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TPS54678: Some question about TPS54678

Jayden Qiu
Intellectual 1225 points
Part Number: TPS54678
Other Parts Discussed in Thread: TPS62867

Hi team,

This is FAE Jayden, ZTE is using TPS54678 in their projects. Here're some question about this part when they test the device.

  • A large stress peak will be detected when switching from full load to no-load during load transient. The no-load and full-load state tests are both within 7v. Why does it exceed 7v when switching?

  • There will be a voltage drop when pre-bias voltage startup is applied at 50% and 85%. What is the reason?
  • The output voltage will fluctuate when the power on. Why?

Many thanks.

Brs

Jayden

  • 0
    TI__Guru*** 146780 points

    Hi Jayden,

    ZTE has used this device for many years (it is old), so I am sure these questions have been accepted before.  For new designs, we recommend the TPS62867.

    For the SW node, there is no issue since the voltage is within the abs max table.  (And it may not be measured properly, with a low inductance probe.)  Does only one PH pulse go up this high?  Can you send a waveform with a few pulses before and after this one to compare them?

    For the pre-bias, how is the pre-bias applied?  Is there a load?  Usually, we apply the pre-bias through a diode to block interaction between the pre-bias supply and the device.  Here is the waveform of the EVM's pre-bias:

    It's not clear what you mean by 'fluctuate'.  Can you provide a zoom in of the last waveform to show the issue?

    This app note explains some of the startup behavior of these devices: https://www.ti.com/lit/an/slva866a/slva866a.pdf 

    Thanks,

    Chris