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TPS54610: TPS54610

Part Number: TPS54610

HI 

Currently using the TPS54610PWPR buck controller for 3V3 output from 5VDC supply. SS for the controller has been selected for a soft start of > than the device fixed SS of 3.35mS however i have noticed that at turn on the controller initially outputs a voltage for a short time period. Any thoughts ? SS cap charge up looks good. device data states that output is enabled once voltage has reached to 90% of set point . Looking at the output prior to LC filter , there appears to be a DC voltage rise followed by clear switching activity (couple of pulses) then turns off, turning back at 90% set point.  

  • Hi Colin,

    I think I do not completely understand your question. Maybe you can attach some oscilloscope measurements to explain the problem in more detail.

    Did you do the measurements in your circuit or on the EVM?

    Best regards,

    Juergen

  • Sorry, unable to post any screen shots however maybe i can provide a clear explanation of problem:

     I'm using the TPS54610PWPR buck controller to provide a 3V3 output rail from a 5VDC supply. The soft start capacitor selected for the device provides a turn on delay of approximately 11mS with a soft start up of 6.5mS. At turn on of the 5VDC supply there appears to be an output voltage present during the initial controller delay period of approximately 1.8VDC. This then falls to 0VDC during the soft start period at which point the output is established at 3V3.

    Regards

    Colin    

     

  • What is connected to the 3.3V rail at the output? Are there any other supply rails on your board which are already enabled and regulating their output voltage?

  • There are a couple of other Bucks used on cct however all voltage rails are isolated from each other. The start up sequence has been changed in relation to other voltage rails without success. The voltage seen on the output of the BUCK appears to be present during the device delay period where the soft start period clearly blanks the output.
  • There are startup waveforms in the datasheet (page 17 figure 19) and in the EVM users guide (page 24 figure 15). The waveforms do not show any increase in voltage during the delay period. Since the device does not have output voltage discharge the output is high impedance while the device is disabled.

    This means any leakage current flowing into the output capacitor will pull the output voltage up. The device starts operating turning on the low side switch. This basically shorts the output through the inductor and can remove remaining charge in the output capacitor. During the on phase of the low side switch the bootstrap capacitor is charged. This charge is supplying the gate driver of the high side switch which can be turned on after the low side switch is turned off. Before that initial charge of the bootstrap capacitor there is no current path from the input to the output which could charge the output capacitor. 

    So from my point of view either the device is damaged or there is a parasitic path in your design which charges the output capacitor while the device is disabled. Since the precharge happens during the ramp up of the 5V I suggest to check the circuit connected between the input and the output of this converter, for example a sense or measuring connection from the 5V to the circuit supplied by this converter.