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Original question:

UCC2818A-Q1: synchronization to external clock

UCC2818A-Q1: Synchronizing the UCC2818A-Q1 to external clock/PWM

Kolbein Blakar
Prodigy 20 points
Part Number: UCC2818A-Q1

Tool/software:

Guys

The typical circuit for synchronizing the UCC2818A-Q1 to an external clock is shown in figure 7 in the specification and is based on adding a pulse on the voltage on CT. The maximum value of the ramp peak voltage in the specification is 5,5V. To ensure synchronization the pulse has to be over 5,5V.  The "Absolute Maximum Ratings" for the UCC2818A-Q1 does not include the maximum value of the voltage at CT. Is there any "Absolute Maximum Rating " given for the voltage at CT?

Regards

Kolbein

  • 0
    TI__Mastermind 40140 points

    Hello Kolbein, 

    I don't have any official specification for the Absolute Maximum voltage rating of the CT input, however, I do happen to know that the oscillator block within the IC is biased by the 7.5V reference voltage. 
     
    My unofficial recommendation is to limit your synchronization pulse peak amplitude to ~1V lower than VREF, which is to say: Keep Vct < 6.5V max.  
    A 6.2V zener diode should be able to serve as a clamp.  

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Hello Ulrich

    Thank you very much for for very fast answer. Zener diode have a couple of  "side effects". 

    - The capacitance will come in parallel with the timer capacitance of the circuit. 6,2V low power diodes like BZX84W6v2 has up to 200pF at 0V. This will have influence of the frequency.

    -I also believe the saw tooth voltage on the timer capacitor is used in the PWM circuit. The value of capacitance from the zener diode depends on the reverse voltage, giving variation of the dv/dt of the saw tooth and thereby a variation in the loop?

    -The leakage current in the zener diode is typical specified at 4V for 6,2V diode, at 25C. This increases as the voltage increases.

    Comments?

    Kolbein

  • 0 in reply to Kolbein Blakar
    TI__Mastermind 40140 points

    Hello Kolbein, 

    The zener diode does not have to be applied directly across the CT cap. 

    A resistor from VCC can establish a bias current into the 6.2V zener (or 5.6V zener) and a 1N4148 diode can connect anode to CT, cathode to the zener. 
    This can clamp the CT voltage through the signal diode, but th every low Cj of the diode will not affect the timing ramp.
    Using a 5.6V zener will accommodate the diode drop.  

    Regards,
    Ulrich