ADT7466
Rev. 2 | Page 27 of 48 | www.onsemi.com
FAN DRIVE
The ADT7466 contains two DACs to control fan speed. The
full-scale output of these DACs is typically 2.2 V @ 2 mA, so
they must be buffered in order to drive 5 V or 12 V fans. The
output voltage of these DACs is controlled by data written to the
DRIVE1 (0x40) and DRIVE2 (0x41) registers.
Since fans do not turn on below a certain drive voltage, a
significant proportion of the DAC range would be unusable;
however, four other registers associated with fan speed control
help the user to avoid this problem.
Fan start-up voltage registers (0x30 and 0x31) determine the
voltage initially applied to the fans at startup. This should be
high enough to ensure that the fans start.
Minimum speed registers (0x32 and 0x33) determine the
minimum voltage that is applied to the fans. This should be
high enough to keep the fans turning and less than the voltage
required to start them.
The speed registers associated with automatic fan speed control
(AFC) are the maximum speed registers (0x34 and 0x35). They
allow the maximum output from the DACs to be limited to less
than the full-scale output.
Some suitable fan drive circuits are shown in Figure 33 and
Figure 34. Basically, voltage amplification is required to boost
the full-scale output of the DAC to 5 V or 12 V, and the
amplifier needs sufficient drive current to meet the drive
requirements of the fan.
Note that as the external transistor increases the open-loop gain
of the op amp, it may be necessary to add a capacitor around
the feedback loop to maintain stability.
04711-032
Q1
2N2219
12V
R3
1kΩ
1/4
LM324
OUT
R1
10kΩ
R2
12kΩ (5V)
43kΩ (12V)
Figure 33. Fan Drive Circuit with Op Amp and Emitter-Follower
04711-033
Q1
IRF9620
5V OR 12V
1/4
LM324
DAC
R3
100kΩ
R1
10kΩ
R2
12kΩ (5V)
43kΩ (12V)
Figure 34. Fan Drive Circuit with P-Channel MOSFET
PWM OR SWITCH MODE FAN DRIVE
Linear dc speed controllers, such as the ones described
previously, waste power, which is dissipated as heat in the power
transistor. To save power and reduce heat dissipation, it may be
desirable to control the fan speed with a more efficient dc-dc
converter or a pulse width modulated (PWM) speed controller.
In this case, the DRIVE outputs of the ADT7466 provide the
reference voltage for this circuit. To maximize efficiency, the
controller can be switched off completely whenever the Fan 1
drive value falls below the value in the V_FAN_MIN register.
When this happens, the FAN1_ON output goes low.
04711-034
ADT7466
V+
DRIVE1
FAN1 ON
DRIVE
VOLTAGE
SHUTDOWN
DC-DC
OR PWM
FAN SPEED
CONTROLLER
Figure 35. DC-DC or PWM Fan Speed Control
FAN SPEED MEASUREMENT
TACH Inputs
Pin 2 and Pin 4 are tach inputs intended for fan speed
measurement. The ADT7466 can measure the speed of 3-wire
fans. Each 3-wire fan has two supply wires and a tach output wire.
Signal conditioning in the ADT7466 accommodates the slow rise
and fall times typical of fan tachometer outputs. The maximum
input signal range is 0 V to 6.5 V, even when V
CC
is less than 5 V. If
these inputs are supplied from fan outputs that exceed 0 V to 6.5 V,
either resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Monitoring 3-Wire Fans
Figure 36 to Figure 39 show circuits for most common 3-wire
fan tach outputs.
If the fan tach output has a resistive pull-up to V
CC
, it can be
connected directly to the fan input, as shown in Figure 36.
04711-035
FAN DRIVE
CC
ADT7466
TACH
TACH
OUTPUT
FAN SPEED
COUNTER
PULLUP
4.7kΩ
TYP.
Figure 36. Fan with Tach Pull-Up to +V
CC
