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LM46000: LM46000 EVM can't power up

Part Number: LM46000

Hi team,

This is FAE Michael. I have 3 LM46000 EVM. But they all can't power up... I only connect the input Vin and Gnd to the power supply 12V, without any load. But the output is charging slowerly from 0.3---1.2V.  Because the MOSFET is not on, the charging output make sense. But what I don't know is why it can't operate normally.

Internal VCC is 3.3V. It is right. 

Actually when I measure the SW waveform, there is one pulse at power up moment about VIN=4V. Then the device can't switch. 

Really don't know why because it is EVM. I think it can be directly used without any change. Do you have this issue before? I didn't change any component on the board... I use multimeter to measure VIN, the VIN connection is good enough and it is 12V normally.

Regards

Michael

  • Michael,

    Please measure the voltage at the pins of the IC and make sure nothing goes against the typical operating conditions.

    I'll order an EVM and check when I get it in a few days.

    -Sam
  • Hi Sam,

    Thanks. I tried more boards... And all are burnt at first power up.

    After check the resistance and behavior, the SS pin is not like what they are like at first. At first, SS pin has some diode connect to the GND. We can measure it with multimeter. But after the power up and power down, it is high-impedance, measured by multimeter. 

    I measured the input voltage. There is only a small spike around 2.5V high at first and then slowly ramp up from 1V to 12V.

    I can't find any other indication so far....

    Regards

    Michael

  • Michael,

    This is strange.  I found an EVM in our lab and it powered up just fine. I'll try out the new one when it comes in the mail. Maybe something was assembled wrong.

    Can you send me a picture of your setup and a close-up picture of your board?

    And I want to confirm, since the EVM is unmarked and can be confusing, that we are both hooking it up the same way:

    -Sam

  • Hi Sam,

    I think I may know why. I use a 24V adaptor and there is no issue. But with digital power supply, the chip will be 100% burnt. 

    I think the digital power supply has some problem with the relay inside. Maybe one is connected too quick than the other, then there may be a very sharp high voltage during VIN?

    Not sure the root cause, but it should be the problem from power supply.

    Michael

  • Michael,

    That sounds like that could be the issue. Thanks for getting back to me.

    -Sam