MAX6620
Quad Linear Fan-Speed Controller
24
Maxim Integrated
The Fan_ Target Drive Voltage consists of 9 bits con-
tained in two bytes. The two bytes must be written in
order in one or two I
2
C transactions with no other I
2
C
writes in between. These target registers are updated
internally at the same time when a second byte (LSB) is
written.
BIT R/W FUNCTION
7
6
5
4
3
2
1
0
R/W
Fan_ Target Drive Voltage D8:D1:
This is a 9-bit value that ranges from 0 to 511 and is contained in two bytes. In DAC mode, write the
desired fan drive voltage to these two registers. The MAX6620 will then ramp the fan drive voltage to
this value at a rate determined by the DAC rate-of-change bits.
In RPM mode, the value contained in this register will be the voltage applied to the fan immediately after
spin-up or after changing the Fan_ Target TACH Count from 2047 (7FFh) to a value lower than 2047
(7FFh). For example, if the fan is currently stopped with spin-up disabled, and a new Fan_ Target TACH
Count corresponding to 60% of the full-scale fan speed is to be selected, the fan voltage can be
programmed to immediately go to 60% of the full-scale drive voltage when the new Fan_ Target TACH
Count is selected from 2047 (7FFh), and then close the RPM control loop starting from that voltage.
The register value is converted to the drive voltage at the fan (or voltage at DACFB_) as follows:
D8:D0 FAN_ DRIVE VOLTAGE (V)
DECIMAL HEX 5V RANGE 12V RANGE
0 000h 0.000 0.000
200 0C8h 1.764 4.486
300 12Ch 2.646 6.729
400 190h 3.527 8.972
480 1E0h 4.232 10.766
511 1FFh 4.506 11.462
The value of the Fan_ Target Drive Voltage at POR depends on state of the DAC_START pin, as shown
below:
D8:D0
DECIMAL HEX
DAC_START
0 000h GND
384 180h Open
511 1FFh VCC
Bit R/W FUNCTION
7RFan_ Target Drive Voltage D0
Fan_ Target Drive Voltage Registers (29h, 2Bh, 2Dh, 2Fh)—POR = X000 0000
Fan_ Target Drive Voltage Registers (28h, 2Ah, 2Ch, 2Eh)—POR = XXXX XXXX
