline measurement

    I'm still getting the waveforms for these observations, but initially removing the ferrite bead L503 doesn't seem to have a constant effect on the observed noise/oscillation.

    below here is the image of the input and output of the LDO that shows that the input is leading the output by 3.128uS

    Maybe you have other ideas where to look for the source of this?

    Thanks,

    Laud...

  • 0 in reply to Laud Laudencia
    TI__Genius 17461 points

    Hi Laud，

    I suggest you adding the input cap of LDO but removing the bead.

    Thanks,

    Colin

  • 0 in reply to Colin Chen
    Prodigy 10 points

    Hello Colin,

    The removing input cap is only for simulation purposes.. but we can focus on the actual bad units behavior from this point forward.

    after removing the ferrite bead, the oscillations were eliminated, but the Vpk-pk still remain, and the failure is still observed.. The effect of high Vpkpk of 2.9V output causes white/bright stripes on the output image since the 2.9V is the analog supply of the image sensor in our application. See below results on the effect of removing L503 on the bad units:

    Basline Removed L503 Returned same L503
    Unit A white stripes observed significantly improved (totally no white stripes) did not returned to baseline measurement, improvement stayed
    Unit B white stripes observed slightly improved (white stripes not toally eliminated) returned to baseline measurement
    Unit C white stripes observed slightly improved (white stripes not toally eliminated) returned to baseline measurement

    Regardsm

    Laud...

  • 0 in reply to Laud Laudencia
    TI__Genius 17461 points

    Hi Laud,

    Below is my comments:

    1. It is necessary to isolate the DC/DC regulator and LDO and let them work separately to narrow down the problem.
    2. I saw that the load in the customer application is very light and the DCDC converter works in PSM mode. Although our converter has Cff integrated inside, it is better to keep the external Cff (C507) installed to improve PSM mode performance. It is also recommended to add a 0.1uF capacitor in Cout to reduce high-frequency voltage spikes.
    3. Considering that LDO can attenuate noise and obtain stable output. There is no need to add a bead(L503) between the DCDC converter and LDO, which can easily cause impedance mismatching problems.

    Colin