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ADM1034ARQ-REEL7

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ADM1034
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22
measurements are the main benefits. The fan is allowed to
run reliably at speeds as low as 30 percent of the full
capability.
The drive signal applied to the fan is synchronized with
the TACH signal. The ADM1034 switches on the drive
signal and waits for a transition on the TACH signal. When
a transition takes place on the TACH signal, the PWM drive
is switched off for a period of time called t
off
. The drive
signal is then switched on again. The toff time is varied in
order to vary the fan speed. If the fan is running too fast, the
toff time is increased. If the fan is running too slow, the toff
time is decreased.
Since the drive signal is synchronized with the TACH
signal, the frequency with which the fan is driven depends
on the current speed of the fan and the number of poles in it.
Figure 34 shows how the synchronous speed drive signal
works. The ideal TACH signal is the TACH signal that
would be output from the fan if power were applied
100 percent of the time. It is representative of the actual
speed of the fan. The actual TACH signal is the signal the
user would see on the TACH output from the fan if the user
were to put a scope on it. In effect, the actual TACH signal
is the ideal TACH signal chopped with the drive signal.
Figure 34. Drive Signal by Using Synchronous Control
ACTUAL TACH
DRIVE
IDEAL TACH
POLE TRANSITION POINTS
t
POLE
t
OFF
DASH = TACH FLOATS HIGH BY PULL-UP RESISTOR
SOLID = TRUE TACH WHEN FAN IS POWERED
Fan Inputs
Pin 2 and Pin 4 are TACH inputs intended for fan speed
measurement. These inputs are open-drain.
Signal conditioning on the ADM1034 accommodates the
slow rise and fall time of typical tachometer outputs. The
maximum input signal range is from 0 V to 5.0 V, even when
V
CC
is less than 5.0 V. In the event that these inputs are
supplied from fan outputs exceeding 0 V to 5.0 V, either
resistive attenuation of the fan signal or diode clamping
must be used to keep the fan inputs within an acceptable
range.
Figure 35 to Figure 38 show examples of possible fan
input circuits. If the fan TACH has a resistive pullup to V
CC
,
it can be connected directly to the fan output.
Figure 35. Fan with TACH Pullup to +V
CC
5 V or 12 V
FAN
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH X
PULLUP
4.7 kW
TYP
ADM1034
DRIVE X
V
CC
100 kW
TYP
If the fan output has a resistive pullup to 12 V (or another
voltage greater than 5.0 V), the fan output can be clamped
with a Zener diode, as shown in Figure 36. The Zener
voltage should be chosen so that it is greater than V
IH
but less
than 5.0 V. Allowing for the voltage tolerance of the Zener,
a value of between 3.0 V and 5.0 V is suitable.
Figure 36. Fan with TACH Pullup to Voltage > 5.0 V,
Clamped with Zener Diode
5 V or 12 V
FAN
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT TACH X
PULLUP
4.7 kW
TYP
ADM1034
DRIVE X
V
CC
100 kW
TYP
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 V
CC
If the fan has a strong pullup (less than 1 kW to +12 V) or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 37. Alternatively, a
resistive attenuator may be used, as shown in Figure 38.
ADM1034
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23
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)
ADM1034
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 V
CC
PULLUP
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)
ADM1034
*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 ADM1034 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 ADM1034 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 ADM1034 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
ADM1034
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RPM = (81920 60) / 6143
Fan Speed = 800 RPM
Alarm Speed
The fan ALARM speed (Bit 6) in Status Register 3
(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.
Fan Response Register
The ADM1034 fan speed controller operates by reading
the current fan speed, comparing it with the programmed fan
speed, and then updating the drive signal applied to the fan.
The rate at which the ADM1034 looks at and updates the
drive signal is determined by the fan response register.
Different fans have different inertias and respond to a
changing drive signal more or less quickly than others. The
fan response register allows the user to tailor the ADM1034
to a particular fan to prevent situations like overshoot.
The user programs the number of updates the ADM1034
can make to the drive signal per second. Table 27 lists the
available options.
Table 27. FAN RESPONSE CODES
Code Update Rate
000 1.25 updates/second
001 2.5 updates/second = Default
010 5 updates/second
011 10 updates/second
100 20 updates/second
101 40 updates/second
110 80 updates/second
111 160 updates/second
Table 28. CONVERSION RATES
Bit # Function
7 Reserved
<6:4> Fan 2 Response
3 Reserved
<2:0> Fan 1 Response
Look-up Table: Modes of Operation
The ADM1034 look-up table has two different modes of
operation used to determine the behavior of the system:
Manual Mode
Look-up Table
Manual Mode
In manual mode, the ADM1034 is under software control.
The software can program the required fan speed value or
the target fan speed to the ADM1034, which then outputs
that fan speed.
Programming Target Fan Speed
In this mode, the user programs the target fan speed as a
TACH count for N poles or a TACH count for one full
rotation of the fan, assuming the number of poles is
programmed correctly in the Configuration 3 Register
(Address 0x03).
Use the following steps to program the target fan speed:
1. Place the ADM1034 into manual mode. Set Bit 7
(Table/SW) of Configuration Register 1
(Address 0x01) = 0.
2. Program the target TACH count (fan speed) using
the following equation:
(eq. 4)
TACH Count + (f 60)ńR
where:
f = clock frequency = 81.92 kHz
R = required RPM value
Example 1: If the desired speed for Fan 1 is 5000 rpm,
program the following value to the TACH count registers:
TACH Count = (f 60)/5000
TACH Count = 983d = 0x03D7
Example 2: If the desired speed for Fan 2 is 3500 rpm,
program the following value to the TACH pulse period
registers:
TACH Count = (f 60)/3500
TACH Count = 1404d = 0x057C
Table 29. REGISTERS TO BE PROGRAMMED
Fan Description Address Value
Fan 1 Look-up Table FS1, LSB 0x2A 0xD7
Fan 1 Look-up Table FS1, MSB 0x2B 0x03
Fan 2 Look-up Table FS2, LSB 0x2C 0x7C
Fan 2 Look-up Table FS2, MSB 0x2D 0x05
Look-up Table
The ADM1034 allows the user to program a
temperature-to-fan speed profile. There are 24 registers in
the look-up table; 8 for temperature and 16 for target fan
speed (each target fan speed is two registers). In total, there
are eight available points. In the default configuration, four
of these are for Fan 1 and four are for Fan 2. However, it can
be configured such that eight points are available and both
fans run from the same table.
There are two options when programming the look-up
table. The ADM1034 can be programmed to make the fan
speed run at discrete speeds and jump to the new fan speed
once the temperature threshold is crossed. Or, it can linearly
ramp the TACH count between the two temperature
thresholds.
Figure 40 and Figure 41 show what the look-up table
looks like if all eight points are used on the one curve for both
fans.
Figure 40 shows the transfer curve when the fan is
programmed to run at discrete speeds. The ADM1034 spins
the fan at its new speed once a threshold is crossed.
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ADM1034ARQ-REEL7

Mfr. #:
Buy ADM1034ARQ-REEL7
Manufacturer:
ON Semiconductor
Description:
IC THERM/FAN SPEED CTRLR 16-QSOP
Lifecycle:
New from this manufacturer.
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