EVAL-ADT7462EBZ ON Semiconductor, EVAL-ADT7462EBZ Datasheet - Page 29

• Manufacturer Part Number: EVAL-ADT7462EBZ
• Description: BOARD EVALUATION FOR ADT7462
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of EVAL-ADT7462EBZ

• Contents: Evaluation Board
• For Use With/related Products: ADT7462
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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.
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
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.
CLOCK
PWM
TACH
Fan speed is measured by the ADT7462, and the result is
To enable continuous measurement for 3−wire fans, set
The method for measuring fan speed for 3−wire fans
1
Figure 47. Fan Speed Measurement
2
3
4
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Driving and Measuring the Speed of Two Fans from
One PWM Output
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.
Driving and Measuring the Speed of One Fan from
One PWM Output
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.
When pulse stretching is enabled, the ADT7462 measures
If four single fans are being controlled and measured by
PWM1
PWM3
Figure 49. Driving and Measuring the Speed on a
Figure 48. Synchronizing Fan PWM Output and
TACH1
TACH5
FAN 1
FAN 5
PWM1
PWM3
TACH1
TACH5
FAN 1
FAN 3
TACH2
TACH6
FAN 2
FAN 6
TACH Inputs
Single Fan
TACH3
TACH7
TACH3
TACH7
FAN 2
FAN 4
FAN 3
FAN 7
PWM2
PWM4
FAN 4
TACH4
TACH8
FAN 8
PWM2
PWM4