On Tue, Mar 16, 2004 at 10:27:28PM -0800, William Chops Westfield wrote:
> On Tuesday, Mar 16, 2004, at 15:32 US/Pacific, Byron A Jeff wrote:
>
> >I think you missed my point. The 78LXX parts are fine for some
> >applications.  However linear regulators will burn through any power
> >budgeted applications like a battery based one.
>
> Nonsense.  As long as your actual application is low current, you CAN
> design a linear regulator that is also low current.  It won't
> necessarily be very efficient, but it can do much better than a 78L05.

I have to pull out an Olinism here. It doesn't matter if you're burning 40
percent at a microamp rate or 40 percent of 10A. In the end you'll waste 40
percent of your power budget.

> The problem with the 78L is not so much the inherent inefficiency of
> linear regulation, but the relatively high "idle" current (5mA, someone
> said?), which is MUCH larger than a PIC in idle mode, for instance.
> You can run for a long time using a linear regulator if the whole thing
> draws an average of 0.5mA, even if the efficiency is relatively poor.
> A simple zener regulator is probably a good example; given reasonably
> accurate ideas of minimum and maximum current consumption of the
> circuit, you can design a zener circuit that is "maximally efficient
> for a linear regulator."

Understood. However my point is still in place: all linear regulators operate
by dropping the excess voltage which converts the power into heat. That's
power that's not available to the target. A switching regulator will deliver
more power to the target, so no matter the current draw, a switching regulator
will last longer.

And if we're talking about 9V batteries, it's critical to deliver every
meager drop of power to the target.

>
> Don't forget that switchers frequently have restrictions on minimum
> current supplied.  While they're great (relatively speaking) for
> getting maximum EFFICIENCY for a relatively high-current application,
> they are NOT always the best solution for a low-current device.  (Of
> course, there are nice chips specifically aimed at high "stand-by"
> times in devices like PDAs and Cell phones that ARE optimized for some
> low-current delivery.  Interestingly enough, sometimes these include
> linear portions for some of their outputs...)

I looked for restrictions, and didn't find any in a cursory glance of
National's simple switchers or the Linear Tech datasheets. But LT's Burst
Mode seems to solve the issue that you are referring to. In short when
the current draw is low, then chip simply stops swtiching and lets the
target draw from the output capacitor until the voltage drops below a preset
level. Then the regulator cranks up again, refills the cap, then cycles.
In this mode the regulator only draws 10 uA from the supply.

The mode is available on both buck (LTC3404) and boost (LTC1700) applications
with input voltages as low a 0.9V! So 2 C cells in an application like this
could power a low current draw circuit virtually forever.

BAJ

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