>> Hello,

>> I designing a DC/DC with LM3477. The data sheet specifies that the duty
>> cycle of the PWM output is about D=Vout/Vin which is independent on the
>> output current (see page 13,
>> http://www.microchip.com/1010/suppdoc/appnote/all/picref2/index.htm).

>> If the duty cycle is fixed and independent of the output current, then
>> how is the regulation done when for example the  load increse its
>> current consumption ?

>> Thanks,

>> Tal

        My original response to the above:

> If Vin remains constant under the increased load, then no increase is
> needed in the duty cycle. This will not happen in real life; Vin will drop
> which is why a regulator is needed. As Vin decreases, D will need to
> increase to compensate for this, keeping the equation accurate.


        This is an amplification of my first response:

      The current in an inductor obeys the equation E = L (di/dt).
      The voltage E is the voltage across the inductor, which is the difference
      between the input voltage Vin and the output voltage Vout: E = (Vin - Vout).
      If Vout decreases because of an increase in load current, E *increases*.
      This increase causes larger increments of current to flow into the inductor
      during the charge phase ( di = E * dt / L, which increases if E increases).
      The inductor integrates these larger pulses, causing the average current in the
      inductor to rise until the new standing current equals the new load current. This
      increase takes place *without increasing the duty cycle*. Once the currents
      reach equilibrium, the *original* duty cycle continues to maintain the ratio
      between Vout and Vin.

      The inductor is just a low pass filter which smoothes the pulses from the switch;
      the DC value of the output must equal the DC value of the input. The DC value
      of the input is Vin * duty_cycle, so Vout = Vin * duty_cycle, or
      duty_cycle = Vout / Vin. Since neither voltage has changed, the duty cycle
      remains constant. In reality, there will be an increase in the ohmic drop in the
      inductor, as well as a drop in Vin, so a change in the duty cycle will occur,
      but only in the non-ideal situation. Also, the regulator may detect a temporary
      drop in Vout and try to compensate by increasing the duty cycle; this change
      will disappear once the inductor current settles at the new value. During the
      interim, there may be instability in the output if the regulator is not compensated
      correctly, but this is not pertinent to the theoretical issue.


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