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LMQ644A2-Q1: LMQ644A2-Q1 Backfeed Behavior in Unpowered Input Condition

Attila Molnár
Prodigy 80 points
Part Number: LMQ644A2-Q1

Tool/software:

Dear Support Team,

We are currently designing a new project utilizing the LMQ644A2-Q1 in single-output mode as our primary power supply. During certain operational scenarios, the input of the LMQ644A2-Q1 may be unpowered while its output is energized by another circuit within the system.

Given space constraints, we aim to minimize the component count and avoid adding external protection circuitry. Therefore, we require clarification on the LMQ644A2-Q1's behavior under this specific condition:

My Question: Can the LMQ644A2-Q1 withstand a backfeed voltage applied to its output when its input is unpowered, without incurring permanent damage?

We understand that the device will not be expected to function as a power supply in this state. Our primary concern is ensuring the device's survivability and its ability to operate normally once the input is energized and the output is loaded.

We appreciate your prompt response and technical guidance.

Sincerely,

Attila

  • 0
    TI__Guru 72525 points

    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.  Alternatively, a diode from output to input will provide a path for any charging current to bypass the regulator.  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

  • 0 in reply to Frank De Stasi
    TI__Guru 72525 points

    Hello

    I will close this post due to inactivity.

    Thanks