LM43603: Powering IC from load side

Hello

Usually it is not a good idea to allow voltage on the output of a regulator when the input is zero volts (or even floating in some cases).  This condition could be caused by a very large output capacitor that is charged when the input is removed, or some kind of active load suppling a voltage, or by a "dual feed back-up" configuration.  There is a diode between the drain(VIN) and source(SW) of the power MOSFET inside the converter.  With a voltage on Vout, and the input held at zero volts, there will be a path from Vout to Vin through the inductor and this diode.  Even if the input is floating, a current will flow and try to charge the input caps, possibly to a high voltage due to resonance with the caps and the inductor.  This phenomenon has been seen in the past under some conditions.  Allowing current to flow through this diode can cause a parasitic to turn on or even damage the diode.  Since it is not a "true" diode but a parasitic formed when the MOSFET is constructed on the die, it is not designed to take current for more than a few 10s-100s nsec.  A blocking diode in series with the load is the best way to prevent any issues from this cause.  TI also has dedicated "diode-OR" controllers that are designed for this type of application.  Please also see app note SLYT689 and SLVAE57 for more information.

Thanks