The MAX6639 employs a register lock mechanism to

avoid getting temperature results from the temperature

register and the extended temperature register sam-

pled at two different time points. Reading the extended

register stops the MAX6639 from updating the temper-

ature register for at least 0.25s, unless there is a tem-

perature register read before the scheduled update.

This allows enough time to read the main register

before it is updated, thereby preventing reading the

temperature register data from one conversion and the

extended temperature register data from a different

conversion.

The MAX6639 measures the temperature at a fixed rate

of 4Hz immediately after it is powered on. Setting bit 7

of the configuration register (04h) shuts down the tem-

perature measurement cycle.

OT Output

When a measured temperature exceeds the corre-

sponding OT temperature threshold and OT is not

masked, the associated OT status register bit sets and

the OT output asserts. If OT for the respective channel

is masked, the OT status register sets, but the OT out-

put does not assert. To deassert the OT output and the

associated status register bit, either the measured tem-

perature must fall at least 5

°C below the trip threshold

or the trip threshold must be increased to at least 5

°C

above the current measured temperature.

THERM

When a measured temperature exceeds the corre-

sponding THERM temperature threshold and THERM is

not masked, the associated THERM status register bit

is set and the THERM output asserts. If THERM for the

respective channel is masked, the THERM status regis-

ter is set, but the THERM output does not assert. To

deassert the THERM output and the associated status

register bit, either the measured temperature must fall

at least 5

°C below the trip threshold or the trip thresh-

old must be increased to at least 5

°C above the current

measured temperature. Asserting THERM internally or

externally forces both PWM outputs to 100% duty cycle

when bit 6 in address 13h (fan 1) or bit 6 in address

17h (fan 2) is set.

ALERT

The ALERT output asserts to indicate that a measured

temperature exceeds the ALERT trip threshold for that

temperature channel. The status bit and the ALERT out-

put clear by reading the ALERT status register. If the

ALERT status bit is cleared, but the temperature still

exceeds the ALERT temperature threshold, ALERT

reasserts on the next conversion, and the status bit sets

again. A successful alert response protocol clears

ALERT but does not affect the ALERT status bit.

TACH1 and TACH2 Inputs

To measure the fan speed, the MAX6639 has two

tachometers. Each tachometer has an accurate internal

clock to count the time elapsed in one revolution.

Therefore, it is counting the time between two tachome-

ter pulses for a fan with four poles. When the PWM sig-

nal is used to directly modulate the fan’s power supply,

the PWM frequency is normally in the 20Hz to 100Hz

range. In this case, the time required for one revolution

may be longer than the PWM on-time. For this reason,

the PWM pulses are periodically stretched to allow

tachometer measurement over a full revolution. Turn off

pulse stretching by setting bit 5 of register 13h or regis-

ter 17h when using a 4-wire fan.

The tachometer count is inversely proportional to the

fan’s RPM. The tachometer count data is stored in regis-

ter 20h (for TACH1) and register 21h (for TACH2).

Reading a value of 255 from the TACH count register

means the fan’s RPM is zero or too slow for the range.

Reading a value of zero in the TACH count register

means the fan’s RPM is higher than the range selected.

Table 2 shows the fan’s available RPM ranges. Use reg-

isters 10h or 14h to select the appropriate RPM range for

the fan being used.

FANFAIL

The FANFAIL output asserts to indicate that one of the

fans has failed or is spinning slower than the required

speed. The MAX6639 detects fan fault depending on the

fan-control mode. In PWM mode, the MAX6639 pro-

duces a square wave with a duty cycle set by the value

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

8

_______________________________________________________________________________________

TEMP (

°C)

TEMP (

°C)

DIGITAL OUTPUT

241

+241

1111 0001

240

+240

1111 0000

126

+126

0111 1110

25

+25

0001 1001

1.50

1

0000 0001

0.00

0

0000 0000

Table 2. Temperature Data Byte Format

FAN RPM

RANGE

INTERNAL CLOCK

FREQUENCY (kHz)

2000

1

4000

2

8000

4

16,000

8

Table 3. Tachometer Setting