MAX6664AEE+T【PDF 12/24 页】IC TEMP MON FAN CNTRL 16-QSOP

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Temperature Monitors and

PWM Fan Controllers

12 ______________________________________________________________________________________

measurement rate, which is controlled by bits [4:2] of

the fan-filter register (23h). Table 5 shows the effect of

the temperature measurement rate control bits. As an

example, assume that the temperature measurement

rate is 2Hz, or 0.5s per monitoring cycle, and the fan-fil-

ter rate is 0.416% per monitoring cycle. For the fan drive

to change from 50% to 100% requires 50% / 0.416% =

120 temperature monitoring cycles. Thus, for a tempera-

ture-monitoring cycle of 0.5s, the time required for the

drive to change from 50% to 100% is 60s.

Fan-Speed Measurement

The fan speed is measured by using the relatively slow

tachometer signal from the fan to gate an 11.25kHz

clock frequency into a fan-speed counter. The mea-

surement is initialized on the starting edge of a PWM

output if fan-speed measurement is enabled by setting

bit 2 of configuration register 2 (01h) to 1. Counting

begins on the leading edge of the second tachometer

pulse and lasts for two tachometer periods or until the

counter overranges (255). The measurement repeats

unless monitoring is disabled by resetting bit 2 in the

configuration register 2 (01h). The measured result is

stored in the fan-speed reading register (08h).

The fan-speed count is given by:

where RPM = fan speed in RPM.

N determines the speed range and is programmed by

bits [7:6] in the fan characteristics register (20h) as

shown in Table 14. When the speed falls below the value

in the speed range column, a fan failure is detected.

The TACH/AIN input can be either a digital signal (from

the fans tachometer output) or an analog signal,

depending on the setting of bit 2 of the configuration

up TACH/AIN as a digital input. For the analog input

(Figure 4), the detected voltage threshold is typically at

250mV, which is appropriate for sensing the voltage of

a sense resistor connected to the ground lead of a 2-

wire fan. The AIN input only responds to pulse widths

greater than 10祍.

Count

RPM N

?/DIV>

675000

Table 12. Spin-Up Time; Bits [2:0] Fan

Characteristics Register (20h)POR = 101

BIT 2

BIT 1

BIT 0

SPIN-UP

TIME (s)

0.2

0.4

0.6

0.8

Table 13. Fan Filter Ramp Rate; Bits [6:5]

Fan Filter Register (23h)POR = 10;

BIT 6

BIT 5

RAMP RATE

x100% / 240

RAMP RATE

% DUTY CYCLE

0.416

0.833

1.667

3.333

FAN CHARACTERISTICS REGISTER (20h) POR = 01

BIT 7

BIT 6

SPEED RANGE (FAIL SPEED)

(RPM)

2647

1324

662

331

Table 14. N Factor for Speed-Range Adjustment (Assuming Two Tachometer Pulses

per Revolution)

Figure 4. Simplified Tachometer Analog Input Structure

100&

VREF 1

0.1礔

TACH

INPUT

VREF 2

CLK

MAX6653

MAX6663

MAX6664

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