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LMR62014 true shutdown implementation

Dmitry Kovalenkov
Intellectual 1210 points
Other Parts Discussed in Thread: LM3017, LMR62014

Hi Team,

My customer is looking to have the output of this device be able to reach 0V instead of staying at the same voltage as input(12V) when shutdown pin is pulled low.

Please let me know what are some of the ways to implement this functionality.

Regards,

Dmitry

  • 0
    TI__Expert 4365 points

    Boost_with_True_Shutdown_Sch.pdfHello Dmitry,

    Thank you for requesting via the forum.

    Attached is a schematic with three approaches. All of the values for the passives need be reviewed for compliance with the actual device selected.

    The first approach places a series PNP into the output power path. While the stage is running, base current to turn on the transistor is provided by the input rail.

    The second approach is similar but uses a series P-Channel MOSFET.

    For these two approaches it is important to evaluate that no turn-on overshoot occurs at input voltage extremes and load current range. This may be an issue if that output voltage is not much higher than the input voltage.

    The third approach is rarely documented but AC couples the switch node through a charge pump. The series resistor of the first diode is to prevent inadvertent tripping of switch current limit at the beginning of each switch cycle as the coupling cap is replenished. This approach has lower efficiency than the first two because of the added diode that subtracts from Vin.

    Alan

  • 0 in reply to Alan Martin
    TI__Intellectual 1210 points

    Hi Alan,

    Thank you for providing the schematic with 3 different implementations of the load disconnect. Are you aware of any parts that would not require a FET or PNP on the output path, basically they would have internal circuitry that would act as true load disconnect? I found one part with "true shutdown" LM3017 but it appears that it would also require a FET on the output.

    Regards,

    Dmitry

  • 0 in reply to Alan Martin
    TI__Intellectual 1210 points

    Hi Alan,

    The customer is looking to implement the LMR62014 using the third approach but is experiencing issues. They are modifying the standard LMR62014 EVM by adding the cap and diode as demonstrated on the schematic with LM2733Y they however cannot get the correct functionality. Our thought is that since LM2733YMF is set to 600khz switching frequency and LMR62014  is at fix fsw of 1.6mhz,  the coupling capacitor doesn't have enough time to charge up? Do you agree that we need a lower cap to use with LMR62014.

    Regards,

    Dmitry

  • 0 in reply to Dmitry Kovalenkov
    TI__Expert 4365 points
    Sorry I didn't see this earlier. Actually capacitor CC_3 should be large enough that it does not substantially charge. In operation it has only a diode or two drop across it. the value of RLs might be too high. The function of that resistor is to prevent early tripping of the switch peak current limit. It the value of RL3 is either too high or two low, there will be startup issues. The value of RL3 should probably be made smaller, say 0.3 or 0.33 ohm. Try 10uF for CC_3 as well. If you can get it to start properly then then you are on the correct track.

    Different thought. They aren't trying to start up into a solenoid load are they? Getting boost converters to start up into full load can be a challenge.
    Alan