PRELIMINARY TECHNICAL DATA
ADM1026
– 1 9 –
REV. PrL
PRELIMINAR
Y
TECHNICAL
DA
TA
PWM OUTPUT
Fan speed may also be controlled using pulse-width
modulation (PWM). The PWM output (pin 18) produces
a pulsed output with a frequency of approximately 75Hz
and a duty-cycle defined by the contents of the Fan 2
Speed Register (address 05h). During automatic fan speed
control, described below, the four MSBs of this register
set the minimum fan speed.
The PWM output operates at normal logic levels and
must be amplified and buffered to drive the fans. Suitable
drive circuits are given in Figures
XX to YY (still await-
ing diagrams).
AUTOMATIC FAN SPEED CONTROL
The ADM1026 offers a simple method of controlling fan
speed according to temperature without intervention from
the host processor.
To enable automatic fan speed control, monitoring must
be enabled by setting Bit 0 of Configuration Register 1
(address 00h).
Automatic fan speed control can be applied to the DAC
output, the PWM output, or both, by setting bit 5 and/or
6 of Configuration Register 1.
The T
MIN
registers (addresses 10h to 12h) contain mini-
mum temperature values for the three temperature chan-
nels (on-chip sensor and two remote diodes). This is the
temperature at which a fan will start to operate when the
temperature sensed by the controlling sensor exceeds
T
MIN
. T
MIN
can be the same or different for all three chan-
nels. T
MIN
is set by writing a two's complement tempera-
ture value to the T
MIN
registers. If any sensor channel is
not required for automatic fan speed control, T
MIN
for that
channel should be set to +127
o
C (01111111).
In automatic fan speed mode, the four MSBs of the Fan
Speed 1 Register (address 04h) and Fan Speed 2 Register
(address 05h) set the minimum values for the DAC and
PWM outputs.
Minimum DAC Code DAC
MIN
= 16
⫻
D
(DAC output voltage = 2.5
⫻
Code/256)
Minimum PWM Duty-Cycle PWM
MIN
= 6.67
⫻
D
where D is the decimal equivalent of bits 7 to 4 of the reg-
ister.
When the temperature measured by any of the sensors ex-
ceeds the corresponding T
MIN
, the fan is spun up for two
seconds with the fan drive set to maximum (full-scale
from the DAC or 100% PWM duty-cycle. The fan speed
is then set to the minimum as defined above. As the tem-
perature increases, the fan drive will increase until the
temperature reaches T
MIN
+20
o
C.
The fan drive at any temperature up to 20
o
C above T
MIN
is
given by:
PWM = PWM
MIN
+ (100 - PWM
MIN
)
⫻
(T
ACTUAL
- T
MIN
)/20)
o r
DAC = DAC
MIN
+ (240 - DAC
MIN
)
⫻
(T
ACTUAL
- T
MIN
)/20)
For simplicity of the automatic fan speed algorithm, the
DAC code increases linearly up to 240, not its full-scale
of 255. However, when the temperature exceeds T
MIN
+20
o
C, the DAC output will jump to full-scale.
P W M
O UT P UT
M IN
T E M P E RA T UR E
T
M IN
T
M IN
+ 20
o
S P IN U P FO R 2 S E CO ND S
100%
T
M IN
- 4
o
Figure 11a. Automatic PWM Fan Control Transfer Function
DA C
O UT P UT
M IN
T E M P E RA T URE
T
M I N
T
M IN
+ 20
o
S P IN U P FO R 2 S E CO ND S
255
T
M IN
- 4
o
240
Figure 11b. Automatic DAC Fan Control Transfer Function
To ensure that the maximum cooling capacity is always
available, the fan drive is always set by the sensor channel
demanding the highest fan speed.
If the temperature falls, the fan will not turn off until the
temperature measured by all three temperature sensors has
has fallen to their corresponding T
MIN
– 4
o
C. This pre-
vents the fan from cycling on and off continuously when
the temperature is close to T
MIN
.
Whenever a fan starts or stops during automatic fan speed
control, a one-off interrupt is generated at the
INT
out-
put. This is described in more detail in the section on the
ADM1026 Interrupt Structure.
