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UC2843A Question

zhong Huo
Prodigy 160 points
Other Parts Discussed in Thread: UC2843A

Hello,

       I build a UC2843A prototype board. I I made FB pin open,(without feedback input)

I can get triangle waveform from pin 4 (8 pin package) but pin 6 no output pulse, Do

I have to send a feedback voltage to FB pin? Which signal I should check next. Thank you

very much.

 

  • 0
    TI__Guru 73265 points

    You may want to refer to the open loop test set up on page 7 of the datasheet. The diagram on page 1 should be reviewed carefully to understand how the device works. To be able to see the pulses from pin 6, a good reference circuit is to take the circuit on page 7 of the datasheet, open-loop laboratory test fixture. You could use a function generator to simulate current signal. But if you skip the needed composite of those shown in the circuit, the pulses cannot be generated.  

  • 0 in reply to Hong Huang
    Prodigy 160 points

    Hi Huang Hong,

     

            Thank you for your reply. I have fixed a output problem and and it can works

    now. My power supply is a veriable output power supply circuit. In some output

    voltage I cannot control the output voltae, output voltage is changed when input

    voltage change. Could you show me that what is Vfb and Isense pin input and comp

    pin voltage range. What are design consideration for these signal? Thank you very much. 

       

  • 0 in reply to zhong Huo
    TI__Guru 73265 points

    Is your converter running in open loop or close loop?

  • 0 in reply to Hong Huang
    Prodigy 160 points

    Hi Huang Hong,

        My converter running in close mode already. Thank you.

    Zhong 

  • 0 in reply to zhong Huo
    TI__Guru 73265 points

    Were you make the feedback control loop stable?

  • 0 in reply to Hong Huang
    Prodigy 160 points

    Yes, It is stable, There are only resistor in feedback circuit. I have a quick question that

     What is relation between output voltage and Vfb pin veltage. For example, If I want to

    Setup output voltage to 15V, What voltage value I should fedback to Vfb pin 

  • 0 in reply to zhong Huo
    TI__Guru 73265 points

    Refer to the diagram on page 1. The Error Amp has a Vref = 2.5V. Then you should set up your feedback VFB = 2.5V when your Vout = 15V. This requires a voltage divider. After done this, the Error Amp Comp will be automatically set up to high or low to correct the error between VFB and Vref to make Vout = 15V in steady state. Some compensation network is needed between VFB and Comp and between VFB and Vout. Let me know how these were designed in your converter.

  • 0 in reply to Hong Huang
    Prodigy 160 points

    Hi Huang Hong,

        Thank you very much for your clear answer for my question. My converter lost

    control between 8.3V to 10.3V input voltage and its current is twice of normal

    current. It works fine from 10.3V and up. I will check it more.

    Zhong   

  • 0 in reply to zhong Huo
    Prodigy 160 points

    Hi Huang Hong,

          I still have problem for my UC2843A output voltage control. I increase input voltage 

    and feedback from output to UC2843a Vfb pin voltage increase too. When UC2843A Vfb

    pin reach 2.5V  output voltage should not increase anymore but I find output voltage

    still increase slightly when increase input. How can I get stable output voltage when

    Vfb pin get 2.5V?  Thank you very much.

     

    Zhong

     

  • 0 in reply to zhong Huo
    TI__Guru 73265 points

    It sounds your feedback loop did not work properly. If Vin increased, Vfb should be adjusted lower through the feedback control. If the feedback control did not adjust Vfb to lower, the output would increase.