Electrical – How to convert Clock signal to PWM signal

Yes you can use a PWM regulator suitable for LEDs to drive a solenoid but don't forget to put a flyback diode in place across the solenoid to restrict back emfs damaging the chip.

You can also use any regular adjustable buck regulator as a constant current source too. Imagine it was a linear fixed voltage regulator of 2.5V and you put a 25 ohm load across the output - the current into the regulator would be 100mA plus what the linear regulator might need internally. Basically a linear voltage regulator can act as a constant current source but, in this example wastes 250 mW in the resistor.

A lot of linear (yes I'm still talking linear at the moment) are adjustable with a sense/feedback pin that needs about 1V from a potential divider attached to the output. Well. if instead of the regular pot divider you used a LED/solenoid/load from the output and a 10 ohm resistor to ground, the adjustable regulator will attempt to put 1 volt across the 10 ohm resistor by lifting its output voltage and forcing 100mA thru the LED/solenoid/load. This time power loss is down at 100 mW.

It's the same for an adjustable buck regulator - pick one that needs a small Feedback voltage level (some as low as 0.6V) and put the load from the bucks inductor-output to the FB point and 6 ohms down to ground - you get 100mA.

However, you want to control things with PWM and this is a different deal entirely. I would seriously still consider voltage controlling the solenoid because it will have resistance that converts volts applied to current through the coil. Use a synchronous buck regulator made from two mosfets - use a driver on the mosfets to be able to control the top-switch much more easily and feed it PWM. The inductance of the solenoid should keep the current fairly "dc" in nature.

I have used analog approaches with voltage control successfully in production (10k/mo) and never had a regulator problem. Simple this 1U high 19"rack was pure analog with an OEM 180W supply that UL dictated a "coke-spill" sealed top, I chose a tiny thermistor epoxied to the SMPS hot-spot to bias a switch to drive the fan ON, above 45deg C. I computed the values and gain in a spreadsheet so the gain was 0 to 100% from 45 to 55'C.

You might find the PWM will work best but alias at some rates with some vendor fans, so test them with a pulse gen. and avoid that PWM rate if using a 2 pin fan.

The problem I had was after a few shipments, fans started to get "stuck" and needed a tiny spin to start up, otherwise they would dither back and forth a couple degrees or simply looked dead. This had nothing to do with analog or PWM control, as I recognized the process design fault as misaligned Hall sensors in the fan. the reason being the max fan power is controlled by sensor magnet alignment and commutation closest to reversal ( before top dead center ) was like a backfire in a piston which made it go back/forth so fast, it stood still. in only 1 or 2 stop positions. So I made a quick fan fail tester with 1 second only 4 seconds off to stop and tested every fan start angle 30 seconds, then after 1 hr found 5 failed in 150 fans. rejected the units. accepted the 145 and sent 1 thousand fans fan to supplier and emailed the Test Design to Distributor& Factory and said if we get 1 more fan failure , they lose our business. That worked. No more stuck fans.

It took me less time to put out this Stop ORder and test 150 fans and send the design procedure than to write up this answer.

Your driver is not linear on V+ high side, and you might want a low side with N type. i.e. the Source to Gnd and Drain to fan(-) and fan(+) to 12V or some other switch. Consider slow startup at 5V for cool and quiet.

Max power dissipation of the fan reduced to 50% at half power and RPM where the driver dissipates the same power, so clamp with isolation to a heatsink or frame. In my case it was only a few Watts out of 5W or use PWM if you prefer 3 pin fans and the thermistor speed control bits and pieces cost me ~$2 added cost to cheaper two-pin twin turbo 1.75" fans. 2 much !