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LP2985: Behavior when output voltage is short-circuited

Franz Schellhase
Expert 3605 points
Part Number: LP2985

Hi team,

Can you comment on the customer inquiry below, please?

In the data sheet of the controller, diagrams (Figures 4, 5 and 6) are shown on page 7, which include the short-circuit current. In the specifications you will also find the following information:


Now the question arises how the controller behaves when the output voltage is short-circuited (for example, via the output capacitor). Does a short-circuit current flow permanently? Is the short-circuit current limited at 350mA when the input voltage is 6V and the output voltage is 3.3V?
Maybe you can give me more information about the behavior of the controller during a short circuit?

Thank you,

Franz

  • 0
    TI__Mastermind 38195 points

    Hi Franz,

    In the event of a short-circuit event, the current through the device will spike briefly before the internal current-limiting circuitry has a chance to kick in, as seen in Figures 4 and 5. Something to note about Figures 4 and 5 is that these short-circuit events are more of a "soft" short-circuit. What that means is that the load impedance is small but not zero; if it were a "hard" short-circuit where the load impedance is zero and the output is shorted directly to ground, the output cannot be maintained at the 3.3V as in the plots. Figure 6 is interpreted the same way - the regulator wants to regulate at 3.3V and is able to do so until the load impedance falls enough such that the load current surpasses ~250mA, and as the load current increases further the output voltage falls off quickly. Figure 6 says that in the event of a "hard" short-circuit event where the output is forced to 0V, the short-circuit current is limited to ~310mA. 

    Franz Schellhase said:
    Now the question arises how the controller behaves when the output voltage is short-circuited (for example, via the output capacitor). Does a short-circuit current flow permanently? Is the short-circuit current limited at 350mA when the input voltage is 6V and the output voltage is 3.3V?

    In the event that an output capacitor fails or something similar, there will be short-circuit current that is maintained at a level that depends on exactly what the load impedance falls to, which will be a point on the plot in Figure 6. The output voltage will also fall if the short-circuit current is sufficiently high. So, the short-circuit current will be permanent in the sense that there is no built-in mechanism to shut the LDO off, but the LDO may cycle in and out of thermal shutdown during a short-circuit event if the short-circuit condition is maintained. This is a reason the PG pin is included as it allows a way to detect events such as this because the output voltage will fall and PG will no longer be good.

    Best regards,

    Nick