max1709 problem.

Discussion in 'General Electronics Chat' started by federico.disante, May 6, 2010.

  1. federico.disante

    Thread Starter New Member

    May 6, 2010
    2
    0
    I have a problem building a step up with variable Vout.

    I first build the circuit with a fixed Vout of 5V, and the IC works perfectly.

    If I try to build a step up with two resistor as described in the data sheet (pages 9
    and 10, fig. 4) the Vout is only about .1 - .2 V over the input voltage, and follow the inout voltage variation. If I change the function mode to output 5V the circuit again works correctly!

    What's wrong? I connected the 3.3/5 to ground and the FB to the center point of the partitor resistor: I choose R4=22k and R3 varying from 50k and 70k (to get a Vout beetwen 4V and 5V).

    There is anyone that used the 1709 with a Vout different fromt 3.3V or 5V?

    Thank you in advance. Best regards,
    Federico
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello Federico,
    You did not post the schematic of your circuit. That's really a mandatory item; unless we can see your schematic exactly how you have wired your circuit, we can spend a lot of time trying to solve your problem when it might have been spotted right off the bat.
     
  3. federico.disante

    Thread Starter New Member

    May 6, 2010
    2
    0
    Hi SgtWookie!

    I hope you can help me. I attached the schematic as you required!
    Thank you for the interest!
    Federico
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    As I said on another website, MAXIM's datasheet schematic does not have any pin numbers so comparing the two circuits will take some translating.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Pin 10 is not connected internally.
    Pin 16 is the feedback pin.
    R3 goes from FB (pin 16) to GND. It must be <50k Ohms. I suggest using a fixed valued resistor of around 5k Ohms.
    R4 goes from FB to Vout.
    The regulator will attempt to keep the FB terminal at around 1.24v.
    R4 = R3 x ((Vout/1.24)-1)
    So if R3=5k and R4=15.15k Ohms, Vout will be about 5v.
     
Loading...