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LM2576: Current being limited

Part Number: LM2576
Other Parts Discussed in Thread: LM2676, LM2596

Hello, 

I am trying to run a lidar at 12V using the LM2576 to get the voltage from 24V to 12V. When I start with an input voltage of 24V the current draw is ~250mA. The lidar attempts to draw more current and the output voltage drops to 8V momentarily and returns back to 12V. When I start at 18V, the current draw is ~600mA. Now when I slowly increase to 24V the current remains at 600mA. But why doesn't the current reach 600mA when I start with 24V? The output of the IC is steady at 12V at all times

  • I think I need some clarification about what you are seeing.
    Are the currents you are quoting from the input supply or the load??
    Please try to explain the exact sequence of your input supply and load start-up.
  • The current I am quoting can be seen on the input supply.
    1. I start up at 24V input, current is at 250mA and above graph can be seen on the oscilloscope. The Lidar is attempting to draw current. Current can be seen to rise to 350mA at the exact time when the suddent drop in voltage is observed. This keep happening continuously.
    2. I start up at 18V input, current seen on the power supply is 600mA. Everything works fine.
    3. I start at 18V, everything works, and then I slowly increase my supply to 24V. The Lidar works fine here as well. The current is 600mA.

    The only problem is why can I not get the right current when I start at 24V
  • It could be that the device is reducing frequency and folding back current limit.
    It would be good if we could see a 'scope shot of the SW pin and Vout during the bad behavour.
    Aslo, if possible a shot of the inductor current.

    As a test, you could try to start the regulator with no load and then apply the load.
    It may be that this device is not optimal for you application.
    Does the load have a constant current characteristic or a constant resistance characteristic?
  • There is no SW pin on this IC. I put in a 0.1 Ohm resistor to measure the current at the output and the circuit started working surprisingly. I don't entirely understand why that happened




  • It seems that the 0.1 Ohm has helped the switcher start-up by raising the effective impedance of the load.
    It seems that you have a difficult load to start-up.
    Can you control your load by disabling it until the switcher can start and then enabling the load?

    You may need a different regulator such as the LM2596 or LM2676 that have different start-up characteristics that may be a better match for you load.