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LM2766: Schottky Diode. Necessary?

Radosav Majstorovic
Prodigy 50 points
Part Number: LM2766

Hallo E2E Team,

The LM2766 says that it needs a diode between Vin and Vout to protect the device from turning on its own parasitic diode and potentially latching up.

Unfortunately, the datasheet does not show, where this parasitic diode acts, what happens or how the component behaves afterwards.

Is the Diode really necessary?

C1=1uF, C2=10uF and Load current about 1,3mA.

Best regards

  • 0
    TI__Guru* 75270 points
    Hi Radosav,
    As indicated in the datasheet it is necessary to have the Schottky diode in place. This part is transferred to us so I cannot tell the details of the parasitic diode at this moment, but the position of the Schottky diode implies that the parasitic diode is between the V+ and VOUT. If you really want to know the exact nature of the parasitic diode, let us know and we can ask our designer to dig into the design file and explain it to you. On the other hand, we always require the users to follow the datasheet recommendations.

    Thanks,
    Youhao Xi, System and Applications Engineering, APP-BMC-BCS, TI
  • 0 in reply to Youhao Xi
    TI__Prodigy 50 points

    We already designed it without the Schottky Diode. We want to know what dangers could come to us and if we need to do a new design, because of that. We need more details about this.

    Best Regards

  • 0 in reply to Radosav Majstorovic
    TI__Guru* 75270 points
    I think you should update the layout. Without the Schottky diode, the circuit can get latched up at startup according to datasheet. We can ask the designer to look into the design to understand how, but this does not change the fact of latching up.
  • 0 in reply to Youhao Xi
    TI__Prodigy 50 points

    Please ask the designer and provide me specific information about the problems, because we arent dealing with any problems without the diode, yet.

    Thanks.

  • 0 in reply to Radosav Majstorovic
    TI__Guru* 75270 points
    The following is our design team's reply regarding your question.

    Internally, from Vin to Vout, there are two switches. If no external diode is used, the output capacitor will charge up during start-up through the back-body diodes of the two internal switches. When running current through the back-body diodes of MOS devices, there is a risk of latchup due to internal pnpn junctions that are not intended to be forwarded biased on normal operation.

    While they do not see an issue in their testing, this does not guarantee that part will never latchup on startup, especially at high temp, where the pnpn latches are easiest to trigger. The external diode bypasses the internal switches at startup and also charges the output cap to a higher potential than the two internal back-body diodes would.

    Hope this closes this case. Again, please ask your customer to add the Schottky diode if they want to rule out the risk of latching up.