Using MLCCs for timing on the EN pin of a regulator

[jolshefsky]

I'm designing a battery-powered device and I want it to turn on with a button press and stay on for some time around 4 minutes. I was working with the MIC5235 regulator and figured I could use its enable pin as the way to keep the circuit alive. I did some quick math and decided a 3.3µF capacitor tied to the enable pin when charged with 9 volts would discharge to 2V (the enable shutoff voltage) after about 4 minutes (given 0.1µV enable-pin current listed in the data sheet). Brilliant, I thought.

Then in laying out the circuit, I wanted to make sure I could get a capacitor. Remembering the discussion about multi-layer ceramic capacitor (MLCC) voltage coefficients, I figured I should make sure my selected 3.3µF really delivers 3.3µF at 9VDC. I started to find there's a hole in SMD capacitance right around the 0.04µF-4µF: Murata, for instance, has a GR3 line of MLCCs with good DC characteristics that get as large as 0.022µF, and then a line of electolytics starting at 6.8µF. Naturally, I can get an MLCC with a wild voltage coefficient ...

So first of all, is this a rabbit hole worth pursuing? Will my enable-pin circuit behave like I expect or should I just scrap it right now. And if it does work, how will that voltage coefficient work out? Should I go with, say, a 6.8µF MLCC with a voltage coefficient of 0.5 at 9 volts so I get a 3.8µF effective capacitance? Or does the voltage coefficient vary with charge voltage (not voltage applied), so as the voltage slowly drops, the effective capacitance would go up meaning I could go with, say, a 4.7µF cap?

[mmagin]

Sounds like a job for tantalum!

[T3sl4co1l]

1. How accurate do you really need this time?
2. How accurate is the actual threshold?  I'm guessing it's not 2.00V precision.  (Also, does it have hysteresis?)
3. How accurate is the internal current sink? Do they specify limits?
4. You didn't mention what voltage you're considering.  Values around 3.3uF are plenty available at 16V+ ratings, X7R, probably in 0805 to 1210 packages.

In any case, if C is dropping off at high voltages, why do you care, as long as it's repeatable?  Use a 4.7uF 10V, or even a 6.8uF 6.3V.

A bigger problem will be aging, so that after some months or years, the delay might be half.

If consistent timing is important, tantalum is ideal; but ONLY if 1-3 are true, because otherwise that sweet 10% tolerance won't get you squat.

Tim

[Alex Trofimov]

First of all applied voltage and charge voltage are the same thing.

If getting a proper value is a problem, get a bigger one and add an additional discharge resistor. In this case if as T3sl4co1l said you need accuracy, you may consider trimming or selecting the resistance.

About effective capacitance calculation, sorry, can't say much about this. Some good datasheets tell how the capacitance vs DC bias was measured, maybe the clue is there.

[danadak]

The enable pin does not have a Schmidt input, no spec on allowed ramp
rate. Makes one wonder how all downstream circuits will work with noisy
power while regulator coming up to power.

To clean that up consider single gate logic from (and a zener to power it)
On Semi, Fairchild, ST as a cheap fix. That would take care of both switch
bounce and C ramp.

Regards, Dana.

[mikeselectricstuff]

Does the device have an MCU ? If so, do it in software. If not, maybe use a CD4060 oscillator/counter
The enable pin current is likely to vary a lot between parts and over temp, so dangerous to rely on it.

[jolshefsky]

1. How accurate do you really need this time?

Not very. 2 minutes to 6 minutes would be fine ... it's just a soft-power switch.

2. How accurate is the actual threshold?  I'm guessing it's not 2.00V precision.  (Also, does it have hysteresis?)

No, not very. The spec is 0.6V max off, 2.0V min on for the logic levels. It doesn't state hysteresis and I imagine it might not have it, but poor behavior in the middle region is okay (except if the output voltage went above spec, which it shouldn't be allowed to do.)

3. How accurate is the internal current sink? Do they specify limits?

Double-checked that:
At 0.6V, -1.0µA min, 0.01µA nom., 1.0µA max.
At 2.0V, 0.1µA nom., 1.0µA max.
At 24V, 0.5µA nom., 2.5µA max.

So yeah, not very tightly specified. If the voltage cutoff works, a much larger capacitor with a high-value bleeder resistor might be a better choice.

4. You didn't mention what voltage you're considering.  Values around 3.3uF are plenty available at 16V+ ratings, X7R, probably in 0805 to 1210 packages.

Battery-powered, 9V specifically, so there are all kinds of X7Rs to use.

In any case, if C is dropping off at high voltages, why do you care, as long as it's repeatable?  Use a 4.7uF 10V, or even a 6.8uF 6.3V.

That's the idea, although according to the linked article, the voltage coefficient on X7Rs will likely drop the capacitance to 20% of its nominal value. Even with a sloppy target of 2 minutes to 5 minutes, calculating the discharge characteristics seems a bit unknown.

Does the device have an MCU ? If so, do it in software. If not, maybe use a CD4060 oscillator/counter
The enable pin current is likely to vary a lot between parts and over temp, so dangerous to rely on it.

Alas, no MCU otherwise that would have been my first choice too. I was not sure if the enable pin/capacitor was a good idea, and it looks like it's leaning toward not good enough. I was looking for a cheap solution to avoid a power switch, and to make sure the batteries wouldn't bleed dry (wet gooey mess, actually) if someone left it on too long. I might be able to use a MOSFET with a capacitor discharge instead which would be reliable enough and offer just enough accuracy. Stay tuned!