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LM27951: Shutdown current much higher than expected

Jonathan Bradshaw98
Prodigy 100 points
Part Number: LM27951

I am working on a design which has an LM27951 LED backlight driver and low power sleep modes.

In normal operation, the LM27951 seems to operate OK; the backlight is illuminated consistently etc.

The problem I have is in low power sleep. I have pulled the LM27951 shutdown pin low and verified that the internal charge pump stops (with DMM and oscilloscope), so supply current should be <1uA per the datasheet. But the chip draws approximately 2900uA in this mode.

Any suggestions for what to do next?

  • 0
    TI__Mastermind 22745 points

    Hi Jonathan,

        Pull down the "EN" pin to ground and serially connect the multimeter between the power supply and LM27951 "VIN" pin. It should enter shutdown mode and should consume <1uA. Remover all the connections or wires or oscilloscopes from the board.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 100 points

    Yes, that's where I started.  With outputs disconnected and EN pin low I measured approx 2900 uA.  Charge pump doesn't seem to be running (as expected).

    (Note: I spent days tweaking test firmware, chopping tracks and scrutinising datatsheets before eventually coming to this chip.)

  • 0 in reply to Jonathan Bradshaw98
    TI__Mastermind 22745 points

    Hi Jonathan,

        That's impossible, there should some circuit that consumes the current and power, you need to check the setup and test method.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 100 points

    I agree that it's very strange.  Here's how I found the issue:

    1. Noted product sleep mode current was quite high
    2. Worked with firmware engineer to deploy deeper sleep mode
    3. Confirmed that all relevant enable pins had pull down resistors
    4. Measured enable pin voltages during sleep mode to confirm they are low  (measured < 10mV above ground)
    5. Blamed a radio module. Cut supply tracks to radio module and installed shunt resistors. Found radio module shutdown current <100uA. Wasn't the radio.
    6. Blamed a USB interface chip. Cut supply track to chip and installed shunt resistor. Found current <100uA. Wasn't the USB chip.
    7. Used a hot air tool to selectively heat components while looking at power consumption
    8. Found current consumption rose a bit when LM27951 heated
    9. Drilled out supply via to LM27951, confirmed open circuit and soldered on test wire with 1R shunt. Measured 2900uA.
    10. Powered LM27951 with separate bench supply (EN pin held low with pulldown resistor) and measured around 2.9mA on bench supply
    11. Measured voltages on charge pump capacitors and Vout capacitor with DMM and oscilloscope: read 0V on all
    12. Measured voltage on Iset resistor: measured 0V

    In case of instrument error:

    • I am using a bench supply with current readout (0.1mA resolution) for basic measurements
    • Confirmed bench supply reads 0.0mA when load disconnected
    • I have cross-checked with 2 digital multimeters

    What can I do next?

  • 0 in reply to Jonathan Bradshaw98
    TI__Mastermind 22745 points

    Hi Jonathan,

        Have you tried that just power on the device and no software run(not use the firmware to configure into sleep mode) and pull down the "EN" pin, what's the power consumption for this situation?

        Or maybe you need to check with another device? Is this one damaged before?

    BR

    Sean