MAX6653 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX6653

Temperature Monitors and PWM Fan Controllers

Maxim Integrated 

Temperature Monitors and

PWM Fan Controllers

Figure 5 shows a schematic using a current-sensing

resistor and a coupling capacitor to derive the

tachometer information from the power-supply current

of a 2-wire fan. This circuit allows the speed of a 2-wire

fan to be measured even though the fan has no

tachometer signal output. The sensing resistor,

RSENSE, converts the fan commutation pulses into a

voltage and this voltage is AC-coupled into the

TACH/AIN input through coupling capacitor C1. The

value of RSENSE is on the order of 1Ω to 5Ω, depending

on the fan, and the value of the coupling capacitor C1

is 0.01µF. When using this method, set bit 2 of configu-

ration register 1 to 1.

Fan-Fault Detection

The FAN_FAULT output is used to indicate fan slow

down or failure. POR disables the FAN_FAULT output

on the MAX6653/MAX6663. POR enables FAN_FAULT

output on the MAX6664. If FAN_FAULT is not enabled,

writing a logic 1 to bit 4 of configuration register 1 (00h)

enables the FAN_FAULT output pin. Either under-

speed or stalled fans are detected as fan faults.

FAN_FAULT is asserted low only when five consecutive

interrupts are generated by the MAX6653/MAX6663/

MAX6664s’ INT due to fan faults. The MAX6653/

MAX6664 apply 100% duty cycle for the duration of the

spin-up time once an INT is asserted. The MAX6663

goes to 100% duty cycle for the duration of the spin-up

time once INT is asserted and status register 1 is read.

Fan-fault detection works by comparing the value of the

fan tachometer high-limit register (10h) with the value of

the fan-speed reading register (08h), which contains the

value of the most recent fan-speed measurement. Note

that the value of the fan-speed reading register (08h)

must exceed the value of the fan tachometer high limit

(10h) by 1 in order to qualify as a fault. The fault gener-

ates an interrupt signal by asserting the INT output, but

does not cause the FAN_FAULT output to assert until

five consecutive failures have been detected. The fan

runs at 100% duty cycle when FAN_FAULT is asserted.

As an example of the function of the fan-fault detection,

assume a fan is stalled or under speed. The MAX6663 ini-

5V

3.3V

PWM_OUT

100Ω

(TYP)

MAX6653

MAX6663

MAX6664

C1

TACH/AIN

2-WIRE

FAN

N

NDT3055L

2Ω RSENSE

Figure 5. Using the MAX6653/MAX6663/MAX6664 with a

2-Wire Fan

tially indicates the failure by generating an interrupt on the

INT pin. The fan fault bit (bit 1) of interrupt status register

1 (02h) is also set to 1. Once the processor has acknowl-

edged the INT by reading status register 1, the INT is

cleared. PWM_OUT is then brought high for a 2s (fan

spin-up default, Table 12) spin-up period to restart the

fan. Subsequent fan failures cause INT to be reasserted

and PWM_OUT to be brought high (following a status

register 1 read) for a spin-up period each time to restart

the fan. Once the fifth tachometer failure occurs, the

FAN_FAULT is asserted to indicate a critical fan failure.

Alarm Speed

For the MAX6663, the alarm speed bit, bit 0 of status

register 1 (02h), indicates that the PWM duty cycle is

100%, excluding the case of fan spin-up. For the

MAX6653/MAX6664, this bit indicates that the THERM

output is low. Once this bit is set, the only way to clear

it is by reading status register 1. However, the bit does

not reassert on the next monitoring cycle if the condi-

tion still exists. It does assert if the condition is discon-

tinued and then returns.

______________________________________________________________________________________ 13

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