I'm back with yet another mystery.
- Why would an input to a voltage clamped circuit de-stabilize and or elevate that voltage clamp's upper rail?
- How do you properly clamp voltage using a lightly loaded LDO supply?
How I got here: I'm in the middle of developing a sensor for use in the field. I'm trying to make this thing robust, so I've added diode clamps to all the inputs with access to the outside world. The diode clamps look like this (input is on the left side, and is fed into an xbee IO port):
The system runs off of a solar source etc so I've got a switching regulator (LMR1403), cascaded onto an LDO (MCP1802) to run everything.
While performing my due-diligence and testing the voltage clamps, I noticed something odd:
- yellow = clamped line
- blue = function generator input
- green = regulated 3.3v:
The test input that I sent into the circuit was "raising" the power rail, thereby causing the voltage clamp to fail (since the diode only holds the input to the rail). The test input was fed with a function generator, 1khz, varying amplitudes, but obviously amplitudes greater than 3.3v.
I simulated the circuit in LT Spice until I could recreate the problem. I used a similar LDO from LT instead of from MCP. I assumed (perhaps in arrogance?, but oh well) that most of the low-power LDO designs were fundamentally (hopefully) the same. Below is my simulation.
It seems that I've correlated at least 2 possible issues:
- Insufficient resistive load.
- Insufficient capacitance on the output rail.
If I mess with either of those in simulation (make them too small, or rather "too big" for the load) I can cause very similar situations to occur. I checked with my circuit, and added more resistive load (circuit runs 30mA), and a MUCH larger output bank capacitor (I only had a 47uF ceramic on hand to swap out with the 1uF that was there) and both "solve" the problem at small input amplitudes, but both still fail for large inputs (20V p-p waves). The wave is properly rectified for the ground loop, but fails to be contained within the regulated 3.3v.
In an attempt to triple check everything, I patched in a bench supply to the 3.3v rail and it behaved in a similar manner.
So my question is basically this.
- How do you protect against lightly loaded circuits? Like, say, a sleeping microcontroller that is only drawing uA.
- Am I missing something fundamental in my voltage clamp design? (I hope so).
- Should I not be using an ldo for this case? I am starting to question whether I should just readjust my switching regulator to feed the circuit 3.3, but was worried that it would not be stable enough for XBEE operations.
Many thanks. I am forever learning new things. Matt
Design Rev #2. I've thrown in a TVS diode for extreme spikes, then increased the value of R22 to limit the current intake through the circuit (thereby reducing the voltage raise). Here's what I've got now:
I'll adjust C1 accordingly for how fast a rise time I need on the signal. Thanks.