• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 779 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28950: Possibility of SYNC signal On/Off etc.

Yoshwa
Expert 4050 points
Part Number: UCC28950

Customer inquired the possibility of SYNC signal On/Off for UCC28950 in slave mode.

Can the SYNC signal be switched between on and off sate for the UCC28950 application operating in slave mode?

 

An 825-kohm resistor is recommended between SS_EN to GND for UCC28950 in slave configuration.

How much fluctuation of 825-kohm resistor value is permitted?

Because, customer said that, in some condition, SS-pin voltage going down then the converter entering into overcurrent protection mode has been measured.

  • 0
    TI__Mastermind 25570 points

    Hello Yoshwa-san

    The SYNC signal may be switched on and off when the UCC28950 is operating in Slave mode - make sure that the SYNC signal stops in the LOW state. The UCC28950 will treat this as a loss of SYNC signal and will continue to run at its own 'normal' frequency as set by the resistor at the RT pin. If you wish to turn the slave device ON and OFF then the correct thing to do is to pull the SS/EN pin low.

    The 825k resistor should be ±5% tolerance.

    The voltage at the SS pin will reduce when the controller enters over current protection (Cycle-by-Cycle ILIM) - the rate of reduction and the final voltage determines whether the controller enters hiccup mode protection - the details of this behaviour are described in the data sheet section 7.3.14.

    Please let me know if you need any further information.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Expert 4050 points

    Colin-san

    Thanks your comments.

    Customer inquired additional questions.

    1) What will be happen for the slave UCC28950, if the SYNC signal stops in the High state by some accident?

     

    2) An oscillator usually starts oscillation from low frequency, then increases frequency up to the set normal frequency.

    What will be happen for the slave UCC28950, during this oscillator frequency changing periods?

    This oscillator starting frequency is 1/10, 1/100 and/or 1/1000 etc. of the set frequency.

  • 0 in reply to Yoshwa
    TI__Mastermind 25570 points
    Hello Yoshwa-san

    If the SYNC signal stops HIGH then the OUT signals will freeze in their current state. The CS signal will ramp up to its maximum and the PWM comparator will terminate the cycle and reset the OUT signals to a 'safe' state (diagonal switches will NOT be on), CS signal will drop to zero. Switching will stop.

    Switching behaviour when the SYNC signal is less than 1.8 times the desired frequency is unpredictable and possibly may damage the power train. The user must take steps to prevent the UCC28950 synchronising to low frequency SYNC signal. For example by holding the SS/EN pin low until such time as the SYNC signal frequency is correct.

    Regards
    Colin