royalmp2001 said:
Correct me but I don't think such a thing exists, either as a chip or
made up discretly.
I want to use a 9v battery to get a variable 9-14V regulated output,
but sticking a regulator on the end of a chip step-up convertor doesn't
seem too efficient.
Anyone know of a chip that will do the whole job that requires just
connecting up a pot to control the output voltage? I don't need much
current, say 20mA.
Thanks
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
www.tech-chat.de)
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