Slowly dimming LED-s
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Triangular PWM generator.

As discussed in the comments, to do what you ask you need a triangle generator to generate the dimming control signal and you need a PWM generator to be controlled by the triangle generator.

The circuit of Figure 1 is intended to get you started but leaves some calculations for you to do.

How it works

Half-supply reference


The circuit works on a single supply voltage. The op-amps need to work about the centre-point of this supply so we generate a half-supply voltage using a voltage divider and capacitor.


Squarewave generator

This works in conjunction with the triangle generator as follows.


OA1 is configured as a Schmitt trigger. If the output is high the '+' input is pulled high by R3. This will keep the output high until the '+' input is pulled low enough by R5. (We'll come back to this.)
When it does switch low the process repeats but in the opposite direction.


Triangle generator


OA2, C1, R4 and R1 form a triangle generator. When the input is high C1 starts to charge, driving the OA2 output negative to maintain the '-' input at the same potential as the '+' input.
Since OA2 is fed back to the Schmitt trigger, at some point it will cause the trigger to toggle. This will start to reverse the charge direction in C1 and the output will rise linearly. Again, at a certain point the squarewave generator will toggle again.


The result is a triangle waveform which we can feed to the PWM generator.

Voltage divider

The PWM generator, LTC6992, accepts a 0 to 1 V signal to generate a 0 to 100% PWM signal. R7 and R8 divide the triangle signal down. You need to adjust these values depending on the output of OA2 which, in turn will depend on the supply voltage. Measure the peak value when you get the triangle generator running.

PWM generator

This is fairly straightforward and the datasheet is clear on its operation.

Work for the OP


Triangle frequency calculations to be calculated. \$\tau = (R1 + R2)C \$ will give a good approximation.
Schmitt trigger switching points to be calculated. Values given should work but may not be optimal.
An opamp that can swing to negative rail will be required to get to 0% PWM.


Further information


Triangle wave generator.
LTC6992 datasheet.