EVAL-ADM1026EB ON Semiconductor, EVAL-ADM1026EB Datasheet - Page 24

• Manufacturer Part Number: EVAL-ADM1026EB
• Description: BOARD EVAL FOR ADM1026
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of EVAL-ADM1026EB

• Contents: Evaluation Board
• For Use With/related Products: ADM1026
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

a totem pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 45. Alternatively, a
resistive attenuator may be used, as shown in Figure 47.
Fan Speed Measurement
directly because the fan speed may be less than 1000 RPM
and it would take several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on−chip
22.5 kHz oscillator into the input of an 8−bit counter for two
periods of the fan tach output, as shown in Figure 47, so the
accumulated count is actually proportional to the fan tach
period and inversely proportional to the fan speed.
2.0 V t V
Totem Pole Output, Clamped with Zener and Resistor
If the fan has a strong pullup (less than 1 kW) to +12 V, or
R1 and R2 should be chosen such that:
The fan counter does not count the fan tach output pulses
Figure 46. Fan with Strong Tach Pullup to > V
Figure 45. Fan with Strong Tach Pullup to >V
Figure 44. Fan with Tach Pullup to Voltage > V
Totem Pole Output, Attenuated with R1/R2
(e.g. 12 V), Clamped with Zener Diode
PULLUP
12 V
12 V
* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 x V
* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 x V
PULLUP TYP
<1 kΩ
12 V
<1 kΩ
PULLUP
4.7kΩ
TYP
R
PULLUP
OUTPUT
TACH
* SEE TEXT
R1*
R2
) R1 ) R2
FAN(0–7)
FAN(0–7)
FAN(0–7)
t 3.3 V STBY
FAN SPEED
FAN SPEED
FAN SPEED
COUNTER
COUNTER
COUNTER
V
V
V
CC
CC
CC
CC
CC
CC
CC
(eq. 15)
http://onsemi.com
CC
or
or
24
monitor bit (Bit 0 of Configuration Register 1). The
INT_Enable (Bit 1) should be set to 1 to enable the INT
output.
channel has been switched to. If the fan tach count reaches
0xFF, the fan has failed or is not connected. If a fan is
connected and running, the counter is reset on the second
tach rising edge, and oscillator pulses are actually counted
from the second rising tach edge to the fourth rising edge.
The measurement then switches to the next fan channel.
Here again, the counter begins counting and is reset on the
second tach rising edge, and oscillator pulses are counted
from the second rising edge to the fourth rising edge. This
is repeated for the other six fan channels.
1.8 seconds after the monitor bit has been set. This is to
allow the fans adequate time to spin up. Otherwise, the
ADM1026 could generate false fan failure interrupts.
During the 1.8 second fan spin−up time, all fan tach registers
read 0x00.
numbers of output pulses per revolution, a prescaler
(divisor) of 1, 2, 4, or 8 may be added before the counter.
Divisor values for Fans 0 to 3 are contained in the Fan 0–3
divisor register (Address 02h) and those for Fans 4 to 7 in the
Fan 4–7 divisor register (Address 03h). The default value
is 2, which gives a count of 153 for a fan running at
4400 RPM producing two output pulses per revolution. The
count is calculated by the equation:
considered to have occurred when the speed drops below
70% of nominal, corresponding to a count of 219. Full scale
(255) is reached if the fan speed fell to 60% of its nominal
value. For temperature−controlled, variable−speed fans, the
situation is different.
CONFIGURATION
CLOCK
REG. 1 BIT 0
22.5kHz
The monitoring cycle begins when a 1 is written to the
The fan speed counter starts counting as soon as the fan
Note that fan speed measurement does not occur until
To accommodate fans of different speed and/or different
For constant−speed fans, fan failure is typically
INPUT
FAN0
START OF
MONITORING
CYCLE
Figure 47. Fan Speed Measurement
Count + 22.5
1
2
FAN0
MEASUREMENT
PERIOD
RPM
3
10
Divisor
4
3
1
60
2
FAN1
MEASUREMENT
PERIOD
3
(eq. 16)
4