- 3.3V 10k pullup will pull down PNP base and turn on fan
R18 must use same 5V but not connected to THERM since IC uses 3.3V
- PNP needs base drive resistor near 220 to 300 Ohms to drive 0.5A fan.
spec p15 "As mentioned above, the THERM signal is open drain and requires a pullup to VDD. The THERM signal must always be pulled up to the same power supply as the ADM1032, unlike the SMBus signals (SDA, SCL and ALERT) that can be pulled to a different power rail. The only time the THERM pin can be pulled to a different supply rail (other than VDD) is if the other supply is powered up simultaneous with, or after the ADM1032 main VDD. This is to protect the internal circuitry of the ADM1032. If the THERM pullup supply rail were to rise before VDD, the POR circuitry may not operate correctly.
The base of the PNP if at 4V should turn on the Fan at 5V. That means the Vbe = 1V which is normally max.
A better solution to avoid the POR issue above and use 2 stages with a low side NPN switch.
simulate this circuit – Schematic created using CircuitLab
I am assuming you are using a small muffin fan.