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UCC28084: UCC28084 CTRL pina dn CS and ISET pin functionality

Murali Krishna
Prodigy 95 points

Part Number: UCC28084
Other Parts Discussed in Thread: TL431

We are using the UCC28084 part in our design applications and which is used in push-pull topology please request you to explain the CTRL and CS and ISET pin functionality its design calculations 

  • 0
    TI__Genius 12905 points

    Hi Murali, 

    CTRL is the control signal provided from the secondary side feedback, it represents the error between the output voltage and the desired output voltage. 

    CS is the current sense signal provided by sensing the current through the FETs in the power stage. The current is sensed through a current sense resistor and a proportional voltage is applied to the CS pin. Current sensing is required for the peak-current-mode control scheme.  

    The CS signal is compared against the CTRL signal in the PWM comparator. When CS exceeds CTRL, one FET is turned off and the other is turned on. 

    ISET provides slope compensation for peak-current-mode control. This is added for additional stability in the control loop. 

    These topics on current-mode control design are discussed in more detail in the application notes below:

    Power Supply Compensation Made Easy - refer to section on Current-mode forward and other buck-derived topologies.

    Understanding and Applying Current-Mode Control Theory - this explains the need/benefit of slope compensation within current mode control.

    To design for these ISET, please refer to the ISET section in the datasheet. 

    To design for CTRL and CS pins, please refer to the example schematics in the datasheet. The CTRL signal is created by the TL431 error amplifier on the secondary side of the transformer, then passed to the primary side through an optocoupler. The CS signal is created by placing a current sense resistor in series with the FETs. You will have to tweak these circuits according to your application conditions. 

    Thanks,

    John