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LM2651 Sleep Mode

Chirag Rasputla
Prodigy 190 points
Other Parts Discussed in Thread: LM2651

Hello,

   Have a 9VDC battery operated device and used LM2651 for 5V output generation. Observed that when there is no load connected on output side, the device consumption is appx 6mA. The questions are below:

1) Can this consumption be reduced (to save battery power)?

2) In datasheet mentions a term "sleep mode", when does LM2651 enter sleep mode?

3) Is there any external control to enter sleep mode?

Please suggests.

Thanks & Regards,

Chirag

  • 0
    TI__Intellectual 1775 points

    Hi Chirag,

    1) 6mA is in line with the expected consumption for this part. The consumption can be improved slightly by adding a schottky diode across the low side FET of the IC (between PGND and SW).

    2) The sleep mode is entered at light load when the current drops below the sleep mode threshold and the feedback voltage rises above the hysteretic threshold. For the -ADJ version it should be around 24mV above the feedback voltage under load. This should translate to close to 100mV above the set output voltage. In this mode the part runs in a hysteretic mode, with a reduced amount of pulses per second to reduce the switching losses.

    Here is an excerpt from the datasheet ("MAIN OPERATION", page9):


    "When the load current is higher than the sleep mode threshold, the part is always operating in PWM mode.

    ...
    When the load current decreases below the sleep mode threshold, the output voltage will rise slightly, this rise is
    sensed by the hysteretic mode comparator which makes the part go into the hysteretic mode with both the high
    and low side switches off. The output voltage starts to drop until it hits the low threshold of the hysteretic
    comparator, and the part immediately goes back to the PWM operation. The output voltage keeps increasing
    until it reaches the top hysteretic threshold, then both the high and low side switches turn off again, and the
    same cycle repeats."

    3) Not really. The FB pin might be offset above the hysteresis level to force the part into sleep mode but I don't see a convenient way of getting a good behavior that way. When FB reaches this threshold, it simply shuts down for as many cycle as needed until the voltage goes back below the hysteresis gap. So in principle you could add a voltage detector and artificially modify the hysteresis levels but I don't see this as being a practical solution.

    Best Regards,

    Florent

  • 0 in reply to Florent Boico
    Prodigy 190 points

    ok thanks Florent.

    Regards,

    Chirag