MAX6620ATI MAXIM [Maxim Integrated Products], MAX6620ATI Datasheet - Page 20

• Manufacturer Part Number: MAX6620ATI
• Description: Quad Linear Fan-Speed Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

Quad Linear Fan-Speed Controller
20
BIT
7
6
5
______________________________________________________________________________________
R/W
R/W
R/W
R/W
Fan_ Speed Range:
The MAX6620 determines fan speed by counting the number of internal 8192Hz clock cycles (using an 11-
bit counter) during one or more fan tachometer periods. Three bits set the nominal RPM range for the fan, as
shown in the table below. As an example, a setting of 010 causes the MAX6620 to count the number of
8192Hz clock cycles that occur during four complete tachometer periods. If the fan has a nominal speed of
2000RPM and two tachometer pulses per revolution, one tachometer period will be nominally 15ms, and four
tachometer periods will be 60ms. With an 8192Hz clock, the TACH count will therefore be equal to 491. With
a fan speed of 1/3 the nominal value, the count will be 1474. If the fan’s nominal speed is 1000RPM, the full-
speed TACH count will be 983. At 1/3 the nominal speed, there will be 2948 clock cycles in four tachometer
periods. This is greater than the maximum 11-bit count of 2047, so four tachometer periods is too many for
this fan; a setting of 001 (two clock cycles) is recommended instead.
The table below shows the full-speed tachometer counts for several combinations of nominal fan speeds
and D7:D5 settings. The shaded combinations will provide the best results. When setting D7:D5, the goal is
to obtain the highest tachometer count without exceeding the maximum count of 2047 when the fan is at the
minimum speed of interest. For example, if the minimum speed of interest is 1/3 of full speed, the maximum
tachometer count will be three times the value shown in the table below:
Tachometer Counts/(Counting Period) (8192Hz Clock Used):
D7:D5
101,
110,
000
001
010
011
100
111
TACH PERIODS
NUMBER OF
Fan_ Dynamics Registers (06h, 07h, 08h, 09h)—POR = 0100 1100
COUNTED
18
32
1
2
4
8
(1920ms)
(120ms)
(240ms)
(480ms)
(960ms)
(60ms)
2047
1966
2047
2047
500
491
983
(120ms)
(240ms)
(480ms)
(960ms)
(30ms)
(60ms)
1000
1966
2047
2047
245
491
983
FUNCTION
(120ms)
(240ms)
(480ms)
(15ms)
(30ms)
(60ms)
2000
1966
2047
122
245
491
983
RPM
(240ms)
(7.5ms)
(120ms
(15ms)
(30ms)
(60ms)
4000
1966
122
245
491
983
61
(3.75ms)
(120ms)
(7.5ms)
(15ms)
(30ms)
(60ms)
8000
983
122
245
491
30
61
(1.875ms)
(3.75ms)
(7.5ms
(15ms)
(60ms)
16000
(30ms
122
245
491
15
30
61