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ADM1022(2003) View Datasheet(PDF) - Analog Devices

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ADM1022 Datasheet PDF : 21 Pages
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ANALOG OUTPUT
The ADM1022 has a single analog output (FAN_SPD) from an
unsigned 8-bit DAC that produces 0 V–2.5 V. The analog out-
put register defaults to 00 during power-on reset, which produces
minimum fan speed. The analog output may be amplified and
buffered with external circuitry such as an op amp and transistor
to provide fan speed control.
Suitable fan drive circuits are given in Figures 5a to 5e. When
using any of these circuits, the following points should be noted:
1. All of these circuits will provide an output range from zero to
almost +VFAN.
2. To amplify the 2.5 V range of the analog output up to +VFAN,
the gain of these circuits needs to be set as shown.
3. Care must be taken when choosing the op amp to ensure that
its input common-mode range and output voltage swing are
suitable.
4. The op amp may be powered from the +V rail alone. If it
is powered from +V then the input common-mode range
should include ground to accommodate the minimum output
voltage of the DAC, and the output voltage should swing below
0.6 V to ensure that the transistor can be turned fully off.
5. In all these circuits, the output transistor must have an ICMAX
greater than the maximum fan current, and be capable of dis-
sipating power due to the voltage dropped across it when the
fan is not operating at full speed.
6. If the fan motor produces a large back ElectroMotive Force
(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.
7. Pulling FAN_SPD/NTEST_IN high externally on power-up
causes NAND Test Mode to be invoked on the ADM1022.
Therefore, a 4.7 kW pull-down resistor should be added
externally to the FAN_SPD pin to prevent ADM1022 inad-
vertently entering the NAND Tree Test Mode.
Figure 5c shows how the FAN_OFF signal may be used (with
any of the control circuits) to gate the fan on and off indepen-
dent of the value on the FAN_SPD/NTEST_IN pin.
5V
FAN_SPD
AD8541
+
R1
10k
R2
15k
Q1
NDT452 P
5V
FAN
Figure 5a. 5 V Fan Circuit with Op Amp
FAN_SPD
ADM1022
12V
AD8519
+
R4
1k
R3
1k
R1
10k
R2
39k
Q1
BD136
2SA968
Figure 5b. 12 V Fan Circuit with Op Amp and PNP
Transistor
12V
FAN_SPD
AD8519
+
R3
100k
R2
39k
3.3V
R1
10k
R4
1k
FAN_OFF
Q1
NDT452 P
Q2
NDT3055L
Figure 5c. 12 V Fan Circuit with Op Amp and P-Channel
MOSFET
12V
R3
100k
R4
100k
Q3
NDT452 P
FAN_SPD
R5
5k
Q1/Q2
MBT3904
DUAL
R2
3.9k
R1
1k
Figure 5d. Discrete 12 V Fan Drive Circuit with
P-Channel MOSFET, Single Supply
12V
R5
100k
R4
100k
Q3
BC556
2N3906
FAN_SPD
R6
5k
Q1/Q2
MBT3904
DUAL
R3
100
R2
3.9k
R1
1k
Q4
BD132
TIP32A
Figure 5e. Discrete 12 V Fan Drive Circuit with Bipolar
Output Single Supply
REV. B
–11–
 

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