ADT7476AARQZ-R datasheet(29/69 Pages) ONSEMI

29 / 69 page

ADT7476A

www.onsemi.com

Laying Out 3-Wire Fans

Figure 40 shows how to lay out a common circuit

arrangement for 3-wire fans.

Figure 40. Planning for 3-wire Fans on a PCB

MMBT2222

TACH

12 V or 5 V

1N4148

PWM

3.3 V or 5 V

TACH Inputs

Pin 9, Pin 11, Pin 12, and Pin 14 (when configured as

TACH inputs) are high impedance inputs intended for fan

speed measurement.

Signal conditioning in the ADT7476A accommodates the

slow rise and fall times typical of fan tachometer outputs.

The maximum input signal range is 0 V to 5.5 V, even

though VCC is 3.3 V. In the event that these inputs are

supplied from fan outputs that exceed 0 V to 5.5 V, either

resistive attenuation of the fan signal or diode clamping

must be included to keep inputs within an acceptable range.

Figure 41 to Figure 44 show circuits for most common fan

TACH outputs.

If the fan TACH output has a resistive pullup to VCC, it can

be connected directly to the fan input, as shown in Figure 41.

Figure 41. Fan with TACH Pullup to VCC

12 V

VCC

FAN SPEED

COUNTER

TACH

OUTPUT

TACH

PULLUP

4.7 k

TYP

ADT7476A

If the fan output has a resistive pullup to 12 V, or other

voltage greater than 5.5 V, the fan output can be clamped

with a Zener diode, as shown in Figure 42. The Zener diode

voltage should be chosen so that it is greater than VIH of the

TACH input but less than 5.5 V, allowing for the voltage

tolerance of the Zener. A value between 5.0 V and 5.5 V is

suitable.

Figure 42. Fan with Strong TACH Pullup to > 5.5 V,

(for Example, 12 V) Clamped with Zener Diode

12 V

VCC

FAN SPEED

COUNTER

TACH

OUTPUT

TACH

PULLUP

4.7 k

TYP

ADT7476A

ZD1*

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× VCC

If the fan has a strong pullup (less than 1 k

W) to 12 V or

a totem-pole output, a series resistor can be added to limit the

Zener current, as shown in Figure 43.

Figure 43. Fan with Strong TACH. Pullup to > VCC

or Totem-Pole Output, Clamped with

Zener Diode and Resistor

5 V or12 V

VCC

FAN SPEED

COUNTER

TACH

ADT7476A

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× VCC

PULLUP

TYP < 1 k

OR TOTEM-POLE

ZD1*

ZENER

TACH

OUTPUT

FAN

10 k

Alternatively, a resistive attenuator can be used, as shown

in Figure 44. R1 and R2 should be chosen such that:

(eq. 4)

2 V

t V

PULLUP

R2 R

PULLUP ) R1 ) R2 t 5.5 V

The fan inputs have an input resistance of nominally

160 k

W to ground, which should be taken into account when

calculating resistor values.

With a pullup voltage of 12 V and pullup resistor less than

1 k

W, suitable values for R1 and R2 are 100 kW and 40 kW,

respectively. This gives a high input voltage of 3.42 V.

Figure 44. Fan with Strong TACH. Pullup to > VCC or

Totem-Pole Output, Attenuated with R1/R2

12 V

VCC

FAN SPEED

COUNTER

TACH

ADT7476A

*SEE TEXT

< 1 k

R1*

R2*

TACH

OUTPUT

ADT7476AARQZ-R Datasheet(PDF) 29 Page - ON Semiconductor

ADT7476AARQZ-R Datasheet(HTML) 29 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69

Datasheet Download

Link URL