## Voltage Regulation

### Voltage Regulation

Looking for a suggestion on a voltage regulator. It will be used in a system with batteries, and running continuously. It will be supplying about 4 or 5 volts, at a current of less than one milla-amp. Looking for minimal drop out voltage and operating power. The normal regulators have idle current of a few mills, which is too much for this application. Thanks again guys.

I like national's web bench - all you have to do is plug in your numbers and it spits out a design. (OK, it's a little morte complex than that, but not much). You don't need to understand SMPS design theory.

http://webench.national.com/appinfo/pow ... /index.cgi

Here's a breakdown of using a switcher:

E1=10

I2=0.005

E2=5

P2=E2*I2

P1=P2/0.85

I1=P1/E1

E1=input voltage

I1=input current

E2=output voltage

I2=output current

P2=E2*I2

P1=P2/eff, eff=0.85

I1=P1/E1

If we calculate all this out, we get:

I1=0.00294

and if we look at the ratio of I2/I1 we see that we get

about 70 percent more runtime if we use a switcher.

This means if we dont use a switcher (use a linear) and we get

1 hour runtime, if we instead use a switcher we get about

1 hour and 42 minutes runtime. Thus, we almost double the

runtime if we use a decent switcher.

The choice is yours....

E1=10

I2=0.005

E2=5

P2=E2*I2

P1=P2/0.85

I1=P1/E1

E1=input voltage

I1=input current

E2=output voltage

I2=output current

P2=E2*I2

P1=P2/eff, eff=0.85

I1=P1/E1

If we calculate all this out, we get:

I1=0.00294

and if we look at the ratio of I2/I1 we see that we get

about 70 percent more runtime if we use a switcher.

This means if we dont use a switcher (use a linear) and we get

1 hour runtime, if we instead use a switcher we get about

1 hour and 42 minutes runtime. Thus, we almost double the

runtime if we use a decent switcher.

The choice is yours....

LEDs vs Bulbs, LEDs are winning.

They come in a tiny SOT-23 package and cost only $0.82

http://para.maxim-ic.com/cache/en/results/5094.html

Many other to choose from but these seemed closest to what you asked for.

THe MAX8880 is worth a look at too as the dropout is only 100mV at 50mA and Iq is 3.5uA and has a slightly larger QFN package

Which is more important, low dropout or low Iq? I suspect the MAX8880 will squeeze more out of a battery.

Ultra low Iq counts as it determines how much current the regulator uses just to operate (i.e. output current =0, you would get this at the input). This is particularly important to squeezing every but of the capacity out of the battery. Not many used more than 1mA

With fresh 4 AA batteries in series you get 6V, 1500mAh (nominally 9Wh available to burn). With a load of 1mA at 5V you loose only 1.002 mA * 1V = ~1mW in the regulator and 5V*1mA= 5mW in the load. Power loss in the regulator decreases as the battery discharges until about 5.3V when it will quit regulating (MAX1726 example). so 9000mWh/6mW= 1500 hours (assuming you can use all 1500mAh of capacity before dropping out)

Or looking at the current only, if the system draws 1.002mA and the voltage drops below dropout at 33% capacity (just a guess) then I get 1000mAh of useful juice or about 998 hours of operation.

That's pretty darn good in any book and at 82 cents per device it beats any switcher for cost and simplicity.

With rechgargables at 1.4V per cell it gets tighter but still dooable. You need to look at the discharge curves for the battery type you plan to use to make a realistic estimate of operating life given the load and minimum operating voltage (dropout V). It would be useless to try to put 5 or 6 batteries in series as you will still have the same capacity but will burn more power across the regulator. If you want longer run time, parallel another bank of 4 batteries. You want the unregulated voltage as close to the regulated voltage as possible to minimize losses across the regulator without going below the dropout.

Another alternative is to redesign the circuit using CMOS 3.3V locgic chips and use a 3.6V coin cell battery without regulator as the primary source.

Using your example though, why not just run with 3 batteries at 4.5V (or 4 nimh for 4.8V) and no regulator at all. you'd get longer life out of that with no wasted energy. you most certainly can't beat the price since you don't need the VR circuit and you save on a battery. There may be some design issues but in this case you'll get more life with no regulator than a linear.

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