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TPS54360 Waveform and Shutdown time issues

Pang
Expert 2415 points

1) Measured the SW pin at EVM, as per understanding that this wave form should be generated in PWM signal but I waveform obtained is non PWM signal.

 

Measurement probe.

 

Waveform at 5ms.

 

 

Waveform at 25us.

 

Waveform is triggered randomly. Please advise why isn't a PWM waveform instead?

 

 

2) EN pin has being tied to ground for shut down but found that the shut down output voltage takes around 18.5s instead of 200us from 5V to 0V.

 

Waveform from EVA board datasheet

 

Signal in Blue is enable, Signal in Yellow is output signal

 

 

 

Kindly please help to assist.

  • 0
    TI__Genius 10115 points

    Hi,

    I am going to move your post to non-isolated dc-dc for any further help on this specific part.

    As to your turn off time and non PWM switching.  It looks like you have no loading on the part.  This puts the part into eco-mode where the switching frequency will decrease dramatically, the part is no longer using Pulse width modulation but a type of Frequency Modulation.  This explains the long time between switching events in your waveform at 5ms.  Additionally because there is very little loading on the vout capacitor it takes a long time to discharge when the part is disabled.

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    TI__Guru**** 191445 points
    I see you do not have any load. Try increasing the load to 1 A or so. At no load you will be in pulse skipping eco-mode. You will not get a meaningful SW waveform at 5 msec due to sampling. At say 2 or 5 usec / dive you should see a regular pulse when you increase the load. Discharge time is also load dependent.