ADM1027ARQZ-REEL ON Semiconductor, ADM1027ARQZ-REEL Datasheet - Page 23

• Manufacturer Part Number: ADM1027ARQZ-REEL
• Description: IC REMOTE THERMAL CTRLR 24QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADM1027ARQZ-REEL

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Fan Speed Counter, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: 0°C ~ 105°C, External Sensor
• Output Type: SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: 0°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 24-QSOP
• Full Temp Accuracy: +/- 1 C
• Digital Output - Bus Interface: Serial (2-Wire, 3-Wire)
• Maximum Operating Temperature: + 105 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

FAN SPEED MEASUREMENTS
TACH Inputs
Pins 11, 12, 9, and 14 are open-drain TACH inputs intended
for fan speed measurement.
Signal conditioning in the ADM1027 accommodates the slow
rise and fall times typical of fan tachometer outputs. The maxi-
mum input signal range is 0 V to 5 V, even where V
than 5 V. In the event that these inputs are supplied from fan
outputs that exceed 0 V to 5 V, either resistive attenuation of
the fan signal or diode clamping must be included to keep
inputs within an acceptable range.
Figures 25a to 25d show circuits for most common fan TACH
outputs. If the fan TACH output has a resistive pull-up to V
can be connected directly to the fan input, as shown in Figure 25a.
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V), then the fan output can be clamped with a
Zener diode, as shown in Figure 25b. The Zener diode voltage
should be chosen so that it is greater than V
input but less than 5 V, allowing for the voltage tolerance of the
Zener. A value of between 3 V and 5 V is suitable.
If the fan has a strong pull-up (less than 1 k ) to 12 V, or a
totem-pole output, then a series resistor can be added to limit
the Zener current, as shown in Figure 25c. Alternatively, a
resistive attenuator may be used, as shown in Figure 25d.
R1 and R2 should be chosen such that
The fan inputs have an input resistance of nominally 160 kW to
ground; this should be taken into account when calculating
resistor values.
REV. A
FAN
FAN
5V OR 12V
5V OR 12V
Figure 25b. Fan with TACH Pull-Up to Voltage
> 5 V (e.g., 12 V) Clamped with Zener Diode
2
Figure 25a. Fan With TACH Pull-Up to +V
V
<
PULL-UP
PULL-UP
V
*CHOOSE ZD1 VOLTAGE APPROX 0.8
4.7k
4.7k
PULLUP
TYP
TYP
¥
TACHO
OUTPUT
TACHO
OUTPUT
R
2
ZD1*
(
R
PULLUP
TACH X
TACH X
+
R
1
IH
+
V
ADM1027
ADM1027
CC
FAN SPEED
FAN SPEED
of the TACH
R
COUNTER
COUNTER
V
V
2
Rev. 3 | Page 23 of 56 | www.onsemi.com
CC
CC
)
>
CC
5
CC
V
is less
CC
, it
–23–
With a pull-up voltage of 12 V and pull-up resistor less than
1 kW, suitable values for R1 and R2 would be 100 kW and
47 kW. This will give a high input voltage of 3.83 V.
Fan Speed Measurement
The fan counter does not count the fan TACH output pulses
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 90 kHz oscillator into the input
of a 16-bit counter for N periods of the fan TACHO output
(Figure 26), so the accumulated count is actually proportional
to the fan tachometer period and inversely proportional to the
fan speed.
N, the number of pulses counted, is determined by the settings
of Register 0x7B (fan pulses per revolution register). This register
contains two bits for each fan, allowing 1, 2 (default), 3, or 4
TACH pulses to be counted.
FAN
FAN
5V OR 12V
5V OR 12V
CLOCK
Figure 25c. Fan with Strong TACH Pull-Up to > V
Totem-Pole Output, Clamped with Zener and Resistor
TACH
Figure 25d. Fan with Strong TACH Pull-Up to > V
or Totem-Pole Output, Attenuated with R1/R2
PWM
PULL-UP TYP
PULL-UP TYP
TOTEM POLE
*CHOOSE ZD1 VOLTAGE APPROX 0.8
Figure 26. Fan Speed Measurement
1
2
<1k
<1k
3
OR
4
TACHO
OUTPUT
TACHO
OUTPUT
*SEE TEXT
10k
R1
R1*
TACH X
TACH X
R2*
ZD1*
V
CC
ADM1027
ADM1027
ADM1027
FAN SPEED
FAN SPEED
COUNTER
COUNTER
V
V
CC
CC
CC
CC
or