ADM1022 datasheet(11/19 Pages) AD

11 / 19 page

REV. 0

ADM1022

–11–

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 14a to 14e. 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 k

Ω pull-down resistor should be added

externally to the FAN_SPD pin to prevent ADM1022 inad-

vertently entering the NAND Tree Test Mode.

Figure 14c 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.

FAN_SPD

NDT452 P

FAN

10k

15k

AD8541

Figure 14a. 5 V Fan Circuit with Op Amp

12V

BD136

2SA968

10k

39k

AD8519

FAN_SPD

Figure 14b. 12 V Fan Circuit with Op Amp and PNP

Transistor

12V

10k

39k

100k

NDT452 P

NDT3055L

3.3V

FAN_OFF

AD8519

FAN_SPD

Figure 14c. 12 V Fan Circuit with Op Amp and P-Channel

MOSFET

12V

NDT452 P

100k

3.9k

100k

FAN_SPD

Q1/Q2

MBT3904

DUAL

Figure 14d. Discrete 12 V Fan Drive Circuit with

P-Channel MOSFET, Single Supply

12V

BD132

TIP32A

100k

FAN_SPD

Q1/Q2

MBT3904

DUAL

3.9k

100

BC556

2N3906

Figure 14e. Discrete 12 V Fan Drive Circuit with Bipolar

Output Single Supply

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