Clamping to Linear regulators

Will be OK if you have a guaranteed minimum consumption of > apr. 210 uA off the 3V.

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

AJ wrote:

Thanks for you reply. What do you think would happen if the regulator had no input?

Regards

AJ

Most power supply type regulators (i.e. 3-terminal ones) are 1-quadrant devices -- they supply current in one direction from a voltage at one polarity. If you just had a 3-terminal regulator, a 24V supply and your resistor the voltage at the output of your regulator would rise, possibly not to 24V but probably well above 3.3V.

If you just want a shunt regulator look for a 'precision zener'. These are IC's that behave like zener diodes, but with much better regulation. If you don't mind wasting a bit of power in your system make sure that you're drawing more current from 3.3V than the maximum that can be delivered from your '24V' rail and resistor, possibly with a simple load resistor on the regulator output.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

Try a TL431

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Nothing I would worry about, as long as the 200-odd microamps have where to go (which is the case whith the regulator on anyway, so I don't think you can have an issue here).

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

AJ wrote:

Depending on the load, it could potentially drag the 3.3v rail upwards. A more universal solution would be to use an opamp set as a unity-gain buffer, powered from the supply into the 3v3 reg, with the input from the 3.3v supply.

Hey- we are truly honored by your willingness to consider our feedback to your esteemed self and advanced work. I cannot tell you just *how* excited we are about the privilege and opportunity. Just the idea of someone like you even glancing at my work adds an overwhelming sense of fulfillment to my life: View in a fixed-width font such as Courier.

. . . . +3.3V . | . [1M] . | . | . 24V >--| V . in clamped . . . . . .

Thanks to everyone for their feed back. The 3.3V rail should have at least

20ma load on it all the time and I also have a 3.6V TVS diode on the rail which I was hoping would prevent the voltage rising. Do you think this would be ok or should I add the OP AMP?

Regards

AJ

Unless you are clamping a LOT of 24V/100K loads to the 3v3 rail it would be fine. The only problem arises when the current you are sinking is more than the load current, in which case the regulator would cease to be conducting and the voltage could rise. As long as there is enough load, the sink current just makes the regulator push slightly less harder to maintain its 3.3v output.