Rainbow Electronics MAX6678 - Manual (Page 14)

MAX6678
Fan Configuration (11h)
The fan-configuration register controls the hysteresis
level, temperature step size, and whether the remote or
local diode controls the PWMOUT2 signal (see
Table
2). Set bit D7 of the fan-configuration register to zero to
set the hysteresis value to 5°C. Set bit D7 to 1 to set the
hysteresis value to 10°C. Set bit D6 to zero to set the
fan-control temperature step size to 1°C. Set bit D6 to 1
to set the fan-control temperature step size to +2°C.
Bits D5 to D2 select which PWMOUT_ channel 1 or
channel 2 controls (see Table 2). If both are selected
for a given PWMOUT_, the highest PWM value is used.
If neither is selected, the fan is controlled by the value
written to the fan-target duty-cycle register. Also in this
mode, the value written to the target duty-cycle register
is not limited by the value in the maximum duty-cycle
register. It is, however, clipped to 240 if a value above
240 is written. The POR state of the fan-configuration
register is 00h.
Duty-Cycle Rate of Change (12h)
Bits D7, D6, and D5 (channel 1) and D4, D3, and D2
(channel 2) of the duty-cycle rate-of-change register set
the time between increments of the duty cycle. Each
increment is 2/240 of the duty cycle (see Table 4). This
allows the time from 33% to 100% duty cycle to be adjust-
ed from 5s to 320s. The rate-of-change control is always
active in manual mode. To make instant changes, set bits
D7, D6, and D5 (channel 1) or D4, D3, and D2 (channel
2) = 000. The POR state of the duty-cycle rate-of-change
register is B4h (1s between increments).
Duty-Cycle Step Size (13h)
Bits D7–D4 (channel 1) and bits D3–D0 (channel 2) of the
duty-cycle step-size register change the size of the duty-
cycle change for each temperature step. The POR state
of the duty-cycle step size register is 55h (see Table 5).
PWM Frequency Select (14h)
Set bits D7, D6, and D5 (select A, B, and C) in the PWM
frequency-select register to control the PWMOUT frequen-
cy (see Table 6). The POR state of the PWM frequency-
select register is 40h, 33Hz. The lower frequencies are
usually used when driving the fan’s power-supply pin as
in the Typical Application Circuit, with 33Hz being the
most common choice. The 35kHz frequency setting is
used for controlling fans that have logic-level PWM input
pins for speed control. The minimum duty-cycle resolution
is decreased from 2/240 to 4/240 at the 35kHz frequen-
cy setting. For example, a result that would return a value
of 6/240 is truncated to 4/240.
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
14 ______________________________________________________________________________________
Table 4. Setting the Time Between Duty-
Cycle Increments
D7:D5, D4:D2
TIME BETWEEN
INCREMENTS (s)
TIME FROM 33%
TO 100% (s)
000 0 0
001 0.0625 5
010 0.125 10
011 0.25 20
100 0.5 40
101 1 80
110 2 160
111 4 320
Table 5. Setting the Duty-Cycle Change
D7:D4, D3:D0
CHANGE IN DUTY
CYCLE PER
TEMPERATURE
STEP
TEMPERATURE
RANGE FOR FAN
CONTROL
(1 °C STEP, 33%
TO 100%)
0000 0 0
0001 2/240 80
0010 4/240 40
0011 6/240 27
0100 8/240 20
0101 10/240 16
… … ...
1000 16 10
... ... ...
1111 31 5
Table 6. PWM Frequency Select
PWM
FREQUENCY (Hz)
SELECT A SELECT B SELECT C
20 0 0 0
33 0 1 0
50 1 0 0
100 1 1 0
35k X X 1
Note: At 35kHz, duty-cycle resolution is decreased from a res-
olution of 2/240 to 4/240.