THMC50

REMOTE/LOCAL TEMPERATURE MONITOR AND

FAN CONTROLLER WITH SMBus INTERFACE

SLIS090 – JULY 1999

12

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

interrupt status register/interrupt status register mirror

The THMC50 contains a pair of interrupt status registers. These registers reflect the state of all of the possible

error conditions that the THMC50 monitors. When an error occurs during the conversion cycle, the

corresponding bit is set in the interrupt status register and the interrupt status register mirror. Once set, the bit

in the interrupt status register is cleared upon reading that register. Reading the interrupt status register does

not clear the mirror register, and vice versa. If the error condition persists after being cleared, the bit is again

set in both the interrupt status register and the mirror register.

analog output - FAN_SPD

The THMC50 has a single analog output, FAN_SPD/NTEST_IN, from an unsigned 8-bit DAC which produces

0 V to 2.5 V. This register is set to 0x00 on RST, which results in the minimal fan speed possible.

Note that if RST is asserted low, it is still possible for the FAN_SPD analog output to go to full scale (0xFF) if

THERM is asserted.

The FAN_SPD output must be amplified with external circuitry in order to achieve an output voltage range of

0 V to 12 V and an output current of at least 250 mA in order to drive a cooling fan. Figure 6 through Figure 11

illustrate external circuits that can be used to drive a fan.

+12 V

+12 V

FAN_SPD/NTEST_IN

LM324

Q1

2N2219A

R1

36 k

Ω

R2

10 k

Ω

_

+

Figure 6. External Fan Drive Circuit Using 1/4 LM324 and NPN Emitter-Follower, Single Supply

+12 V

+12 V

FAN_SPD/NTEST_IN

LM324

R4

1 k

Ω

R3

1 k

Ω

R1

39 k

Ω

R2

10 k

Ω

Q1

BD136

2SA968

+

–

Figure 7. External Fan Drive Circuit Using 1/4 LM324 and PNP Transistor, Single Supply