LTC6992 PWM duty cycle error

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
Hello,

I made an PWM circuit using LTC6992. I used the schematic and resistors as per datasheet (the same as bellow with added capacitor). After I started troubleshooting I also checked my resistor values according to this online calculator and they mach. I have the same values for R1, R2 and used an 250 k trimmer for Rset in order to gain some frequency adjustment. Bellow you can see the screenshot from the calculator with the desired frequency of 4 kHz.

Screen shoot from the calc

Basically I made a 0-10 V to PWM converter so in order to convert the voltage to 0-1 V range I made and adjustable voltage divider.

I have a problem with an error of the output duty cycle. I measured the following values:
  1. 2 V ~14 % duty cycle,
  2. 5 V 50 % duty cycle, so dead on,
  3. 8 V ~86 % duty cycle.

As can be seen in the datasheet the error should be lower than 3,7 % and clearly it is not.

I wonder why is that happening I tried replacing the IC and also disconnect the output from the other circuitry and nothing changed. The voltage on the MOD pin is dead on, so the issue is not in the divider. Otherwise the output waveform looks superb and stable.

Thank you very much for help in advance.
 
Last edited:

jpanhalt

Joined Jan 18, 2008
10,917
Which model of 6992 do you have? (dash 1 or dash 2?)
It would help if you showed calculated and observed values for your duty cycle. Also show resistances actually used and tolerance. That is, a table with t least 4 columns:
Calculated DC, Observed DC, R1, and R2. Is you supply 5.00 V?
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
Which model of 6992 do you have? (dash 1 or dash 2?)
It would help if you showed calculated and observed values for your duty cycle. Also show resistances actually used and tolerance. That is, a table with t least 4 columns:
Calculated DC, Observed DC, R1, and R2. Is you supply 5.00 V?
I have dash 1 model and yes my supply is 5 V. My R1 (+-0,1 %) and R2 (+-1 %) are as on the schematic which I posted before. Cap between + and DIV is ceramic with 0,1 uF. Did you mean duty cycle with DC? Duty cycles which I wrote down before were measured with oscilloscope at the output without a load. Frequency was set to 4 kHz.

1608042281808.png
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
I am amazed neither you nor anybody else has spotted it yet, what do you think the voltage range of the MOD input is ?
Nobody had spotted it, because it's just fine and I mentioned it in my first post. You can even see the divider on te MOD pin on my last image.
Basically I made a 0-10 V to PWM converter so in order to convert the voltage to 0-1 V range I made and adjustable voltage divider.
 

fourtytwo

Joined May 2, 2017
80
Nobody had spotted it, because it's just fine and I mentioned it in my first post. You can even see the divider on te MOD pin on my last image.
Your last image is not a complete schematic so the voltage source at the top of R4 is completely unknown.
In your 1st post you say :-
"Basically I made a 0-10 V to PWM converter so in order to convert the voltage to 0-1 V range I made and adjustable voltage divider.

I have a problem with an error of the output duty cycle. I measured the following values:

  1. 2 V ~14 % duty cycle,
  2. 5 V 50 % duty cycle, so dead on,
  3. 8 V ~86 % duty cycle.

As can be seen in the datasheet the error should be lower than 3,7 % and clearly it is not.

I wonder why is that happening I tried replacing the IC and also disconnect the output from the other circuitry and nothing changed. The voltage on the MOD pin is dead on, so the issue is not in the divider. Otherwise the output waveform looks superb and stable."

So why would any sane person NOT think you were trying to poke 8V into the MOD pin.
Good luck
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
Your last image is not a complete schematic so the voltage source at the top of R4 is completely unknown.
In your 1st post you say :-
"Basically I made a 0-10 V to PWM converter so in order to convert the voltage to 0-1 V range I made and adjustable voltage divider.

I have a problem with an error of the output duty cycle. I measured the following values:

  1. 2 V ~14 % duty cycle,
  2. 5 V 50 % duty cycle, so dead on,
  3. 8 V ~86 % duty cycle.

As can be seen in the datasheet the error should be lower than 3,7 % and clearly it is not.

I wonder why is that happening I tried replacing the IC and also disconnect the output from the other circuitry and nothing changed. The voltage on the MOD pin is dead on, so the issue is not in the divider. Otherwise the output waveform looks superb and stable."

So why would any sane person NOT think you were trying to poke 8V into the MOD pin.
Good luck
I take it back. I hope it is clear now.
 

jpanhalt

Joined Jan 18, 2008
10,917
Thank you for the Table. For Duty Cycle, you show "V" do you mean % or are you integrating/low-pass filter the output?

I haven't studied the datasheet that carefully, but doesn't this graph suggest that 14% DC is about right for MOD/VSET= 0.2V?

1608116498635.png
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
Thank you for the Table. For Duty Cycle, you show "V" do you mean % or are you integrating/low-pass filter the output?

I haven't studied the datasheet that carefully, but doesn't this graph suggest that 14% DC is about right for MOD/VSET= 0.2V?

View attachment 225080
Thank you for all the help.
;)

Yes I mean %, I made a mistake while making the table.

I did checked the datasheet but mainly looked at the errors and didn't noticed that graph you posted which clearly shows that the correlation between Vmod and output DC is not 1. I guess that's it and we'll need to implement a transformation function on the PLC in order to make it work.
 
Top