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LM2674: MX-5.0 - No output

Yuhin Wu
Prodigy 50 points
Other Parts Discussed in Thread: LM2674

Issue is resolved

  • 0
    TI__Mastermind 25590 points
    Yuhin,

    Regarding the VIN issue:
    If the supply is not current-limited (and I don't think so if you also tried with a battery with the same result) then something must be drawing a lot of current. Measure VIN at the battery, at the J1 pin 1 terminal, at the LM2674 VIN pin, and at the end of the trace connecting to the other part of the circuit cut off by the layout picture. Are they all 3V?

    Regarding the layout:
    I do not think your layout will work. I don't have all the info (how many layers, if there are vias that I can't see) but here are my thoughts:
    1. The inductor should be as close as possible to the SW node to create as small a loop as possible (SW, through L, through COUT to GND pin of IC).
    2. The diode D3 is connected to GND but that GND plane doesn't seem to connect to any other GND plane. This should also be a short loop (SW through D3 to GND pin of IC)
    3. Every power-carrying trace (VIN, VOUT, SW) should be much thicker. At least 30 mil but ideally as large as reasonable. These look to be ~10.
    4. and more. Check the datasheet for layout guidelines.
  • 0 in reply to Yuhin Wu
    TI__Mastermind 25590 points
    Yuhin,

    I think there's something wrong with the supply. You're saying the battery measures 12V but as soon as you hook up the board (and draw a few milliamps) the voltage measured at the battery drops to 3V?

    Can you try again but instead of connecting the board, connect a 1k resistor. That should be 12mA. If that kills the battery voltage down to 3V, you know there's something wrong with the battery.

    You can also check the VIN with an oscilloscope to ensure it's not a high-frequency thing like you mentioned. But I agree with you, this is unlikely.

    -Sam
  • 0 in reply to Yuhin Wu
    TI__Mastermind 25590 points
    Yuhin,

    Can you also try connecting a 1k resistor across the power supply to see if it behaves the same?

    Thanks,
    -Sam
  • 0 in reply to Yuhin Wu
    TI__Mastermind 25590 points
    So the battery is 12V when nothing is connected.

    Connecting 1k across the battery draws 12mA and the battery stays at 12V.
    Connecting the board draws a few milliamps and the battery drops to 3V.

    Is that all correct?
  • 0 in reply to Yuhin Wu
    TI__Mastermind 25590 points
    Yuhin,

    Can you send me a picture of your test setup? All of this isn't adding up. Here are the only possible reasons I can think of:

    The only way for the battery/supply voltage to drop from 12V to 3V is to pull a lot of current. Make sure you are measuring current directly at the supply.

    If the supply isn't actually dropping to 3V but you're measuring 3V at the board, you may have a few thousand ohms in series with the connector between supply and board. This would drop your 12V supply to 3V by the time it reaches the board.

    -Sam