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LM2596: Reverse protection of Output pin 2?

6001060
Prodigy 10 points
Part Number: LM2596

If an LM2596 is used to charge, say, a 12V lead acid battery, and the input comes from, say, a 36V solar panel that will generate no voltage during the night, is it necessary to protect the LM2596 against the battery voltage when no input voltage is present?

The data sheet does not include a spec for Vbe max of the internal NPN switching transistors. If this was >14V I think no such protection would be needed. If it is less than 14V then a Schottky diode between output and battery would be needed.

(Yes I know there are bespoke MPPT chips, but I have my reasons for contemplating LM2596 use here)

  • 0
    TI__Mastermind 29250 points

    Hello,

    It is suggest to follow ABSMAX table. I cannot advise otherwise for liability reasons.

  • 0 in reply to Marshall_Beck
    Prodigy 10 points

    What do you mean? As I pointed out, the datasheet does not include an absolute maximum rating for Vbe. It does say "Output voltage to ground, steady-state... -1V", but as this value is negative it doesn't seem to be relevant.

  • 0 in reply to 6001060
    TI__Guru 57555 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).  There is a diode between the (VIN) and (SW) of the power NPN 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 NPN is constructed on the die, it is not designed to take current for more than a few 100 nsec.  One way to avoid this issue is to use a diode from input to output to provide a path for the output capacitors to discharge around the device.  A Schottky diode with cathode connected to Vin and anode connected to Vout can be used.  A blocking diode in series with the load can also be used.  Please also see app note SLYT689 for more information.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 10 points

    Thanks. That app note is about parasitic diodes in power mosfets between VIN and SW. The LM2596 doesn't have a power mosfet and it doesn't have a SW pin. It seems to have power NPN transistors. I don't think there would be a parasitic diode from emitter to collector.

  • +1 in reply to 6001060
    TI__Mastermind 29250 points

    Hello.

    Output is SW pin. It does seem that the block diagram shows different than typical output  stage...perhaps a simplified block diagram.

    I do agree with Frank's comment that their is a possible conduction path between output pin and vin. He would be the most knowledgeable person within TI on this device.

    We recommend design in such case as your below