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Original question:

TPS53355: Vout waveform

TPS53355: Vout waveform

Yuta Nishie
Genius 5015 points
Part Number: TPS53355

Hi, 

Sorry for the late reply. I received a reply from the customer yesterday.

I created a new thread because the following thread was locked.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1264508/tps53355-vout-waveform/4812307

The customer found a solution for Vout fluctuations. However, the customer wants to know the cause of TPS53355 output voltage fluctuations. Therefore, could you tell me why the output voltage fluctuates? Is OCP the cause?

The output voltage waveform and circuit image are as attached.

TPS53355 Vout waveform.pdf

＜Background＞

When 1.2V (TPS53355) and 1.8V (Other Power IC) connected to the load start up at the same time, the load current is drawn significantly, causing this problem. The load current is about 18A.

Output voltage fluctuations continue to occur after startup.

The generation rate is in the 6/200 range.

Best Regards,

Nishie

  • 0
    TI__Genius 11315 points

    Hey Nishie-san,

    What was the solution that fixed the vout fluctuations in the original thread?

    Based on the pdf you provided, the load (I'm assuming some sort of CPU) activates as the other two ICs are activated, and pulls an 18A load from the TPS53355? From the original thread, I remember you said that the DC-DC converter that provides 12Vin was acting irregular. Has it been fixed? and have we verified that there is not a VIN droop as the TPS53355 ramps up to Iout-max? 

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Genius 5015 points

    Hi Caleb-san,

    Thank you for your reply.

    Caleb Perez said:
    What was the solution that fixed the vout fluctuations in the original thread?

    By setting so that the load does not become active during the power supply startup, the voltage drop at the moment of startup was prevented. This also prevented the output voltage from fluctuating. It seems that excessive current flowed when the load started, causing the voltage drop.

    Caleb Perez said:
    Based on the pdf you provided, the load (I'm assuming some sort of CPU) activates as the other two ICs are activated, and pulls an 18A load from the TPS53355?

    According to the customer, when the TPS5335 starts at 1.2V and the other IC starts at 1.8V at the same time, the load pulls 18A from the TPS53355.

    Caleb Perez said:
    I remember you said that the DC-DC converter that provides 12Vin was acting irregular.

    Is it about VDD pins? I reconnected the GND of the condenser for VDD, but there was no change in the operation.

    Caleb Perez said:
    have we verified that there is not a VIN droop as the TPS53355 ramps up to Iout-max? 

    There seems to be no VIN  large drop (fluctuation) even if TPS53355 ramps up to Iout_max.

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    TI__Genius 11315 points

    Hi Nishie-san,

    Okay, I understand where we are at now. There are a few things from the last thread that we did not close out that may lead to a solution.

    You stated:

    "After removing the ferrite bead (BF1) from the schematics posted in the past, the output voltage is normal. If you press the FB pin of the DCDC on the front stage of the TPS53355 with your finger, the output voltage is normal. However, the FB1 cannot be removed in the customer's circuit."

    Does removing the ferrite bead still fix the output voltage regulation in this situation? Does pressing on the DCDC at the input of the TPS53355 still fix this issue?

    Can you verify VIN with an oscilloscope capture at startup and during abnormal operation? Please also include Inductor current, vout, and vdrv on the scope shot.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Genius 5015 points

    Hi Caleb-san,

    What does Vdrv stand for? Is it the drop voltage of the output voltage?

    Also, in a normal device, the problem did not occur when the load current of 18A was drawn at startup.

    At this time, can you think of anything why this problem is caused by the load current(18A)  at startup?

    Best Regards,

    Nishie

  • +1 in reply to Yuta Nishie
    TI__Genius 11315 points

    Hey Nishie-san,

    My apologize. I meant "VDD" not "VDRV".

    My thoughts are as follows: The DC-DC converter powering the TPS53355 is suffering from a voltage drop when there is an 18A load at startup. I noticed on your layout that the VDD capacitor is pretty far from the device. I'm wondering if a voltage drop on VDD is causing the part to shut down intermittently. I am asking that you verify my thoughts by using a differential probe to measure the voltage.

    Please collect an oscilloscope screen capture at startup. Capture should include Vin, Inductor Current, Vout, and VDD.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Genius 5015 points

    Hi Caleb-san,

    Thank you for sharing your opinions.

    I will make a request to the customer, so I would appreciate it if you could give me some time.

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    Genius 5015 points

    Hi Caleb-san,

    The customer said they didn't have a differential probe and a current probe. They also said he would close this issue once to control IC connected after TPS53355.

    I appreciate your great support.

    Best Regards,

    Nishie