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Original question:

LM43603: LM43603 unstable output voltage

LM43603: LM43603 Circuit - Critical Issue in Board Bringup

Sridevi K93
Prodigy 190 points
Part Number: LM43603

Hello,

I am working to bring up one of my boards which has the LM43603 regulator as the primary voltage regulator.

I am facing a vague and critical issue with LM43603 circuit, which is designed with the following specification:

Vin = 24V

Vout = 5V

Iout = 3A

Please find attached the circuit & layout details for your reference & review. 

For testing the circuit, the typical load i.e., MCU and the core peripherals, shall be around 100mA to 400mA.

The following problems are observed:

1. Initially powered ON the regulator at no load. During no load, the output is stable and is 5V. Then when the load is connected, the output voltage drops to around 1.8V. I dont observe any drop in Vin or over current error in the PSU.

2. Whether with or without load, at any case once after the first switch ON, the successive power cycle does not happen immediately. Only after multiple trials at some instant the regulator gets turned ON again. As per observation, when the VOUT totally drops to 0V and then when the power cycle is done, the device shall turn ON.

3. When such power ON happens with load, then initially 5V shall come up for few ms and then drop to 1.8V again.

4. As per the circuit the default switching frequency is 500kHz. But at SW node, the frequency seem to be varying and different.

Could anyone please advice on what could be the cause of the above stated issue wrt LM43603 circuit and how this could be resolved?

Looking forward for your immediate support in this regard.

Thank you.

With Best Regards,

Sridevi

  • 0
    TI__Guru 62887 points
    Hello - I forwarded your enquiry to the product expert.

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Prodigy 190 points
    One mistake in my previous post. Please ignore the point 2 observation of earlier post and replace point 2 observation as follows.

    "2. Once after Vout starts to drop, then whether with or without load, at any case once after that, the successive power cycle does not happen immediately. Only after multiple trials at some instant the regulator gets turned ON again. As per observation, when the VOUT totally drops to 0V and then when the power cycle is done, the device shall turn ON."
  • 0 in reply to Sridevi K93
    Prodigy 190 points
    Hello,
    The problem is resolved.
    The value of capacitor on VCC pin was wrong as per design. So as per datasheet recommendation it was changed to 2.2uF, 10V and the now the regulator seems to be work fine thereafter.
    Thank you for the support.