1) 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
2) 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.
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This will also work for higher fan voltages.
I am assuming you are using a small muffin fan.