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LM2599 Shutdown (SHDN)

Sam DeBruin
Prodigy 130 points
Other Parts Discussed in Thread: LM2599

Hello,

I am using a LM2599 to buck my voltage from 7.4V to 5V. The regulator functions correctly in normal operation, but it doesnt work in shutdown (SHDN driven low).

Briefly, my setup is a 0.1uF capacitor between SHDN/SS and GND, and a switch that can drive it low. I included a diode between the LM2599 and the switch, but because the datasheet lists 0-1.3V as the shutdown range this seemed fine. When I flick the switch, the voltage on SHDN/SS correctly goes to 300mV (the forward drop of the diode) but the output doesnt go to 0V. Instead, it hovers around 0.8V.


Perhaps most strangely, I have implemented this same design on another board (EAGLE's 'cut' command) and it works fine - output voltage goes to 0V when SHDN goes to 300mV.

Any ideas as to what I am doing wrong? Thank you

Sam

  • 0
    TI__Genius 16025 points

    Hi Sam,

    Did you try swapping in a new IC? Examine both layouts and look for any differences in components or layout. Granted both being exactly the same and one layout working, there are very few things we can suggest as potential solutions. 

    Your implementation looks fine. Additionally, I would try disconnecting the diode and attempting to pull the SHDN pin directly to GND and observe the behavior.

    Let me know if this helps.

    Thanks,

    Anston