ADM1033ARQZ-REEL7 ON Semiconductor, ADM1033ARQZ-REEL7 Datasheet - Page 22

• Manufacturer Part Number: ADM1033ARQZ-REEL7
• Description: IC THERM/FAN SPEED CTRLR 16-QSOP
• Manufacturer: ON Semiconductor

Specifications of ADM1033ARQZ-REEL7

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 125°C, External Sensor
• Output Type: SMBus™
• Output Alarm: Yes
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-QSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Fan Speed Measurement
pulses directly. This is because the fan may be spinning at
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 81.92 kHz oscillator into the input of a 16−bit
counter for one complete revolution of the fan. Therefore,
the accumulated count is actually proportional to the fan
tachometer period and inversely proportional to the fan
speed.
Configuration Register 3 (Address 0x03). Bits <3:0> set the
number of poles for Fan 1, and Bits <7:4> set the number of
poles for Fan 2. This number must be an even number only,
because there cannot be an uneven number of poles in a fan.
A TACH period is output for every two poles. Therefore, the
number of poles must be known so that the ADM1033 can
measure for a full revolution.
assuming that the fan outputs an ideal TACH signal. In
reality, the TACH signal output by the fan is chopped by the
drive signal. However, since the drive and the TACH signal
are synchronized, there is enough information available for
the ADM1033 to measure the fan speed accurately.
Fan Speed Measurement Registers
registers.
CLOCK
IDEAL
TACH
Figure 39. Fan Speed Measurement for a 4−Pole Fan
Figure 38. Fan with Strong TACH. Pullup to >V
The fan counter does not count the fan TACH output
The number of poles in the fan must be programmed in
Figure 39 shows the fan speed measurement period,
These 16−bit measurements are stored in the TACH value
Totem−Pole Output, Attenuated with R1/R2
12 V
MEASUREMENT
PERIOD
FAN
<1 kΩ
OUTPUT
TACH
R1*
*SEE TEXT
R2
FAN(0–7)
ADM1033
FAN SPEED
COUNTER
V
CC
CC
http://onsemi.com
or
22
Reading Fan Speed
each measurement. The low byte should be read first. This
freezes the high byte until both high and low byte registers
have been read, preventing erroneous fan speed
measurement readings.
number of 12.2 ms period clocks (81.92 kHz oscillator) gated
to the fan speed counter, for one full rotation of the fan,
assuming the correct number of poles is programmed. Since
the ADM1033 essentially measures the fan TACH period,
the higher the count value, the slower the actual fan speed.
A 16−bit fan TACH reading of 0xFFFF indicates that the fan
has stalled or is running very slowly (< 75 rpm).
Calculating Fan Speed
that the number of poles programmed in the Configuration
Register 3 (Address 0x03) is correct for both fans.
Example:
Alarm Speed
(Address 0x51) is set whenever the fan runs at alarm speed.
This occurs if the device is programmed to run the fan at full
speed whenever the THERM temperature limits are
exceeded. The device runs at alarm speed, for example, if the
Boost Disable bit (Bit 1) of the Configuration 2 Register
(Address 0x02) is not set to 1.
Table 22. TACH Value Registers
Reading back fan speeds involves a 2−register read for
The fan tachometer reading registers report back the
Fan speed in rpm is calculated as follows. This assumes
Fan Speed (RPM) = (81920 x 60)/Fan TACH Reading
Fan TACH Reading = 16−bit Fan TACHometer Reading
What is Fan 1 speed in rpm?
The fan ALARM speed (Bit 6) in Status Register 3
Register
where:
TACH1 High Byte (Reg. 0x4A) = 0x17
TACH1 Low Byte (Reg. 0x4B) = 0xFF
Fan 1 TACH Reading = 0x17FF = 6143d
RPM = (f x 60) / Fan 1 TACH reading
RPM = (81920 x 60) / 6143
Fan Speed = 800 RPM
0x4A
0x4B
0x4C
0x4D
TACH1 Period, MSB
TACH2 Period, MSB
TACH1 Period, LSB
TACH2 Period, LSB
Description
Default
0xFF
0xFF
0xFF
0xFF