G768B View Datasheet(PDF) - Unspecified
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G768B Datasheet PDF : 15 Pages
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Global Mixed-mode Technology Inc.
G768B
Twisted Pair and Shielded Cables
For remote-sensor distances longer than 8 in., or in
particularly noisy environments, a twisted pair is rec-
ommended. Its practical length is 6 feet to 12feet (typi-
cal) before noise becomes a problem, as tested in a
noisy electronics laboratory. For longer distances, the
best solution is a shielded twisted pair like that used
for audio microphones. Connect the twisted pair to
DXP and DXN and the shield to GND, and leave the
shield's remote end unterminated.
Excess capacitance at DX_limits practical remote
sensor distances (see Typical Operating Characteris-
tics), For very long cable runs, the cable's parasitic
capacitance often provides noise filtering, so the
2200pF capacitor can often be removed or reduced in
value. Cable resistance also affects remote-sensor
accuracy; 1Ω series resistance introduces about + 1°C
error.
Low-Power Standby Mode
Standby mode disables the ADC and reduces the sup-
ply-current drain to less than 10µA. Enter standby
mode via the RUN/STOP bit in the configuration byte
register. In standby mode, all data is retained in mem-
ory, and the SMB interface is alive and listening for
reads and writes. This is valid for temperature sensor
only.
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 2. 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
To stop the fan, program the fan speed register to 255. This
also makes the fan controller into power saving mode.
Standby mode is not a shutdown mode. With activity
on the SMBus, extra supply current is drawn (see
Typical Operating Characteristics). In software
standby mode, the G768B can be forced to perform
temperature measurement via the one-shot command,
despite the RUN/STOP bit being high.
Supply-current drain during the 125ms conversion
period is always about 500µA. Slowing down the con-
version rate reduces the average supply current (see
Typical Operating Characteristics). In between con-
versions, the instantaneous supply current is about
25µA due to the current consumed by the conversion
rate timer. In standby mode, supply current drops to
about 3µA. at very low supply voltages (under the
power-on-reset threshold).
Fan Controller
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:
Controlling Fan at Lower Speed
For stably controlling fans at lower rotataion 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.
Reset Immunity Negative-Going VCC Transients
In addition to issuing a reset to the microprocessor (µP)
during power-up, power-down, and brownout condi-
tions, the G768B is relatively immune to short duration
negative-going VCC transients (glitches).
Typically, for the G768B, a VCC transient that goes
100mV below the reset threshold and lasts 20µs or
less will not cause a reset pulse. A 0.1µF bypass ca-
pacitor mounted as close as possible to the VCC pin
provides additional transient immunity.
Ver 1.3
Oct 28, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
8
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