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LM5022: LM5022 isolated flyback design questions

Neo.Wang
Intellectual 960 points
Part Number: LM5022
Other Parts Discussed in Thread: LM5156

Tool/software:

Hi TI experts,

With LM5022 isolated flyback design, i have a few questions:

1. There is an internal 1.4V offset at COMP pin, but datasheet 7.3.5 mentions :"The PWM comparator polarity is such that 0 V on the COMP pin results in a zero duty cycle at the controller output.". Is this true? 0V at COMP means -1.4V divided by 3 at internal PWM generator.

2. What is the desired COMP voltage variation range for isolated flyback design? Is Duty cycle proportional to COMP voltage?

3. COMP is internally pulled up to 5V through a 5kohm resistor. What's the current limitation of internal 5V reference?

4. As FB is grounded in isolated flyback design, Is there any additional bias/leakage current which drawn from 5V reference? or any additional current source to opto Ic current?

Thanks

Neo

  • 0
    TI__Mastermind 30410 points

    Hi Neo,

    Thanks for using the e2e forum.
    1. The datasheet shows a block diagram of the internal structure. Here you see the 1.4V offset.

    Additional specs for COMP are given in the electrical characteristics.

    It should be true that if COMP is connected to ground (0V), the duty cycle should be 0%.

    2. If FB is short to GND, the duty cycle depends on the COMP voltage level. A higher COMP level will create a larger duty cycle, but protection like OCP or maximum on-time are still active and can limit the duty cycle.

    3. The COMP pin short circuit current is 1.1mA typical per datasheet.

    4. There are no additional internal pull-up sources at the COMP pin that I am aware of.


    You will find a lot of Flyback reference design within our library, which should be helpful.
    https://www.ti.com/reference-designs/index.html#search?keyword=lm5022

    If you are in an early stage of designing and part selection is not final, I would also recommend to switch to the LM5156 device, as it is more modern than LM5022 and comes with improved features.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 960 points

    Hi Niklas,

    Thanks. Is there a equation to calculate the COMP pin voltage for a given VIN/VOUT/IOUT/transformer parameters?

    This is to evaluation the Opto coupler life time for such kind of conditions.

    Thanks.

    Neo.

  • 0 in reply to Neo.Wang
    TI__Mastermind 30410 points

    Hi Neo,

    Unfortunately, there is no equation given for the COMP pin voltage.
    However, it should be possible to calculate the expected duty cycle based on the parameters for VIN, VOUT, IOUT and transformer.
    As the COMP pin voltage level is connected to the PWM duty cycle, this should be the best approach for defining the expected operation conditions for the optocoupler.

    Best regards,
    Niklas