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TPS65000-Q1: Switching from PFM to PWM mode causes the SW pin to stop oscillating.

? ?
Prodigy 10 points
Part Number: TPS65000-Q1
Other Parts Discussed in Thread: TPS65000

Tool/software:

Why does the DC-DC converter integrated in the TPS65000 experience a brief output interruption when switching from PFM mode to PWM mode?
At this time, the shutdown time of SW is approximately 250 microseconds.
  • 0
    TI__Expert 5575 points

    Hi, 

    There should not be an interruption in the transition from PFM to PWM mode.

    Please refer to the expected waveform in the datasheet:

    This SW shutdown behavior may be indicative of another issue. 

    I believe I am in contact with an FAE looking this same issue - Jerry Hu? Can you confirm?

    Best Regards, 
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 10 points
    您好，感谢您的回复。随附的图 1 显示了 1.2V 电源轨上电时发生的电压降波形。图 2 是图 1 的扩展视图。在测试过程中，我观察到 1.2V 输出的每个电压降都伴随着从 PFM 到 PWM 模式的转换。但是，当TPS65000切换到强制 PWM （FPWM） 模式时，电压不再下降。因此，PFM/PWM 模式切换与电压降之间似乎存在很强的相关性。

  • 0 in reply to ? ?
    Prodigy 10 points
    Hi, thank you for your reply. The attached Figure 1 shows the waveform of the voltage drop occurring at the moment of power-on for the 1.2V rail. Figure 2 is an expanded view of Figure 1. During the test, I observed that each voltage drop of the 1.2V output is accompanied by the transition from PFM to PWM mode. However, when the TPS65000 is switched to forced PWM (FPWM) mode, the voltage no longer drops. Therefore, it appears that there is a strong correlation between the PFM/PWM mode switching and the voltage drop.
  • 0 in reply to ? ?
    TI__Expert 5575 points

    Hello, 

    Thank you for providing the images. It appears that there is a correlation between the mode switching and the voltage drop. 

    However, the mode switching itself should not cause this significant drop. 

    I would like to try to figure out why the device is switching, is there a change in load at this time?
    What is the expected load current on this rail?

    Best Regards, 
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 10 points
    The reason for the mode switch of the 65000 is that the load voltage is 1.2V, and the current demand has increased from 108mA to 168mA. The expected operating current range for this power rail is 60mA to 250mA.
  • 0 in reply to ? ?
    TI__Expert 5575 points

    Hi, 

    Can you tell me more about the 60 mA load being connected to the 1.2V rail at this time?

    Is it possible to try increasing the output capacitance at the 1.2 output rail to 22 uF to see if this helps with the drop?

    From waveforms that Jerry Hu shared with me, it also appears there is a small drop in the VSYS and 2.8V output at this same time 
    and there was no/low input capacitance for VINLDO1 & VINLDO2.
    Please add at least 2.2 uF to the input for best operation.

    Best Regards,
    Sarah