temp based PWM fan control

Hi there I am planning to do a temperature based PWM control ciruit.

The requirements are:
20mA source/sink
5V powered and ~5V out (voltage out not too important as long as its high impedance)
scales from 50-95% PWM with temperature
small board size (smaller = better but no actual limit)
2kHz +/-20% (*edit*)
no software
would like to be able to set the top (95% PWM) and bottom (50% PWM) temperature

I was planning to do this with a 555timer and an NTC/PTC. I figured I could use a pot to set a minimum duty cycle, but working out the resistance values is proving to be a pain.
Then I would put a schmitt trigger on the output to get my high impedance and required current.

I was wondering if anyone else would go about this differently.

I have read several articles online saying that PWM works best at lower frequencies, between 20 and 60Hz or so. I recently got an IC from microchip that is capable of driving an external fan using PWM based on temerature. It has a lower cut off limit, a low end PWM limit and a built in fault detector. I have made some test circuits using the IC and it works great. The only thing that I can't seem to work out is in one of the equations it says something about the beta value of the thermistor. I can't remember where in the data sheet I saw that but I am at a loss for what this means. If you get someone to explain that, we'll both be set to go. Here is a link to the data sheet: http://www.datasheetcatalog.org/datasheet/microchip/21444c.pdf

This should explain it Dragon.
**broken link removed**

hi DT,
The d/s reference to Beta is for transistor Q1. the Gain for the design example is 50.

For a Thermistor 'B' is a function of the material used... the formula is: Rnew = R0*exp(-B*((1/273)-(1/(273+T1))))
R0 = resistance at Ref tempr , usually 25C
Typically B is around 4000.

That thing looks pretty awesome, but my fan has a required frequency of 2kHz +/- 20% (I've updated the original post)

It can be modified to work at a higher frequency. The capacitor that opperates the frequency just needs to be modified from 1uF to another value. The capacitor is connected to pin 2 labeled Cf. There is an equation in the data sheet that will allow you to calculate what size cap you need.

You can also get it from microchip as a free sample. The max number of parts is 5, just order 2 or 3 so you don't look like a mooch. I always order at least 2 so that if something goes wrong with the first one, I don't have to stop working while I wait for a replacement in the mail.

hmm i dont see an equation or limitations but also they recommend using the value for 30 Hz
I would find it odd if it also operated normally at 2000Hz

Equation or no equation, it's just a simple RC oscillator. There is nothing in the data sheet that says you can't run it at 2kHz. Just start lowering the capacitor value until you get a 2kHz output. It should be within the +/- 20% tolerance range you are looking for.