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TPS92520-Q1: The difference between CHxEN=0 and UDIM=0

zhiyuan zhang
Intellectual 620 points
Part Number: TPS92520-Q1

Hello expert,

    What is the different between we set CHxEN=0 and we pull down the UDIM PIN? I think UDIM just control the output current and the internal function will remain,but CHxEN=0 will not only disable the output current but also disable the internal function(such as temperature sensor,output voltage measurement), am I right?

    And if we use UDIM to implement the PWM dimming(CHxINTPWM=0), do we still need to configure the register 0x0C~0x10 (fPWM, PWM WIDTH) and the 'PWMPH' bit of 0x00?I think the value of these register make no sense in this situation,right?

  • 0
    TI__Mastermind 20885 points

    Hello Zhiyuan,

    I like to think of UDIM as the master control.  Nothing will be output if UDIMx is not above the VUDIMx(EN)rising voltage of 1.22V (typ.)  If that is above 1.22V, then the output will turn on if CHxEN = 1 and if CHxINTPWM is set to 0 (external signal on UDIMx pin sets PWM).  You can also use the internal PWM signal (CHxINTPWM = 1), CHxPWM regiser (to set duty cycle of PWM), and PWMDIV (to set PWM frequency) to control it and it will control the PWM of the output, but CHxEN must be 1 and UDIMx still must be above 1.22V.  

    If using UDIMx to control PWM (CHxINTPWM bit = 0) then it it doesn't matter what the CHxPWM register (used to set duty cycle of internal PWM) is set to or what the PWMDIV register (used to set PWM frequency) is set to.  It will be ignored.   

    UDIMx is used to set the UVLO of the channel, thus is it the master control. UDIM thus has both UVLO and PWM control.     

    -fhoude

  • 0 in reply to fhoude
    TI__Mastermind 20885 points

    Here is a set of waveforms that shows correlation of UDIMx, CHxEN, CHxINTPWM, CHxPWM.  Let me know if this is helpful.

  • 0 in reply to fhoude
    Intellectual 620 points

    Clear for me,thank you!

  • 0 in reply to fhoude
    Intellectual 620 points

    Clear for me,thank you!

  • 0 in reply to zhiyuan zhang
    Intellectual 620 points

    Hello expert,

        One more question, in limp home mode, what happened if the MCU port to the UDIM is high impedance. Will UDIM below 1.22V and the output OFF?

        When the MCU is break off(SPI lost,and IO port high impedance), we want this device enter the limp home mode and maintain the output current as set before in the NVM.

  • 0 in reply to zhiyuan zhang
    TI__Mastermind 20885 points

    Hello Zhiyuan,

    I general you should never leave inputs floating.  You should have either a pullup or pulldown resistor to establish the state you want in those conditions.  Generally we use a resistor divider to setup the UVLO based on the input of the supply to the buck.  This makes sure the output never turns on until the input voltage is high enough.  That divider on UDIM can be driven by an IO from the MCU.  Here is an example seen in datasheet.

    If you don't want to use that circuit then do what I said with respect to either a pullup or pull down on UDIM.  

    If you want to maintain output current in limp home mode then you need to put the settings you use in normal mode into the limp home settings.  If you loose SPI then it will go into Limp Home and regulate as it did before.