Square wave oscillator with LT1818

Thread Starter

captoro

Joined Jun 21, 2009
109
Hello

I have a Square wave oscillator with LT1818 chip.
It is working but the wave form does not go to 0v on each cycle. With a 12V input, the minimum it will go down to is 1v. How do I get down to 0v ?
I also made the circuit on a breadboard and its actually worst, it would oscillate between 9 and 11 volts only !
Any help would appreciated, thanks
See attached files .

Ken
 

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Audioguru again

Joined Oct 21, 2019
3,201
Look at its datasheet. It has complementary emitter-followers on the output that cannot go to ground or to the positive supply.
Use a modern rail-to-rail opamp instead.
 

MisterBill2

Joined Jan 23, 2018
8,793
The best solution depends on how much output current you need for the application. If it is not much, then a fast CMOS digital buffer may be adequate. and how close to actual 0.00 volts you need to reach also matters. And how close to the 12 volt level is also important.
 

sparky 1

Joined Nov 3, 2018
544
The op amp astable multivibrator would be a square wave generator. I think wave shape and timing can be addressed under load conditions.
https://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html

Some like to use the Arduinos. They are concerned about damaging the controller. So they interface with an opto coupler to the TC4420 mosfet driver. The TC4420 can drive large mosfets and there are configurations that can handle inductive loads. The load and the driver may or may not have issues.
The spec sheet will show the recommended waveforms that are known to work well. Always start easy and avoid transients.
 
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MisterBill2

Joined Jan 23, 2018
8,793
Sparky has inspired the solution. Use a negative supply for the negative supply terminal of the comparator. Just one volt negative should do it.
 

Bordodynov

Joined May 20, 2015
2,870
It is extremely unreasonable to load an oscillator with a 1 μF capacitor. Your circuit produces an edge of about 100µs. i>1uF*12V/50ns=1.0e-6*12./50.0e-9 = 240.
To get an edge of 50 nsec you need an output current of 240 amps.2021-01-21_15-28-11.png2021-01-21_15-34-13.png2021-01-21_15-49-37.png
 
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MisterBill2

Joined Jan 23, 2018
8,793
Certainly "B" is correct. Remove C2 and see what you get.
And has this circuit actually been built and tested, or is it just a simulation? AND try putting a capacitor across R2, the positive feedback resistor. That should speed up the state change.
 

Audioguru again

Joined Oct 21, 2019
3,201
An opamp switches slowly because it has a frequency compensation capacitor that cuts its high frequency response so that it has no gain at the high frequencies that cause oscillation when negative feedback is added.
A logic circuit switches very fast because it does not have a frequency compensation capacitor in it.
 

Thread Starter

captoro

Joined Jun 21, 2009
109
That MIC1555 chip looks pretty, although I could not find in the datasheet how much peak current on its output.
Also I do have a -5 volt in my circuit, I can use my LT1818 and and feed -5v instead of ground.
 

Thread Starter

captoro

Joined Jun 21, 2009
109
Also , I intend to use the output of the LT1818 circuit to feed into the MOSFET driver ISL55110. The ISL55110 datasheet does not mention if damages occurs if a negative voltage is supplied to the input of the ISL55110 IC.
Anyone can tell me that ?
... I think I should start a new thread for that last question.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
8,793
Also , I intend to use the output of the LT1818 circuit to feed into the MOSFET driver ISL55110. The ISL55110 datasheet does not mention if damages occurs if a negative voltage is supplied to the input of the ISL55110 IC.
Anyone can tell me that ?
... I think I should start a new thread for that last question.
The bias on the circuit can be adjusted to set yje output low point to just zero volts.
 

Thread Starter

captoro

Joined Jun 21, 2009
109
I realized that the LT1818 was getting hot after just a minute.
I just read this in the datasheet:
The inputs can withstand differential input voltages of up
to 6V without damage and without needing clamping or
series resistance for protection. This differential input
voltage generates a large internal current (up to 50mA),
which results in the high slew rate. In normal transient
closed-loop operation, this does not increase power dissipation
significantly because of the low duty cycle of the
transient inputs. Sustained differential inputs, however,
will result in excessive power dissipation and therefore
this device should not be used as a comparator.
I am getting 12v differential voltage, therefore I dont think I can use this chip for this purpose.
Anyone can suggest an alternative ? I chose this IC because it gave me 1.5ns rise time.
 
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