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UC2844 inside logical circuit_ current sence and protection

Changqi You
Prodigy 40 points
Other Parts Discussed in Thread: UC2844

As we all know that UC2844 is a current mode PWM controller, pin3 is used for current sense and circuit protection. Quote on datasheet: 

"An external series resistor, RCS, senses the current and converts this current into a voltage that becomes the
input to the ISENSE pin. The ISENSE pin is the noninverting input to the PWM comparator. The ISENSE input is
compared to a signal proportional to the error amplifier output voltage"

Based on diagram below:

Based on my understanding, the feedback of output voltage and reference voltage make up the voltage, comparing with Visense, if Visense is larger,  the output of IC will be low level, which means MOSFET will not work.

Then in the circumstance  that output voltage has hit the set voltage, which means error of voltage is 0, dose it mean the limitation of Visense is 0V at that moment?

And another question is, how this IC limit the power of circuit?

Thank you all

 

  • 0
    TI__Genius 9130 points
    Changqi,

    The error amplifier will adjust (increase) the output level at the COMP pin as much as needed, until the feedback level at the FB pin equals the internal reference level (2.5 V). So a resistor divider at the output voltage will scale Vout down to 2.5 V to match the REF. Remember that an opamp will change its output voltage up or down as needed to try to keep the same voltage at both inverting and non-inverting input pins.

    If the load is heavy, then the COMP pin voltage will go to a higher level in order to get a higher peak current level at the CS pin, to satisfy the load. Similarly, if the load is very light, the COMP level will fall, and this will programme a much smaller peak current level at the CS pin.

    If the COMP level is commanding a small level of peak current at CS, and the load increases, this will cause Vout to fall. Thus the FB pin level will fall, and COMP will then increase, commanding higher peak current at CS, and COMP will increase to whatever level is required to get the right peak current level at CS to make FB = REF.


    I hope this helps to answer your question, if so, please click the "verify answer" button.

    Thanks,
    Bernard