Undercurrent (AC Component)

at least 1s (Figure 2). The fan remains powered during

undercurrent failures.

Minimum Speed and

Locked-Rotor Detection

The MAX6684 asserts FAIL if the fan-current fluctuation

frequency is below 25Hz, which corresponds to a fan

speed of approximately 700rpm. The fan remains pow-

ered during a locked rotor or an under-speed failure

condition (Figures 3 and 4).

The MAX6684 can be designed to detect fan failure

Because of the complexity of fan-current waveforms,

must be verified by bench testing. The guidelines of

Figure 5 are only appropriate for the test signals used

and do not represent all possible fan waveforms. They

are to illustrate the ability of the MAX6684 to discrimi-

nate failure due to low fan speed. As a rule, failure typi-

cally occurs when the amplitude measured at pin 4 of

the MAX6684 drops below 70mV.

Overcurrent Protection

If an overcurrent condition begins and continues for 2ms,

fan failure is signaled for 60ms. During this 60ms period,

the power to the fan is turned off. If the part does not enter

thermal shutdown and the overcurrent condition contin-

ues, power to the fan is turned on every 62ms for 2ms

(see Overcurrent Operation in Typical Operating Char-

acteristics). Once the overcurrent condition is removed,

the fan is powered continuously. A 0.1µF capacitor

between SENSE and PGND prevents the internal DMOS

switch from being damaged by back EMF current.

Thermal Shutdown

A die temperature in excess of +160°C initiates thermal

shutdown. In thermal shutdown, the MAX6684 shuts off

the fan and the FAIL output asserts. While in thermal

shutdown, the MAX6684 monitors the die temperature.

Once the die has cooled to below +145°C, the MAX6684

exits thermal shutdown and power is returned to the fan.

A thermal shutdown fault condition has precedence over

all other failure modes. While the MAX6684 die is over

temperature, power is not cycled to the fan, as occurs

during overcurrent failure.

Fan-Failure Detector with Integrated

Power Switch

_______________________________________________________________________________________

5

Figure 3. MAX6684 Commutation Fault Timing Diagram

FAIL

100Hz

ƒ

Figure 4. Current Fluctuation and Commutation Frequency

Diagram

FAIL HIGH

FAIL LOW

0

10

20

30

40

50

60

70

80

0

100

200

300

400

CURRENT FLUCTUATION

vs. COMMUTATION FREQUENCY

CURRENT COMMUTATION FREQUENCY (Hz)

MAX6684

OFF

SENSE

APPROXIMATE FAILURE FREQUENCIES:

FUNCTION

GENERATOR*

50mA TO 300mA DC COMPONENT

PGND

GND

FC+

µF < 25Hz

µF < 86Hz

µF < 250Hz

10k

Ω

FC-

3.3V

FAIL

Figure 5. Test Circuit Demonstrates Failure Frequency as a