help step down converter

Discussion in 'The Projects Forum' started by nandax, May 4, 2009.

  1. nandax

    Thread Starter Member

    Nov 20, 2008
    12
    0
    I am using TPS62056 http://focus.ti.com/lit/ds/symlink/tps62056.pdf
    i would like to test it with button connect to enable pin.
    I connect the output to the resistor and LED to see if the led on when enable is high. th eproblem is th eled start blinking and then off..
    when I check the connection with multimeter suddenly pin 9 (SW) and pin 10 (PGND) got connected together internally,
    i DOnt know the cause, can somebody tell me why?
     
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    the SW pin is the drain for the internal power MOSFETS, the other is power ground. It's not unlikely that the internal MOSFETS have broken down and conducting straight to ground.

    In other words your chip is fried if thats the case.
     
  3. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    If you look at the schematic of the chip you'll see that the SW pin is connected between the mosfet transistors. SW and PGND should only be open circuit while the transistor between SW and PGND is conducting which it shouldnt while the chip is not powered.

    So if SW and PGND is open when you measure while the chip is not powered it most likely means that the mosfet is fried and conducting straight trough.
     
  4. nandax

    Thread Starter Member

    Nov 20, 2008
    12
    0
    but what can be the cause? because when i Check the connection of my circuit , it is ok. but suddenly when i testing it, pin 9 and 10 got connnected internally
     
  5. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    You need to post a image of your circuit before anyone here can point out errors in the setup or you need to refer to the step down converter datasheet for example circuits.

    But how much voltage are you feeding the converter ?
     
    Last edited: May 7, 2009
  6. nandax

    Thread Starter Member

    Nov 20, 2008
    12
    0
    here i attached the schematic, i use 9 V for power supply. i would like to test it with push button to enable and disabled the step down converter. keyboard in the schematic is push button. I connect keep_power to VCC to keep the voltage high in enable pin when i release the button.
    it works when i press push button and i release it, it still enable the step down converter. the problem occur when i connect keep_power to GND, it stll enable the step down converter, actually i expect the step down converter to be disabled.
    and sometimes if I would like to testing it again (turn off the power supply and turn it on again), pin 9 and pin 10 of step down converter is connected internally. thats why i confused
     
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  7. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    At first glance i dont see anything major. However i did notice that you seem to be missing a pullup resistor between PG and the output.

    PG (power good) is connected directly to ground. In the manual they state that it should be pulled high with a pullup resistor between power good pin and the output.
     
  8. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    Have you read the section on EN pin pushbutton configuration in the manual, it shows a simple circuit for implementing it and the requirements.

    It's mentioned there that the resistor between EN and ground must be able to sink 0.3uA of current with a minimum voltage drop of 1.3V for it to operate correctly. Make sure your setup fullfills that requirement.
     
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