LTC6991 oscillator not oscillating

Papabravo

Joined Feb 24, 2006
17,296
Did you connect the SET to GROUND through a High Value resistor, or did you imagine that it was the same as connecting directly to GROUND? It sounds like you have no idea how to interpret the block diagram on page 8 to realize the importance of that resistor and what purpose it serves. Connecting SET to V+ has absolutely got to the most wrong thing you can do.
 

Thread Starter

captoro

Joined Jun 21, 2009
135
Did you connect the SET to GROUND through a High Value resistor, or did you imagine that it was the same as connecting directly to GROUND? It sounds like you have no idea how to interpret the block diagram on page 8 to realize the importance of that resistor and what purpose it serves. Connecting SET to V+ has absolutely got to the most wrong thing you can do.
Of course. I have 400k ohm resistor connected from Set to ground.
 

Papabravo

Joined Feb 24, 2006
17,296
Of course. I have 400k ohm resistor connected from Set to ground.
That was certainly not clear from your original post. I'm going to say this just once. We need a schematic diagram of what you have built, complete with reference designators, part numbers and values. Do not under any circumstances point us to a picture from a datasheet or another website. Those pictures and your verbal descriptions do not contain the information necessary to help you. Comply and help may be forthcoming. Ignore this request and you are on your own.
 

Thread Starter

captoro

Joined Jun 21, 2009
135
That was certainly not clear from your original post. I'm going to say this just once. We need a schematic diagram of what you have built, complete with reference designators, part numbers and values. Do not under any circumstances point us to a picture from a datasheet or another website. Those pictures and your verbal descriptions do not contain the information necessary to help you. Comply and help may be forthcoming. Ignore this request and you are on your own.
 

Thread Starter

captoro

Joined Jun 21, 2009
135
Hi, I tried it, I shorten the wire, I even added 3 wire total from SEt to ground. Still no output. I also used another LTC6991, same output.
 

ronsimpson

Joined Oct 7, 2019
1,657
SET through resistor to VCC works and I think it should not work.
SET through resistor to GND and it does not work but should.
How strange.
You must have got you bread board from the same place I got mine. Looks the same. lol
1625147648973.png
 

Thread Starter

captoro

Joined Jun 21, 2009
135
On your breadboard, I see no connection between the - rail at the bottom and the - rail at the top.

Bob
Hello Bob,

The connections are on the extreme right. OUt of range of the picture.
I tested the voltage with a voltmeter and its 5V.
 

du00000001

Joined Nov 10, 2020
85
Had a look into the DS: capacitance parallel to the SET resistor has to be kept below 100 pF (the lower, the better - see P.7) to provide stable oscillations.
And these fancy breadboards are notorious for their parasitic capacitances. Sometimes it's better not to use a breadboard but wire a circuit "free".
 

RPLaJeunesse

Joined Jul 29, 2018
193
Here is the schematic
FWIW your schematic doesn't show pin numbers. Did you just assume the pins were depicted in the correct numeric order? That's a BAD assumption. Did you look at the data sheet to get the proper pin numbers? Not apparent. How do you know you have the pins used in the way the schematic indicates? Also, the 0.1uF bypass cap should be across the device power pins, right at the device, with short leads. I didn't even see it in your picture. Could explain your problems.
 
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