Fan speed control - Voltage (0-5V) to PWM (0-100%) with minimum value

Question

I need to control 12V fans (w/ PWM so 4 pin) with a laptop motherboard which originally had 5V fans (3-pin only so voltage-controlled), so I want to:

• use external supply of 12V (or internal from laptop but 12V, don't want to mess with stepping up the 5V)
• use LTC6992 (https://cds.linear.com/docs/en/datasheet/69921234fc.pdf) to map 0-5V from laptop motherboard to PWM for my new 12V fans
• use a voltage divider (2 resistors - R1=10kohm, R2=2.5kohm) to map 0-5V to 0-1V input of LTC6992
• I want to have a minimum value (eg. 50% of fan speed) always and never go below that - was thinking to use another voltage divider to bring 0.5V to the analog input of LTC6992 and only leave the 1st voltage divider to raise that 0.5-1V - anything you see wrong with that approach? Perhaps I need some diodes to not mix those two?

Any other comments/suggestions to the above-mentioned?

Note: I do NOT want to use 5V fans here, I'm making liquid cooling for a laptop (and essentially turning it into half-desktop), already have 12V fans and everything working, this is the last piece :-)

PS: for anybody interested, this is the MSI GS70 Stealth with retro-fitted Corsair liquid coolers (laptop was too noisy when under load and I never take it anywhere anyway :P).

https://www.dropbox.com/s/pnm01z1gvzbdrmb/removing_radiators.jpg?dl=0 https://www.dropbox.com/s/ps5ih695et8ii9x/retrofitting_water_blocks.jpg?dl=0

Thanks! Lukas

Answer 1

You may wish to try adding a common diode in series with your divider network between it and ground. The diode will raise the lower leg of your divider by 0.3 to 0.6 volts depending on the type of diode selected.

If this method isn't granular enough for your application, consider an op amp circuit with an adjustable offset.

Note that either solution would change your resistive divider calculation to keep the input range <= 1 volt.