LM2524 PWm controller

Discussion in 'General Electronics Chat' started by shoka, May 23, 2009.

  1. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    hello,
    i try to use LM2524 PWM controller , i know that the error amplifer in these IC is connected as PI controller.
    but in this IC there is no feed back , an R C branch in connected between the output of the error amplifier and the ground
    any one can explain what is the effect of loading the o/p with this RC circuit
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    Did you read the datasheet.
    A lot of information can be found there.
    In the schematics you have the resistor Rf, feedback resistor.
    There is also sometimes the Cf resistor for current feedback.

    Greetings,
    Bertus
     
  3. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    Of course LM2524 has a feedback and an error amplifier. Any PWM controller needs that. Pin 1 is the inverting input of the error amplifier. Pin 2 is the non-inverting input and pin 9 is the error amplifier output called Compensation in this circuit. So you have full access to the error amplifier and you can build any PI circuit around it. On page 13 of the datasheet you can find a section called "Basic Switching Regulator Theory and Applications" where there are schematics and equations that can help you with your application.
     
  4. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    i know that but,in the data sheet , a R C is connected from the load to the ground , i mean " No feed back"
    tha data sheet mention that the erroe amplifier is transconductance ap amp
    i wanna know what does it mean plz
    thanks in advance
     
  5. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    A transconductance amplifier outputs a current, and is driven with a voltage signal. Because of that, its transfer function has units "mho" (ohm^-1). Its output impedance is high (megohms). In order to output a current the amplifier has a feedback current-to-voltage and that is why the amplifier load appears in the closed-loop transfer function. Therefore, its bandwidth, and stability for that matter, is determined by the output load, so you will see a transconductance amplifier compensated with a reactive load. In the LM2524 datasheet they show an RC series load. So, to answer your question, the feedback is internal, the RC pair is the compensation and the voltage feedback from the circuit output (not error amp output) is determined by RF and the 5k resistor to ground.

    Hope this helps.
     
  6. shoka

    Thread Starter Active Member

    Mar 12, 2009
    39
    0
    thanks for all,
    i wanna know info about the curves in this data sheet:
    specially
    refrence transistor peak o/p current Vs Ta
    output transistor emitter voltage Vs Ta ( ambient temp )
    stand by current vs voltage
    why when the Ta increase the sense voltage increase(page 7 last curve )
     
  7. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    It is the nature of silicon grown components to have characteristics that vary with temperature. Take those curves as face value and design your circuit accordingly.

    The manufacturer asks you not to go over the Reference Transistor Peak Output Current, shown in the curve depending on the ambient temperature. This is due to the maximum power dissipation on that transistor. Going over that current will certainly damage the part. Same with the Output Transistor Emitter Voltage.

    Standby Current vs Voltage is self explanatory. The standby current increases as the input voltage increases, up to a certain value, when the current is almost constant.

    The Current Limit Sense Voltage increases with temperature due to the variation of the input offset of the CL amplifier with temperature. In fact, at specifications, under Current Limit, there is TC-Vsense (thermal coefficient of Vsense) which is 0.2mV/degC.
     
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