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).