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LM5576 Negative 5V output does not shutdown after power cycle

Javcel
Intellectual 470 points
Other Parts Discussed in Thread: LM5576, LM22675

Hello everyone,

I am having an issue regarding the LM5576 device.

I have used one of the WebBench designs and it seems to work fine. When I plug in the power (24V) and I press the ON switch, the LM5576 turns ON fine but when I turn OFF the circuit the -5V remains there.

The way the board works is that the ON switch enables the 3V3 volts and this 3V3 enable +5V and -5V simultaneously.

I am using a LM22675 for the +5V and it shuts down fine.

I have measured the voltage at the SD pin of the LM5576 and it goes low, but the -5V somehow remains there.

Any Ideas?

PS:This is the Schematic

  • 0
    TI__Genius 15810 points

    Hello,

    Your SD is referenced to GND instead of -Vo. Please look at the attached example schematic for proper connection in a buck-boost topology. In the B-B topology the reference of the IC is taken to -Vout. 

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Intellectual 470 points
    Dear Akshay,

    Thank you very much for your answer.

    Your suggestion makes sense, however I am struggling to determine a way to make the SD circuit works....So far either turns On and never turns OFF or it doesn't turn ON at all.

    The issue is that the SD point (3V3) is pulled down to GND (I cannot change this unfortunately) This means that when the board is power off SD points goes to GND and because -Vout is -5V at this stage, the circuit remains ON.

    I have tried also a voltage divider connected to 3V3 at the top and -5V at the bottom and middle point connected to SD, but I am struggling to find the resistor values which fulfil both conditions (SD < 0.7V and SD > 1.225V).

    Any suggestions?
  • 0 in reply to Javcel
    Intellectual 470 points

    Actually I just found out that you need a level shifter to achieve that.

    I have found an example on the slva469 document...in case somebody is having the same issues.