SG3525 as square wave generator? - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th March 2007, 12:20 AM   #1
diyAudio Member
 
Join Date: Mar 2006
Location: croatia
Default SG3525 as square wave generator?

I am planing to make a little more "stable" sircuit for square wave generations insted of NE555 using SG3525.

Using NE555 I have problems that is chaniging of frequency is related to duty cycle change: What I want is that duty cycle ALWAYS stays at the SAME PERCENTAGE no matter if I vary frequency..

What I want is resistor trimable square wave generator with freq and duty cycle change with resistors and switches for capacitor select (frequency range)

In the SG3525 datasheed it stays that MAX duty cycle is 50 %: So is that meant both A and B outpouts together 50 % or all together 50 %: ???
I mean if placing diode at the B output and conecting A and B across diode to the same otput cause I want duty cycle vary from 0 to 100 % (by trimmer change)


So than is that possbile to make vith SG3525 and is it good for that... Searching the net I didnt find any related schematics for such need

SG3525 is pretty cheap and can handle 300 khz frequency...
  Reply With Quote
Old 16th March 2007, 02:50 AM   #2
diyAudio Member
 
Join Date: Feb 2007
Location: Lake Tahoe, NV
Medogrizli,

I have not used this chip since 1990, so my memory has faded a little. But, you could diode "OR" the outputs to get a full range duty cycle. The 50% max on each output is derived from F/F steering of the PWM signal to each output, sequentially. That is, each output is forced low every other osc cycle. So there is at least 50% off time for each output. The weakness would be that the positive edge will be xistor driven and the negative edge will be controlled by a pulldown and the load impedance unless you build an output stage buffer. You could also use a logic 2-in OR gate and parallel unused gates to increase output drive. Cmos would give better rail-to-rail swing at moderate output levels.

Controlling the duty cycle by controlling the error voltage input is possible, but you may find that it is not linearly proportional, nor entirely freq independent between low freq and high freq operation. Certainly at 100KHz and above I would expect delay times in the PWM logic to cause shifting of the duty cycle due to delays being a significant and growing portion of the osc period.

Another problem is that the outputs are not rail-to-rail and will shift quite a bit between no load and 100mA plus loading (the 555 has similar problems). Also, the chip power dissipation will be a factor for higher drive currents (see datasheet).

Depending on how high in freq you must go and how low an impedance you need to drive, a cmos 555 (say TI TLC555) can be made to be a good adjustable duty cycle generator up to 10KHz and not so good up to 100KHz if you run it at 15V. There is a standard hookup for a single pot duty cycle control for a 555 that goes back 25 to 30 years. It does have diodes which contribute to temperature sensitivity and to duty cycle shift at high freq operation.

The cmos 555 does exhibit good rail-to-rail output for loads up to 10mA and progressively poorer performance out to 100mA (worry about power dissipation at higher current) or so. The output stage does have asymmetry in the p and n channel mosfets so at high freq the rise time will be obviously slower than the fall time.

I hope this is helpful,
VSR
  Reply With Quote
Old 16th March 2007, 03:22 AM   #3
Tim__x is offline Tim__x  Canada
diyAudio Member
 
Join Date: Jun 2004
Location: Edmonton area, Alberta
Just thinking out loud here...

I'd probably do it with a 555 followed by a divide-by-2 stage to give you a 50% duty square wave, and if you need not just steady but an adjustable duty cycle, follow it with an integrator and then a comparator.
  Reply With Quote
Old 16th March 2007, 03:58 AM   #4
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
If sg3525 produced a 300kHz 50% duty cycle square, then it would be a mosfet burner instead of driver. Maybe try a PLL..., hah?
  Reply With Quote
Old 16th March 2007, 06:52 PM   #5
diyAudio Member
 
Join Date: Mar 2006
Location: croatia
Quote:
Originally posted by darkfenriz
If sg3525 produced a 300kHz 50% duty cycle square, then it would be a mosfet burner instead of driver. Maybe try a PLL..., hah?
I could use mosfet driver

sherelec

Quote:
Controlling the duty cycle by controlling the error voltage input is possible, but you may find that it is not linearly proportional, nor entirely freq independent between low freq and high freq operation. Certainly at 100KHz and above I would expect delay times in the PWM logic to cause shifting of the duty cycle due to delays being a significant and growing portion of the osc period.

