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MAX6639 Datasheet(PDF) 16 Page - Maxim Integrated Products

Part No. MAX6639
Description  2-Channel Temperature Monitor with Dual, Automatic, PWM Fan-Speed Controller
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Manufacturer  MAXIM [Maxim Integrated Products]
Direct Link  https://www.maximintegrated.com/en.html
Logo MAXIM - Maxim Integrated Products

MAX6639 Datasheet(HTML) 16 Page - Maxim Integrated Products

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Fan 1 and 2 Pulses and Min RPM (24h and 25h)
D[7:6]: This sets the number of tachometer pulses per
revolution for the fan. When set properly, a 2000RPM fan
with two pulses per revolution has the same tachometer
count as a 2000RPM fan with four pulses per revolution.
Table 10 lists tachometer pulses per revolution.
D[5:0]: This sets the minimum allowable fan tachometer
count (maximum speed). This limits the maximum
speed of the fan to reduce noise at high temperatures.
For reasonable speed resolution, the fan RPM range
should be set so this value is between approximately
30 and 60. If a maximum RPM limit is unnecessary, this
value can be set to the full-speed tachometer count.
Fan 1 and 2 Duty Cycle (26h and 27h)
These registers contain the present value of the PWM
duty cycle. In PWM fan-control mode, the desired (tar-
get) value of the PWM duty cycle can be written directly
into this register.
Channel 1 and Channel 2 Fan-Start Temperature
(28h and 29h)
These registers contain the temperatures at which fan
control begins (in automatic RPM mode).
Applications Information
Fan-Drive Circuits
A variety of fan-drive circuit configurations can be used
with the MAX6639 to control the fan’s speed. Four of
the most common are shown in Figures 6 through 10.
PWM Power-Supply Drive (High Side or Low Side)
The simplest way to control the speed of a 3-wire (sup-
ply, ground, and tachometer output) fan is to modulate
its power supply with a PWM signal. The PWM frequen-
cy is typically in the 20Hz to 40Hz range, with 33Hz
being a common value. If the frequency is too high, the
fan’s internal control circuitry does not have sufficient
time to turn on during a power-supply pulse. If the fre-
quency is too low, the power-supply modulation
becomes more easily audible.
The PWM can take place on the high side (Figure 6) or
the low side (Figure 7) of the fan’s power supply. In
either case, if the tachometer is used, it is usually nec-
essary to periodically stretch a PWM pulse so there is
enough time to count the tachometer pulse edges for
speed measurement. The MAX6639 allows this pulse
stretching to be enabled or disabled to match the
needs of the application.
Pulse stretching can sometimes be audible if the fan
responds quickly to changes in the drive voltage. If the
acoustic effects of pulse stretching are too noticeable,
the circuit in Figure 8 can be used to eliminate pulse
2-Channel Temperature Monitor with Dual,
Automatic, PWM Fan-Speed Controller
16
______________________________________________________________________________________
REGISTERS 24h
OR 25h D[7:6]
TACHOMETER PULSES PER
REVOLUTION
00
1
01
2
10
3
11
4
Table 10. Tachometer Pulses per
Revolution
VCC
PWM1
4.7k
4.7k
TACH1
3V TO 5.5V
TACH
OUTPUT
VFAN
(5V OR 12V)
Figure 6. High-Side PWM Drive Circuit
VCC
TACH1
4.7k
4.7k
PWM1
3V TO 5.5V
3V TO 5.5V
TACH
OUTPUT
VFAN
(5V OR 12V)
Figure 7. Low-Side Drive Circuit


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