Regulated DC Step-up Convertor with variable output voltage?

A switching converter requires more than a chip, since they are based on string energy in an inductive component, and filtering pulses with capacitors.

Here is a data sheet for a controller that shows about the minimum involved.

Make that: A switching converter requires more than a chip, since they are based on storing energy in an inductive component, and filtering pulses with capacitors (at least).

As Mr. Popelish suggested, you can't use a saw to add wood to the board. You need something that will either store energy (an inductor with a switching regulator) or pump up the voltage (a voltage doubler).

There are also other considerations involved here, though. The two biggest problems that come to mind are efficiency (your 9V battery will have to be replaced occasionally) and your skillset as a relative newbie. Switching regulators are pretty efficient (typically over 75% of the battery power is used by the load). However, it can be a real bear for a newbie to get one working at anything near the manufacturer's specs for line and load regulation. These ICs want to be bad. If you don't have a good ground plane and nice short current paths, you'll get smoke, oscillations or just a really bad regulator.

I'd stick with the good old 555 voltage doubler followed by a linear regulator, like the one that was suggested to you by several posts when you asked back in March (view in fixed font or M$ Notepad):

| | VCC VCCVCC | + + + | | | | VCC | .-. | | + | 2.2K| | .--o--o--. | | | | | 8 4 | D V | '-' | | C - _____ | | | | ||+ | D | | Vo = 9V to 14V | o------o7 3o---||---o->|-o----|LM317|-o-------o-----o | | | | || +| |_____| | | | .-. | LMC555 | --- | .-. 0.1uF| | 15K| | | | C --- | | | --- | | | .--o6 | | | | |1.2K --- | '-' | | | === | '-' | | | | | | GND | | | | o---o--o2 | .----o----' === |0.01uF| | 1 5 | .01uF| | GND | --- '--o--o--' --- .-. | --- | --- | |7.5K | | | | | | | === === D=1N4002 === '-' | GND GND C=220uF, 25V GND | | .----o | | | | Minimum load current | .-. | of 4mA '-->| |5K | | | | '-' | | | === | GND | (created by AACircuit v1.28.6 beta 04/19/05

You've got a relatively low component cost here, and you should get over 30% efficiency with a 20mA load.

If you do decide to try your hand at a switcher, try to get one in a DIP or TO220-5 package and use the "dead-bug" method of construction. Use a piece of bare copper-clad FR-4, glue the IC pins-up on the copper (hence the "dead bug -- the legs are sticking up), and use point-to-point wiring, with the non-grounded components sticking up in the air. Keep your current paths short and your ground plane generous, and you might be OK. Be sure to check your work with an oscilloscope before you pronounce it OK.

The variable output will probably be a bit of a problem with an LM3524 switcher, though. Typically, you'll want to use the pot as part of the voltage feedback Rf. All pots have capacitance and inductance, and I'd guess you'll want to be able to adjust output voltage from a front panel, which usually means several inches of wire. No chance with a standard switcher, sir. Keep Rf close to the IC and use a small non-wirewound low inductance tweaker pot. You can't mount astandard pot on the front panel here, where you can with the LM317, particularly with the small cap between the 1.2K-7.5K node and GND.

Good luck Chris

Thanks for the detailed techy reply...and what a good memory you have of my earlier post...thanks

Thanks, John. A nice chip indeed, with a regulated output that does not depend on input voltage. I take my hat off to you, sir. BTW, I have seen other similar chips that ONLY require two external caps. e.g.

No problem, and my memory is far from that good -- you just click "View Profile" at the top of the post for Google Groups to look at prior posts.

I kind of ran out of time last night -- it notes on the ASCII schematic that the above minimum current LM317 configuration is only good if you have a minimum of 4mA of load current. If not, just divide by 5 -- replace the 1200 ohm with a 240, the 7.5K with a 1.5K, and the 5K pot with a 1K. Keep the small cap if you've got a remote pot. Corresponding increase in wasted power, of course.

Your initial idea that this is a very inefficient way to get this higher voltage is a very perceptive one. Switching regulators are crafty solutions to this problem, and I didn't want to discourage you from trying one. If you have an application that didn't run on a 9V battery, I'm sure Mr. Popelish would have suggested one of National Semiconductor's "Simple Switcher" ICs. They do have adjustable ones that would work very well for a hobbyist using the "dead bug" method of prototyping. Same caveats about remote pots would apply, though. And the biggest problem is that they're not made for battery operation -- they're kind of current hogs.

Try one sometime you don't have to worry about battery milliamp-hours.

Cheers Chris

That one just charges a cap up to supply voltage, and then stacks the charges cap on the supply, as if it were a second battery. It doesn't regulate and is not so efficient, since when it is connected across the supply, half of the energy it eventually holds is lost as heat in the switches that connect it. It also doesn't provide any regulation for supply changes, or load caused sags. But if efficiency isn't important, it can almost double the supply voltage, so you can add a linear regulator to waste the extra and give a regulated output, that way.

You could mount a one-turn cermet pot in a way that you could attach a knob to a stick, coupled to the little slot in the pot's rotor. :-)

Cheers! Rich