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LM723: Negative Switching Configuration

Tyler Krochmalny
Prodigy 60 points
Part Number: LM723


Hi,

I had a question about the CL and CS pins in the schematic shown on page 13 of the datasheet. It seems they are not connected to anything, so should they be grounded, or just left open?

Thanks.

  • 0
    Prodigy 60 points

    Sorry, meant to add a screenshot of the schematic.

  • 0 in reply to Tyler Krochmalny
    TI__Mastermind 37135 points

    Hi Tyler,

    The current in the switching regulator is flowing through the switching elements.
    The LM723 only provides current limiting of the LDO IC in this configuration.
    In this circuit the LM723 should be providing only minimal current to turn on and off the T1 BJT.
    The current limit is not needed.

    The current limit is achieved by sensing the voltage across an external resistor, and applying that across the base-emitter junction of an internal BJT.  Once the sufficient current limit has been sensed, the internal BJT will conduct from the collector to emitter and pull down on the Error Amplifier.  Leaving these pins floating is sufficient in typical applications as the noise required to charge the pins to something above the base-emitter ON voltage is significant - that is, this is unlikely to happen.  However, you may safely tie both CL and CS pins to GND in this configuration, which will prevent the internal BJT from turning on and also prevent it from being affected by stray noise, if that is a concern for your system.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 60 points

    Great, thanks for the detailed reply.