G760A View Datasheet(PDF) - Unspecified

Part Name

Description

Manufacturer

G760A

Fan Speed PWM Controller

Unspecified 

Global Mixed-mode Technology Inc.

Pin Description

PIN NO.

1

2

3

4

5

6

7

8

PIN NAME

OUT

F.G.

ON/OFF

GND

CLK

SMBCLK

SMBDATA

VCC

PIN FUNCTION

PWM output, connect to fan

FAN pulse Input

ON/OFF control pin, 0V means shutdown

Ground

Clock input for fan speed measurement

SMBus clock line

SMBus data line

Supply voltage

G760A

Description

The fan speed is measured by counting the number of

the CLK pin period between the rising edges of two

fan speed pulses on FG pin. In this way, we are actu-

ally measuring the period of the fan speed. To avoid

the cost of doing division to obtain the speed, this

count number, N, is used in the PWM control algo-

rithm, thus, the desired fan speed should be pro-

grammed by writing the corresponding count number.

The count number is given by:

N = (CLK x 30) / (rpm x P)

N : Count Number

P : FG pulses number per revolution of fan.

For CLK = 32768Hz, P = 2

⇒N = 491520 / rpm

For CLK = 16384Hz, P = 2

⇒N = 245762 / rpm

Some selected count numbers are shown below

Table 1. Count numbers for P=2

rpm

968

1935

2000

3000

4000

5000

6000

7000

8000

9000

10000

20000

30000

CLK=32768Hz

---

254

246

164

123

98

82

70

61

55

49

25

16

CLK=16384Hz

254

127

123

82

61

49

41

35

31

27

25

12

8

When pin 3 (On/Off) is a logical 1, the fan speed will

be controlled according to the setting in the pro-

grammed fan speed register. There are two ways to

stop the fan, (1)setting pin 3 (On/Off) to 0V, (2) pro-

gram the fan speed register to 255. These also makes

the fan controller into power saving mode.

Controlling Fan at Lower Speed

For stably controlling fans at lower rotation speed,

three schemes are recommended as below:

1.Use larger decoupling capacitors between FAN_OUT

and GND.

2.Shunt a capacitor of 1µF-2µF on FG pin to GND.

3.Use fans with open-collector FG outputs.

When controlling fans under lower rotation speed, the

output voltage of FAN_OUT would be too low for fan

to generate recognizable FG signals.

Using decouple capacitors on FAN_OUT and FG is to

increase the SNR on FG pins. While using fans with

open_collector FG outputs can thoroughly solve the

problem, because the logic high level of FG would be

fixed to 5V.

Ver 4.3

Oct 28, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

3

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