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Fortunately, that is not required for the pass transistor to be “on.” All that is required is that the base be low.
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Looking at the DC sweep, there is, of course, a region of operation where the comparator is not under power. The output is connected through a current limiting resistor to the base of the pass transistor. The input voltage is monitored by a voltage divider on the noninverting input. Ī Zener diode is used to set a voltage reference at the inverting input.Looking at the connections to the comparator: Small package size is achieved even though the transistor is passing high current, its voltage drop is so low that the overall power dissipated in the transistor is low. The pass transistor is a PNP switching type, optimized for low collector–emitter voltage drop and high current. This, of course, is a nonrecoverable condition that may be appropriate for some applications, but why design a nonrecoverable circuit when it is just as easy to design a recoverable one? There are two types of overvoltage protection circuits-one that opens the voltage path and does not allow voltage to pass to the load, and a shorting or “crowbar” type of protection that shorts overvoltage to ground, thus blowing any fusible element on the input voltage. If both the regulator and overvoltage protection circuitry fail at the same time, the whole system should come under suspicion because something really dramatic has happened. But playing a game of odds, anything short of a lightning stroke or direct connection to line voltage will probably not produce such a failure. Granted, there are no absolutes, and there are always circumstances where the regulator and the protection circuitry fail at the same time. This is almost always catastrophic, after all, if raw unregulated voltage would not cause a problem why not use it in the first place? Fortunately, it is easy to add an overvoltage protection circuit to greatly reduce the likelihood of this happening. Bruce Carter, Ron Mancini, in Op Amps for Everyone (Fifth Edition), 2018 22.6 A Companion CircuitĬonsider the case when a voltage regulator fails- allowing unregulated input voltage to be applied to the load.