ADM1033
http://onsemi.com
22
R1 and R2 should be chosen such that
(eq. 3)
2Vt V
PULLUP
R2ń(R
PULLUP
) R1 ) R2) t 5V
The fan inputs have an input resistance of nominally
160 kW to ground. This should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and pullup resistor less than
1 kW, suitable values for R1 and R2 would be 100 kW and
47 kW. This gives a high input voltage of 3.83 V.
Figure 37. Fan with Strong TACH. Pullup to >V
CC
or
Totem-Pole Output, Clamped with Zener and Resistor
12 V
V
CC
FAN SPEED
COUNTER
FAN (0−7)
ADM1033
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 V
CC
PULL-UP
TYP < 1 kW
OR TOTEM-POLE
ZD1*
Figure 38. Fan with Strong TACH. Pullup to >V
CC
or
Totem-Pole Output, Attenuated with R1/R2
12 V
V
CC
FAN SPEED
COUNTER
FAN (0−7)
ADM1033
*SEE TEXT
< 1 kW
R1*
R2
TACH
OUTPUT
Fan Speed Measurement
The fan counter does not count the fan TACH output
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.
The number of poles in the fan must be programmed in
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.
Figure 39 shows the fan speed measurement period,
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.
Figure 39. Fan Speed Measurement for a 4-pole Fan
CLOCK
IDEAL
TACH
FAN
MEASUREMENT
PERIOD
Fan Speed Measurement Registers
These 16-bit measurements are stored in the TACH value
registers.
Table 26. TACH VALUE REGISTERS
Register Description Default
0x4A TACH1 Period, LSB 0xFF
0x4B TACH1 Period, MSB 0xFF
0x4C TACH2 Period, LSB 0xFF
0x4D TACH2 Period, MSB 0xFF
Reading Fan Speed
Reading back fan speeds involves a 2-register read for
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.
The fan tachometer reading registers report back the
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
Fan speed in rpm is calculated as follows. This assumes
that the number of poles programmed in the Configuration
Register 3 (Address 0x03) is correct for both fans.
Fan Speed (RPM) = (81920 60)/Fan TACH Reading
where:
Fan TACH Reading = 16-bit Fan TACHometer Reading
Example:
TACH1 High Byte (Reg. 0x4A) = 0x17
TACH1 Low Byte (Reg. 0x4B) = 0xFF
What is Fan 1 speed in rpm?
Fan 1 TACH Reading = 0x17FF = 6143d
RPM = (f 60) / Fan 1 TACH reading
