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How to synchronize a UC38xx control chip with a function generator?

Siyu He
Expert 1040 points
Other Parts Discussed in Thread: UCC38C40

Hi,

I am trying to synchronize a UC3886 and a UCC38C40 using the technique introduced in the UNITRODE APP NOTE U-111, injecting SYNC pulse signal through a 24 ohm resistor connecting between CT and ground. The problem is that the output PWM is not stable in the open loop performance. Here is how I set up the circuit.

The UC3886 is programmed running at 100KHz and a 1.9V voltage source is connected to the pin 1 CAO/ENBL to output a 50% PWM. The circuit was running as it was designed. After this, a 24 ohm resistor is put between CT and ground. A 110KHz ,10% duty cycle and 1V peak pulse signal is injected at the connection of 24 ohm and CT. The output PWM was not stable at all, the duty ratio varied cycle by cycle in big range(about 20%).

What should I do with this technique?

He,Siyu 

  • 0
    TI__Mastermind 34465 points

    The CAO/ENBL pin is  intended as a means of disabling the PWM output and not as an analog sigtnal. So this is probably why you see this instability.

    To get a 50% duty cycle you must force the EA output to be half way on the ramp signal, i.e. about 1.7V.

    The data sheet for the UCC38C40 shows how the run the ic open loop.

  • 0 in reply to John Griffin1
    Expert 1040 points

    Hi John,

    Thanks for your comment. From the block diagram of UC3886, it seems like the pin 1 CAO/ENBL is used as the output of current amplifer, the output duty ratio is determined by the voltage at pin 1 and the oscillator. Am I right?

    I have not set up the test circuit of UCC38C40 yet, thanks for datasheet.

    He, Siyu

  • 0 in reply to Siyu He
    TI__Mastermind 34465 points

    Hi Siyu,

    Yes you are correct. The CAO/ENBL pin is the output of the current amp and it can be disabled by pulling the pin low.

    Regards,

    John

  • 0 in reply to John Griffin1
    Expert 1040 points

    Hi John,

    Thanks for letting me know. Is there any simple circuit of UCC38C40 to test the synchronization? The open loop circuit diagram on datasheet is way more complicated than that of UC3886, I am wondering is it possible to set up a simple circuit that outputs stable PWM with UCC38C40?

    He, Siyu