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UCC28C44: blocking capacitor

Noriyuki Takahashi
Genius 10470 points
Part Number: UCC28C44

Hi team,

Customer undestands that the blocking capacitor is removed from the schematic in the datasheet. 

But it seems that the blocking capacitor is sitll required as below. Does the device still require the cap? And could you also tell the role of the cap and when the cap is required?

Regards,

Noriyuki Takahashi

  • 0
    TI__Expert 7900 points

    Hi Noriyuki,

    The low value of the RT/CT triangle wave is not 0V.  The low value is not specified in the datasheet, but let's say it is about 1.0V.  Without a DC blocking capacitor, the "starting voltage" at the ISENSE pin will be 1.0V - VBE = 0.4V.  A duty cycle of 0% will require COMP to be 0.4V.  The maximum voltage to the ISENSE pin is 1.0V, so the usable range for duty cycle control is 1.0V - 0.4V = 0.6V.  This is the magnitude of the ramp to the control loop.  To obtain a larger usable range a DC blocking capacitor can be added.  With the DC blocking capacitor the usable range (magnitude of the ramp) will be close to 1.0V.  Note: the gain of Voltage Mode loop is inversely proportional to the magnitude of the DC ramp.

    Regards,

    Eric