ADT7462ACPZ-R7 datasheet(29/81 Pages) ONSEMI

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ADT7462

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fans are in use, power is continuously applied and removed

from the fan, thereby chopping the TACH information. As

a result, every time a fan speed measurement is to be made,

the fan must be switched on for a long enough period of time

that a measurement can be made. This is called pulse

stretching. With 4−wire fans, power is always applied to the

fan, so fan speed measurements can be made continuously,

and there is no need for pulse stretching. Pulse stretching is

also not necessary when driving a 3−wire fan with a dc input.

The Fan Speed Measurement with Pulse Stretching section,

which describes how fan speed is measured both when pulse

stretching is required and when it is not.

Fan Speed Measurement Without Pulse Stretching

Fan speed is measured by the ADT7462, and the result is

stored in the fan TACH value registers. The fan counter does

not count the fan TACH output pulses directly because the

fan speed can be less than 1000 rpm, and it would take

several seconds to accumulate a reasonably large and

accurate count. Instead, the period of the fan revolution is

measured by gating an on−chip 90 kHz oscillator into the

input of a 16−bit counter for N periods of the fan TACH

output (see Figure 47), so the accumulated count is actually

proportional to the fan tachometer period and inversely

proportional to the fan speed.

Figure 47. Fan Speed Measurement

CLOCK

PWM

TACH

To enable continuous measurement for 3−wire fans, set

the corresponding dc bit for the TACH input in the TACH

configuration register. This bit is set automatically when the

HF PWM is in use with 4−wire fans.

Fan Speed Measurement with Pulse Stretching

The method for measuring fan speed for 3−wire fans

requiring pulse stretching is similar to the method described

in the Fan Speed Measurement without Pulse Stretching

section for continuous measurements. The main difference

is that the PWM drive must be synchronized to the TACH

input so that the ADT7462 knows that pulse stretching is

taking place while the TACH is being measured.

PWM1 is synchronized with TACH1 and TACH2.

PWM2 is synchronized with TACH3 and TACH4.

PWM3 is synchronized with TACH5 and TACH6.

PWM4 is synchronized with TACH7 and TACH8.

Driving and Measuring the Speed of Two Fans from

One PWM Output

When pulse stretching is enabled, the ADT7462 measures

fan speed once a second. The counter then counts up from

the first to the third TACH pulse; this value is stored in the

TACH value register. The PWM drive returns to its previous

programmed value. Each TACH input is synchronized to a

particular PWM output. The PWM and TACH pins must be

connected as shown in Figure 48 to ensure that pulse

stretching is synchronized between the PWM output and the

TACH inputs, and an accurate fan speed measurement is

made on each fan.

Figure 48. Synchronizing Fan PWM Output and

TACH Inputs

PWM1

PWM2

PWM3

PWM4

FAN 1

FAN 2

FAN 3

FAN 4

TACH1

TACH2

TACH3

TACH4

FAN 5

FAN 6

FAN 7

FAN 8

TACH5

TACH6

TACH7

TACH8

Driving and Measuring the Speed of One Fan from

One PWM Output

If four single fans are being controlled and measured by

the ADT7462, the following configuration should be used.

This applies only to 3−wire fans controlled using low

frequency PWM with pulse stretching enabled.

Fan 1 is driven by PWM1 and measured using TACH1.

Fan 2 is driven by PWM2 and measured using TACH3.

Fan 3 is driven by PWM3 and measured using TACH5.

Fan 4 is driven by PWM4 and measured using TACH7.

Figure 49. Driving and Measuring the Speed on a

Single Fan

PWM1

PWM2

PWM3

PWM4

FAN 1

FAN 2

TACH1

TACH3

FAN 3

FAN 4

TACH5

TACH7

ADT7462ACPZ-R7 Datasheet(PDF) 29 Page - ON Semiconductor

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