ADM1026
•
•
In all these circuits, the output transistor must have an
ICMAX greater than the maximum fan current, and be
capable of dissipating power due to the voltage dropped
across it when the fan is not operating at full speed.
If the fan motor produces a large back EMF when switched
off, it may be necessary to add clamp diodes to protect the
output transistors in the event that the output goes from
full scale to zero very quickly.
12V
R2
100kΩ
R2
100kΩ
Q3
IRF9620
R3
39kΩ
Q1/Q2
MBT3904
DUAL
12V
R4
10kΩ
DAC
1/4
LM324
DAC
Figure 39. Discrete Fan Drive Circuit with P-Channel MOSFET, Single Supply
Q1
2N2219A
+12V
R2
100kΩ
R2
36kΩ
Q3
IRF9620
R1
10kΩ
R3
39kΩ
Q1/Q2
DAC
MBT3904
DUAL
Figure 36. Fan Drive Circuit with Op Amp and Emitter-Follower
R4
10kΩ
12V
R1
4.7kΩ
1/4
LM324
R4
1kΩ
–12V
Q1
BD136
2SA968
DAC
Figure 40. Discrete Fan Drive Circuit with P-Channel MOSFET, Dual Supply
R3
1kΩ
PWM Output
Fan speed may also be controlled using pulse width modulation
(PWM). The PWM output (Pin 18) produces a pulsed output
with a frequency of approximately 75 Hz and a duty cycle
defined by the contents of the PWM control register (Address
05h). During automatic fan speed control, described below, the
four MSBs of this register set the minimum fan speed.
R2
39kΩ
R1
10kΩ
The open drain PWM output must be amplified and buffered
to drive the fans. The PWM output is intended to be used with
an NMOS driver, but may be inverted by setting Bit 1 of Test
Register 1 (Address 14h) if using PMOS drivers. Figure 41
shows how a fan may be driven under PWM control using an
N-channel MOSFET.
Figure 37. Fan Drive Circuit with Op Amp and PNP Transistor
12V
1/4
LM324
R3
100kΩ
+V
DAC
Q1
IRF9620
3.3V
5V OR 12V
FAN
R2
39kΩ
10kΩ TYP
Q1
NDT3055L
PWM
R1
10kΩ
Figure 41. PWM Fan Drive Circuit Using an N-Channel MOSFET
Figure 38. Fan Drive Circuit with Op Amp and P-Channel MOSFET
Rev. A | Page 23 of 56
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