Help Understanding This Voltage Regulator Circuit

[t1d]

I found an interesting circuit at Linear Technology and I don't understand what is going on with the voltage regulator section. The circuit is powered by a 9 volt battery and the VR drops it down to 5 volts. What I don't understand is the LED in series in the power rail and the resistor in parallel to the LED. Maybe they are using the LED to drop the voltage, before the regulator? It is a diode... "Dunno," as they say... Obviously, I am not an EE...

Thanks for your help...

1 - Voltage Input
2 - Ground
3 - Shut Down
4 - Bypass
5 - Voltage Output

[edpalmer42]

it's not part of the regulator itself, it's just a sneaky way to get a 'power on' light without wasting any current.  You have to drop the voltage anyway so might as well include the LED instead of just dropping it in the regulator.  To get 2.0V dropped across the resistor, you'd have a load current in the range of 25 -30 ma.

Where is this circuit from?

Ed

[t1d]

Thank, Ed...

Well, it is a clever approach and I like those...

The circuit is a Differential Probe:
http://www.linear.com/solutions/6058

Thanks for your help!

[Zero999]

The main downside is it reduces the minimum operating voltage by a couple of volts, which could be an issue with a small 9V battery. The traditional approach but with a high efficiency LED with a forward current of 1mA might give better battery life.

Nice circuit. It's a shame it's AC coupled only though.