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Simple single-supply, negative ramp/sawtooth generator idea?
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ebastler:
Well, lets face it -- my analog design skills are marginal. ::)  So I could use some help...

From an FPGA, I would like to drive a simple negative sawtooth output: With approx. 3 kHz repetition rate, set the output "high" (rise time up to 1µs), then let it fall reasonably linearly to the "low" level (for approx. 300µs). "Resonably linear" means within a couple %, so no high precision requirements, but exponential capacitor charge/discharge curves won't be good enough.

I have a 3.3V supply available. To drive/trigger the ramps, I can provide one or two (complementary?) digital signals with whatever duty cycles makes sense, toggling them between 3.3V and 0V, or high-Z if that helps. The output does not have to operate rail-to-rail (0V to 3.3V), but at least 1Vp-p output amplitude would be desirable.

There must be a simple solution to this, using an op-amp or two, or an NE555 -- but I have not found any "cookbook" solutions, and can't figure out anything simple myself.  Any ideas or pointers are much appreciated!
temperance:
I see several options:

-A SPI DA converter if you have four pins available
-A resistor network DA converter if you have 6 or more pins available.
-A resistor network DA converter connected to the outputs of a 74xx595 like shift register if you three pins available.
-A PWM output with a low pass filter. (one op-amp, two or four resistors and two capacitors)
-An integrator with the non inverting input biased around 1V. (the integrator input is pulled up to the op-amp VCC. To reset the integrator, the input is pulled low with an open collector output with 100R in series connected to the inverting input. Make the integrating capacitor just a few nF)

MT:
Dont really understand what you want but you can rearrange a 7555 (dont use 555) to generate up or down ramps or drive it from negative voltage and do the same but then ramps are -V going. Some 7555 can be driven from +2V.


temperance:
Attached you will find the integrator based version.

For the op amp you will need to select a rail to rail op amp like the OPA350 or something alike.
magic:
ICs are boring :P
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