Maker Pro
Maker Pro

Regulated DC Step-up Convertor with variable output voltage?

R

royalmp2001

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
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
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.
http://cache.national.com/ds/LM/LM2733.pdf
 
J

John Popelish

Jan 1, 1970
0
John said:
A switching converter requires more than a chip, since they are based on
string energy in an inductive component, and filtering pulses with
capacitors.

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).
 
C

Chris

Jan 1, 1970
0
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
 
R

royalmp2001

Jan 1, 1970
0
Thanks for the detailed techy reply...and what a good memory you have
of my earlier post...thanks
 
R

royalmp2001

Jan 1, 1970
0
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.
http://www.catsemi.com/datasheets/660.pdf
 
C

Chris

Jan 1, 1970
0
royalmp2001 said:
Thanks for the detailed techy reply...and what a good memory you have
of my earlier post...thanks

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
 
J

John Popelish

Jan 1, 1970
0
royalmp2001 said:
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.
http://www.catsemi.com/datasheets/660.pdf

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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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
 
Top