ADM1026JSTZ-R7 datasheet(22/55 Pages) ONSEMI

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ADM1026

http://onsemi.com

Figure 37. Fan Drive Circuit with Op Amp and

P-channel MOSFET

IRF9620

DAC

12 V

10k

1/4

LM324

100kW

39kW

Figure 38. Discrete Fan Drive Circuit with P-channel

MOSFET, Single Supply

Q1/Q2

MBT3904

DUAL

12 V

DAC

39k

10k

IRF9620

100kW

Figure 39. Discrete Fan Drive Circuit with P-channel

MOSFET, Dual Supply

Q1/Q2

MBT3904

DUAL

+12 V

DAC

39k

10k

4.7k

–12 V

IRF9620

100kW

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

described below, the four MSBs of this register set the

minimum fan speed.

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 40 shows how a fan may be driven under

PWM control using an N-channel MOSFET.

Figure 40. PWM Fan Drive Circuit Using an

N-channel MOSFET

NDT3055L

PWM

3.3 V

10k TYP

5.0 V OR 12 V

FAN

Automatic Fan Speed Control

The ADM1026 offers a simple method of controlling fan

speed according to temperature without intervention from

the host processor. Monitoring must be enabled by setting

Bit 0 of Configuration Register 1 (Address 00h), to enable

automatic fan speed control. Automatic fan speed control

can be applied to the DAC output, the PWM output, or both,

by setting Bit 5 and/or Bit 6 of Configuration Register 1.

The TMIN registers (Addresses 10h to 12h) contain

minimum temperature values for the three temperature

channels (on-chip sensor and two remote diodes). This is the

temperature at which a fan starts to operate when the

temperature sensed by the controlling sensor exceeds TMIN.

TMIN can be the same or different for all three channels.

TMIN is set by writing a twos complement temperature value

to the TMIN registers. If any sensor channel is not required

for automatic fan speed control, TMIN for that channel

should be set to 127C (01111111).

In automatic fan speed control mode, (as shown Figure 41

and Figure 42) the four MSBs of the DAC control register

(Address 04h) and PWM control register (Address 05h) set

the minimum values for the DAC and PWM outputs. Note

that, if both DAC control and PWM control are enabled

(Bits 5 and 6 of Configuration Register 1 = 1), the four MSBs

of the DAC control register (Address 04h) define the

minimum fan speed values for both the DAC and PWM

outputs. The value in the PWM control register (Address 05h)

has no effect.

Minimum DAC Code DACMIN = 16  D

(eq. 12)

DAC Output Voltage + 2.5

Code

256

Minimum PWM Duty Cycle PWMMIN = 6.67  D

where D is the decimal equivalent of Bits 7 to 4 of the

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