Hello,

I've been looking into switched capacitor such as the LTC1043 and the VCO in the datasheet has grabbed my attention so I've been trying to analyze but can't seem to figure out how it works.

So here's my current understanding of the circuit:

-When the Op Amp hits negative rail the 330K charges the 1uF capacitor until the pnp transistor turns on...

-I see positive feedback but never have I seen with capacitor by itself, maybe a filter...

-I believe that the 2.5V zener triggers the capacitor switch when reached..

This what I could figure out any help is great if anyone has used the LTC1043 help would be appreciated.

Thanks

 

I forgot to add a picture of the circuit but here it is...

 

The circuit is explained here on pages 10-11:

http://cds.linear.com/docs/en/application-note/an03f.pdf

There are a couple typos in it, don't let them confuse you (the part number of the zener and one mention of the internal capacitor).

 

I have read over a 100 times before but I don't quite get the part where once the 0.01uF capacitor gets charged, how does it switch back to turn the op amp output to the negative rail?

 

The output of the opamp is connected to pin 16. If you read the introduction at the top of the application note, it says that by driving that pin with an external source you can synchronize the internal, non-overlapping clocks (which control the switches) with an external circuit. There might be more information on how that works in the datasheet.

 

After further analysis I now understand that the capacitor at the inverting input of the opamp charges and once it reaches about 20mV the Op amp goes to negative rail causing the LTC1043 to switch which then the 0.01uF capacitor discharge the cap at the inverting input which will then restart the cycle. Thanks for the hints they did help me reach to conclusions.

 

I'm not positive, but I think the 1μF capacitor at the inverting input is just there for high-frequency noise filtering. The 0.01μF capacitor is charged by the current going from the input through the resistors to the inverting input, and is the capacitor that controls the voltage-to-frequency function. If the capacitor at the inverting input had any significant role to play in the function of the circuit, I'd think it would have been mentioned.

 

Oddly enough though when I change its value so does the frequency... Heres an LT Spice file if you'd like to see it.

 

Check this picture out, letter A, when the inverting input reaches 20 mV via the capacitor it switches states forcing the 0.01uF cap to discharge the 1uF cap to discharge.

 

I see. Too bad the application note doesn't mention that capacitor, and too bad I don't know enough to figure out exactly what effect it has.

 

Hey you helped me out and I appreciate it, but yes it is unfortunate that they don't mention that capacitor.

Thanks