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UCC28722: CC mode of the IC

Part Number: UCC28722
Other Parts Discussed in Thread: UCC28704,

Hello,

In the datasheet it is given that Vccr is 330 mV. I want to understand if this a calculated value or a fixed reference for CC mode of the CS Pin voltage. Vcst (max) is given as 780mV, I want to know how the CS pin reference changes from 780mV to 330mV suddenly or is 330mV simply calculated using tdm/tsw and 780mV. 

What is switching frequency and CS pin reference while operating in CC mode?

  • Hello,

    In UCC28704, the secondary side operating duty is clamped to 0.475 once the system enters CC mode and thus, Vccr = 0.475*750=356mV is the used to calculate the Iocc&Rcs. Is a similar methodolody with duty=0.423 (330/780) used in UCC28722? Please confirm.

  • Hello,

    The UCC28722 and UCC28704 are DCM converters that use FM/AM/FM modulation schemes.  The peak current, maximum frequency, transformer primary magnetizing inductance and transformer turns ratio are set at the minimum input voltage.  These equations can found in section 8.2.2.3 of the UCC28722 data sheet.  With the UCC28722 DMAGCC is limited to 0.425.

    When design this way at maximum load Dmag = 0.425 and Ipp(max) is equal to VCST(max)/Rcs = Ipp(max)

    To help understand the FM/AM/FM modulation DCM control scheme please refer to figure 15 of the UCC28722 data sheet.

    At full load the internal EA is at 5V and the peak current is VCST(max)/Rcs and is operating at fsw(max).  As the load decrease the EA voltage will decrease from 5V down to 3.5V.  To control the duty cycle the converter will decrease the switching frequency while maintaining Ipp(max).

    As the load continues to decrease the EA output will reach less than 3.5V and operate at a fixed frequency of 28 kHz.  As the load continues to decrease the EA output will move from 3.5V to 2.2V modulating the primary peak current from VCST(max)/Rcs = Ipp(max) down to VCST(min)/RCS = Ipp(max)/4.  This is the current amplitude modulation region (AM).

    The reason why the AM modulation region is present is to decrease the energy in the transformer ((Lpm*Ip^2)/2) to help reduce audible noise when the converter is operating below 20 kHz.

    As the load drops the EA output will transition from 2.2V down to 0.75V while maintaining IPP(max)/4 and will once again decrease the duty cycle by decreasing the frequency.  This is the lower FM band.  Once the EA out  drops down to 0.75V the converter will operate at fsw(min) of roughly 650 Hz.

    Regards,