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LM5030: Usage of COMP and VFB pins

Gabriele Ulissi
Prodigy 130 points
Part Number: LM5030

Hello,

My question concerns the LM5030 IC. In the application at hand I wish to measure the rectified and filtered output voltage of a transformer and regulate the duty
cycle applied to the primary (full bridge) as consequence.

By reading the datasheet it is not clear to me what the relation is between the VFB pin and the COMP pin. It would seem that the op amp that is inputed VFB and 1.25 V
 works as a comparator, in which case the effect of the COMP pin is not evident.
On the other hand if the COMP pin is driven externally I do not understand what the effect of applying VFB is.
This is based on the figure at page 10 of the LM5030 datasheet.
 I have also considered the different suggested/typical applications, but this has still not provided me with an answer.

Thanks

Gabriele Ulissi

  • 0
    TI__Mastermind 25570 points
    Hi Gabriele

    VFB is one input to the error amplifier, the other input is the 1.25V reference voltage. COMP is the output of the error amplifier and is used to set a control (or demand) level for the PWM comparator. The PWM comparator will terminate the ON time when the current signal (ramp) at the CS pin reaches this control level. COMP is not driven externally, it will take up whatever level is needed so that the ON time it demands makes the VFB (feedback) pin equal to the reference level. There is no direct relation between the COMP and FB pins other than the one through the feedback loop. COMP is brought out to an external pin because it is the pin where the loop compensation components are connected. Have a look at the information at www.ti.com/.../login.shtml and especially the control loop cookbook article at www.ti.com/.../slup113.pdf to gen anoverview.

    Regards
    Colin