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LMR61428 Startup Issue

Other Parts Discussed in Thread: LMR61428

               LMR61428 Schematic.pdfLMR61428 Startup Waveforms.zipAs attached are the related layout and schematic. The input voltage range of LMR61428 is 3.5V to 6V, and the output voltage is set as 8V by the resistor divider. Attached waveforms shows the startup waveforms when no load. It shows that the peak voltage goes up to 18V which would damage our IC and the output load (laser lamp). This startup issue happens every startup. Could you give any suggestions to avoid this peak voltage?

               Thank you very much.

  • Hello,
    What is the end application here? Your schematic doesn't have any component values noted on it. Would you kindly send us the BOM used?
    The LMR61428 is a gated oscillator regulator. This means that the duty cycle will be fixed around 70% and the device will stop switching when the output goes out of the range of the window. Would you also please send scope shots of SW, VDD, BOOT, and input voltage? The typical startup time is less than 1ms (about 500 to 600us). Your scope shots are of much larger time duration. Your schematic shows BOOT connected to VOUT. But the datasheet says BOOT should be tied to VOUT only when the VOUT is in the range of 2.5V to 5V. Do you have another rail that you could use to bootstrap the regulator? You could alternatively use a 5V zener to limit the voltage on the BOOT and VDD pins. I wonder if the devices are damaged considering the abs max on BOOT and VDD is 10V and you are above that for about 300ms.

    Regards,
    Akshay
  • Akshay,

             Thank you for your support.

              The component values are shown in the schematic pdf document (you can click the component one time). Also, the device are not damaged, it can work well after the startup. 

    BTW, is there any potenial risk when the VDD voltage is over 5V ?

  • Hello,

    Voltage higher than 5V at VDD would exceed datasheet recommendation. I am not sure what exact misbehavior will manifest.

    Regards,
    Akshay

  • Soccer,
    Looking closely at your schematic we think your COUT might be de-rating and at no load condition, the output might fly up before the comparators react to it. Please experiment with tantalum capacitors as well for this schematic.
    Regards,
    Akshay