Question about LDO regulators

Discussion in 'The Projects Forum' started by KansaiRobot, Nov 5, 2015.

  1. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    Hello and thank you always for your help.

    Apparently I made I mistake and posted a question on the homework forum, which was not homework. I am trying to correct my mistake by putting the question here. My deepest apologies. :oops:


    I have a question related with LDO regulators . Please take a look at these circuits:

    circuitoBacklight.jpg

    Can someone explain me these circuits, and specially the part that I marked with a black elipse??

    The IC being used is
    http://www.torexsemi.com/products/discon/item/53-XC6209_12.pdf

    I understand that CE is some kind of a enable pin. The datasheet actually provides the left circuit as a "typical application circuit".

    But regarding the right circuit, I suppose VCC-3V3 refers to a 3.3V so what happens if I put another voltage to BL_CTRL?? what happens if I put GND there? and what happens if I apply a PWM signal there??

    I understand voltage dividers when they go from VCC to ground so I guess R8 and R9 are somehow dividing the voltage, but ok, if this is so, then what happens when this divided voltage enters CE???

    My understanding so far is tha CE enables or disables the function of this circuit but I guess that would be with a 5V or GND signal. What I have absolutely no idea and I cant't even guess is what happens with lower voltages or with a PWM signal there and why do they put a voltage divider there...


    Any help will be greatly appreciated

    Thanks in advance
     
    Last edited: Nov 5, 2015
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,734
    759
    CE...is chip enable.
    A voltage close to Vin will turn on the LDO and off it when low
    BL_CTRL is used to shut down the LDO when Low or grounded. When BL_CTRL is open, LDO turns on. NO PWM what so ever
     
  3. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    Thanks for your reply.

    But even if BL_CTRL is zero, due to the resistors the voltage that would enter is 0.3V and in the datasheet it says that CE minimum is 0.25V, so even with zero it would be on. or not????

    I ll try and experiment
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Actually there is probably a no-man's land between the maximum value of a LOW input and the minimum value of a HIGH input. This is common for TTL, CMOS that emulates TTL, and devices with Schmitt Trigger inputs.
     
  5. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    Well, I tried and with both GND and Vin, the LDO is on. Nothing changes. ... ummmmm
     
    Last edited: Nov 10, 2015
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,734
    759
    Then change R8 to 470R and try
     
  7. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    I have to report that my previous post was incorrect. I didnt realize that I left the pin unconnected :eek:. So now the results are the following

    1) with the pin unconnected, the LDO is on. This is connected to a LCD so that is also on
    2) with a potentiometer, it is ON/OFF abruptly. (the LCD becomes on or off)
    3) with PWM the LCD becomes off slowly.

    thanks a lot for the help:)
     
    Last edited: Nov 9, 2015
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