TPS40170: Vout is connected to capacitor through cable

Hi Kohei,

Can you share the schematic and layout files for this board? I want to understand your set up for these measurements.

Best Regards,
Katelyn Wiggenhorn

Katelyn-san,

Thank you for your reply.
I will attach the test schematic. Layout file is customer information, so please e-mail me directly if you need it. I will send you the file.

my e-mail address : sasaki-k@macnica.co.jp

TPS40170 test circuit.pptx

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

Thank you for sharing your schematic.

I want to clarify your waveforms. Are you measuring VOUT/IOUT at the 560uF capacitor, or before the wire?

Best Regards,
Katelyn Wiggenhorn

Katelyn-san,

Yes, Imeasured VOUT/IOUT at the 560uF capacitor.
IOUT is measured at cable by current probe.

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

Can you share a waveform of the output voltage before the wire? I want to see if the output is also oscillating before the wire.

Can you also remove the wire and take a waveform of the output voltage again?

Best Regards,
Katelyn Wiggenhorn

Katelyn-san,

I update material.
Please check attached file.

3857.TPS40170 test circuit.pptx

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

I believe that the 560uF output capacitor is effecting the loop response of the TPS40170, and the system is unstable with this capacitor. To verify this theory, can the customer test the board with 560uF capacitor before the wire?

Thank you,
Katelyn Wiggenhorn

Katelyn-san,

I tested with one 560uF. However our customer condition is two 560uF capacitor.
The capacitor is 35CE560ZC. ()

Could you tell me the recommended COMP components value for two 560uF capacitor?

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

Were you able to confirm if the oscillations remain after moving the 560uF (1 or 2) to before the cable?

I used the TPS40170 calculator, and without the bulk capacitance the loop response is marginally stable. With the bulk capacitance the loop response is no longer stable.  

With no bulk capacitance:

With bulk capacitance:

Calculated compensation values: (You can see the calculated compensation values in blue below)

Best Regards,
Katelyn Wiggenhorn

Katelyn-san,

The output voltage swing level and current swing level became low by changing comp components value.
And this phenomenon is resolved by flowing load current.

However this phenomenon was not resolved completely at light or no load condition.

Do you have any concern that we use this device with this condition, about current swing or reverse current etc...?

Best Regards,
Kohei Sasaki

Hi Sasaki-san,

What is the voltage swing and current swing level at light or no load? The concern is if the system is stable under these conditions. Can you measure the loop response of this design?

Best Regards,
Katelyn

Katelyn-san,

At actual customer board, voltage swing is about 1Vp-p, current swing level is about +/-3.5A, 7Ap-p.

We have not measure loop response. If we can measure loop response and that result is no problem, can we judge there is no concern?

Best Regards,
Kohei Sasaki