02-08-2015, 03:46 PM
All those show the LR8 followed by an emitter follower, or some variation thereof. They should work, but the voltage regulation - if that matters - won't be as good as what I had in mind. Here's what I was thinking:
The resistors surrounding the LR8 can be determined from the datasheet.
The input current flows through the resistor before reaching the LR8 - this can be chosen so that when a certain current is reached - say, 5mA - the voltage across it causes the MOS-FET to pass current. From this point, it takes over from the LS8, so it bears the heat burden.
The zener and resistor connected to the gate of the MOS-FET are optional protection components. I tend to err on the side of caution when it comes to the gate of a MOS-FET, and like to fit protection. It's optional, but for what it costs... The zener voltage would be determined from the datasheet of the chosen MOS-FET...
Finally, the BJT is there to provide current limiting. It samples the voltage across the resistor in series with the MOS-FET, and once that voltage reaches 0.6V, then it starts to rob the MOS-FET of drive voltage. 12 ohms gives around 50mA...
This sort of circuit is used all the time with voltage regulators, and works well. As shown, it's just the bare bones; protection diodes and decoupling capacitors might well be needed...
The resistors surrounding the LR8 can be determined from the datasheet.
The input current flows through the resistor before reaching the LR8 - this can be chosen so that when a certain current is reached - say, 5mA - the voltage across it causes the MOS-FET to pass current. From this point, it takes over from the LS8, so it bears the heat burden.
The zener and resistor connected to the gate of the MOS-FET are optional protection components. I tend to err on the side of caution when it comes to the gate of a MOS-FET, and like to fit protection. It's optional, but for what it costs... The zener voltage would be determined from the datasheet of the chosen MOS-FET...
Finally, the BJT is there to provide current limiting. It samples the voltage across the resistor in series with the MOS-FET, and once that voltage reaches 0.6V, then it starts to rob the MOS-FET of drive voltage. 12 ohms gives around 50mA...
This sort of circuit is used all the time with voltage regulators, and works well. As shown, it's just the bare bones; protection diodes and decoupling capacitors might well be needed...
