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Original question:

LM61480: Dual Voltage Power Supply using LM61480

LM61480: Question about LM61480

Jimmy Huang
Genius 3505 points
Part Number: LM61480

Tool/software:

Hello：

Display no current and power PCB test record (1)

1. The abnormal device is powered on for measurement. After the pump is turned on, when the output power is 1W, the host computer displays that the pump output current is 0, and the circuit board makes a dripping sound. The waveform of the SW output pin of the DCDC chip LM61480 is measured as shown in the figure below, indicating a hiccup protection phenomenon.

  

2. Measure the voltage at each position in the pump drive circuit and find that all devices at each position have abnormal test data as follows:

2.66V R5 2.39V R6 2.14V R25 1.90V R22 1.66V R27 0.841 R31 0.25V

3. The U13 chip was irradiated with X-RAY and no welding short circuit was found.

4. Remove the U13 power chip LM61480 and replace it with a new LM61480. When the pump is turned on and the output power is 1W, the upper computer displays a pump output current of 5.2A, which returns to normal. Test the waveform of the SW output pin of the DCDC chip as shown in the following figure.

  

  • 0
    TI__Mastermind 21960 points

    Hello,

    Is it possible for you to provide higher-resolution images of the layout and schematic? The images as posted are too blurry to review.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Genius 3505 points

    Hi Ridge:

    I post images again, is it clear?

    Display no current and power PCB test record (1)

    1. The abnormal device is powered on for measurement. After the pump is turned on, when the output power is 1W, the host computer displays that the pump output current is 0, and the circuit board makes a dripping sound. The waveform of the SW output pin of the DCDC chip LM61480 is measured as shown in the figure below, indicating a hiccup protection phenomenon.

      

    2. Measure the voltage at each position in the pump drive circuit and find that all devices at each position have abnormal test data as follows:

    2.66V R5 2.39V R6 2.14V R25 1.90V R22 1.66V R27 0.841 R31 0.25V

    3. The U13 chip was irradiated with X-RAY and no welding short circuit was found.

    4. Remove the U13 power chip LM61480 and replace it with a new LM61480. When the pump is turned on and the output power is 1W, the upper computer displays a pump output current of 5.2A, which returns to normal. Test the waveform of the SW output pin of the DCDC chip as shown in the following figure.

       

  • 0 in reply to Jimmy Huang
    TI__Mastermind 21960 points

    Hello,

    I am able to see the schematic more clearly. On the schematic I see that the FB pin of the buck is connected to the pump circuitry. This could potentially be causing issues with the buck converters regulation. What is the intended purpose of the connection from the pump circuitry to the FB pin?

    The the SW node waveform for the first device is typical of light load or no load. The device is operating in discontinuous conduction mode and is not in hiccup. Please refer to Figure 9-29 in the datasheet for a similar light load waveform. 

    The switching waveform that is shown for the second device is unstable. 

    The inductor could be increased slightly, but it looks OK. The output capacitance should be increased to help with stability. Here is an example design I made in Webench with similar conditions to yours: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=18DCCDE16F6B4315

    What is the expected output current of the buck converter?

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    TI__Mastermind 21960 points

    Hello,

    Since we have not heard from you in a while I will be closing this thread. If you have more questions, you can re-open this thread by replying or create a new thread.

    Best regards,

    Ridge