But I can use for example 12 V divided by resistors in series: So compared ti internal voltage source SG would change duty cycle tompared to that voltage?

than are there any other posibilites instead 555 or SG

Maybe it is somehow possible to make circuit with 555 so that DUTY cacle in percentage could be constant



  Reply With Quote
Old 16th March 2007, 08:15 PM   #6
diyAudio Member
 
Join Date: Feb 2007
Location: Lake Tahoe, NV
Medogrizli,

I believe that at some increasing freq, a fixed voltage applied to the error amp (SG3525) will see shifts in the duty cycle. How much shift and at how high a freq, I do not know. You could experiment. You have not provided any specifications for your freq range of interest or how stable the duty cycle must be for changes in freq. Nor any specs for rise/fall time, drift with temp, etc.

If you search the internet for 555 application circuits or even some original Signetics 555 application/specsheets you should find a circuit for a one pot variable duty cycle 555 square wave generator. I know of a few techniques for making such a circuit more stable, but the 555 has limited drive and limited freq. As I indicated previously: a TLC555 might give pretty good results up to 10KHz if running at 15V, but less desirable performance as you go towards 100KHz.

There are PWM chips that are cmos, single output, 0-100% duty cycle, and good to several 100Khz. These are not as cheap as the SG3525, or as easy to find! Look at TI SMPS PWM controllers on the internet.

There have been several function generator chips designed. I do not have the numbers off the top of my head. I believe Intersil, Exar may be manufactures. An internet search for DIY function generators should give you lots of ideas. There are several kits available using these chips. Most of these are 0-100KHz type performance. Most optimistic 0-1MHz.

Finally, your idea is not bad, but without knowing your specifications; I suspect the SG3525 approach may have several features that you will find to be bugs.

Best Wishes,
VSR
  Reply With Quote
Old 16th March 2007, 08:51 PM   #7
eris is offline eris  United States
diyAudio Member
 
Join Date: Jan 2007
Location: rocky mountains
Send a message via AIM to eris
Hi,

If all you want is square wave generation and nothing else, there are perhaps easier ways to do it.

If you need a function generator that does sine, square and triangle wave generation, I justed posted an article that should show up any second about an inexpensive function generator based on the Atmel AVR microcontroller. It generates rock-solid functions up to almost 300khz. Excellent for testing and really easy to build and control. Here it is...

http://www.myplace.nu/avr/minidds/

eris
__________________
----
Don't believe everything you think!
  Reply With Quote
Old 18th March 2007, 02:12 AM   #8
diyAudio Member
 
Join Date: Mar 2006
Location: croatia
Quote:
Originally posted by sherelec
Medogrizli,

I believe that at some increasing freq, a fixed voltage applied to the error amp (SG3525) will see shifts in the duty cycle. How much shift and at how high a freq, I do not know. You could experiment. You have not provided any specifications for your freq range of interest or how stable the duty cycle must be for changes in freq. Nor any specs for rise/fall time, drift with temp, etc.

There are PWM chips that are cmos, single output, 0-100% duty

Finally, your idea is not bad, but without knowing your specifications; I suspect the SG3525 approach may have several features that you will find to be bugs.

Best Wishes,
VSR


No it is not importat so much precise in PWM:
For Example if I change from 10 khz to 20 khz freq: duty cycle can vary for example 10%

Max freq 100-200khz

what about ICL8038
  Reply With Quote
Old 18th March 2007, 02:13 AM   #9
diyAudio Member
 
Join Date: Mar 2006
Location: croatia
Quote:
Originally posted by eris
Hi,

If all you want is square wave generation and nothing else, there are perhaps easier ways to do it.

If you need a function generator that does sine, square and triangle wave generation, I justed posted an article that should show up any second about an inexpensive function generator based on the Atmel AVR microcontroller. It generates rock-solid functions up to almost 300khz. Excellent for testing and really easy to build and control. Here it is...

http://www.myplace.nu/avr/minidds/

eris
I think it can not change duty cycle ... ?!
  Reply With Quote
Old 18th March 2007, 03:18 AM   #10
eris is offline eris  United States
diyAudio Member
 
Join Date: Jan 2007
Location: rocky mountains
Send a message via AIM to eris
Medogrizli said...

"I think it can not change duty cycle ... ?!"

Perhaps not. I haven't seen its user interface when running on Windows, but changing duty cycle would be an almost trivial additional feature if not. Probably a minor change to the software.

eris
__________________
----
Don't believe everything you think!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sine wave - Square & Triangle wave generator using Transistors / OP-Amps lineup Solid State 20 9th October 2006 12:15 AM
Sine and square wave function generator Bricolo Parts 14 7th March 2003 11:45 PM
Circuit diagram for "Auto Sweeping" square wave generator patwen Digital Source 2 8th February 2002 04:07 AM


New To Site? Need Help?

All times are GMT. The time now is 05:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2