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G768B Datasheet(PDF) 6 Page - List of Unclassifed Manufacturers

Part No. G768B
Description  Remote/Local Temperature Sensor, 2 Fan Controllers with SMBus Serial Interface and System Reset Circuit
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G768B Datasheet(HTML) 6 Page - List of Unclassifed Manufacturers

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Ver 1.3
Oct 28, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
6
G768B
Global Mixed-mode Technology Inc.
Fig 1. Functional Diagram
A/D Conversion Sequence
If a Start command is written (or generated automati-
cally in the free-running auto-convert mode), all three
channels are converted, and the results of all meas-
urements are available after the end of conversion. A
BUSY status bit in the status byte shows that the de-
vice is actually performing a new conversion; however,
even if the ADC is busy, the results of the previous
conversion are always available.
Remote-Diode Selection
Temperature
accuracy
depends
on
having
a
good-quality, diode-connected small-signal transistor.
Accuracy has been experimentally verified for all of
the devices listed in Table 1. The G768B can also di-
rectly measure the die temperature of CPUs and other
integrated
circuits
having
on-board
tempera-
ture-sensing diodes. The transistor must be a
small-signal type with a relatively high forward voltage;
otherwise, the A/D input voltage range can be violated.
The forward voltage must be greater than 0.25V at
10µA; check to ensure this is true at the highest ex-
pected temperature. The forward voltage must be less
than 0.95V at 200A; check to ensure this is true at the
lowest expected temperature. Large power transistors
don't work at all. Also, ensure that the base resistance
is less than 100
Ω. Tight specifications for forward
current gain (+50 to +150, for example) indicate that
the manufacturer has good process controls and that
the devices have consistent VBE characteristics.
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the G768B's
effective accuracy. The thermal time constant of the
SSOP-16 package is about 140sec in still air. For the
G768B junction temperature to settle to within +1°C
after a sudden +100°C change requires about five
time constants or 12 minutes. The use of smaller
packages for remote sensors, such as SOT23s, im-
proves the situation. Take care to account for thermal
gradients between the heat source and the sensor,
and ensure that stray air current across the sensor
package do not interfere with measurement accuracy.
Self-heating can significantly affect the measurement
accuracy. Remote-sensor self-heating due to the di-
ode current source is negligible. For the local diode,
the worst-case error occurs when auto converting at
the fastest rate and simultaneously sinking maximum
current at the ALERT output, and both fans are work-
ing at low speeds.
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER
MODEL NUMBER
Philips
PMBS 3904
Motorola(USA)
MMBT3904
National Semiconductor(USA)
MMBT3904
Note:Transistors must be diode-connected (base short
-ed to collector).
DXP2
FAN CONTROL
FAN CONTROL
CONTROL
LOGIC
SMBUS
REGISTERS
MUX
ADC
RESET
CIRCUIT
OUT1
FG1
OUT2
FG2
SMBCLK
SMBDATA
ALERT
RESET
V
CC
+
+
+
DXP1
DXN
+
INTERNAL GROUND
CLK
DXP2
FAN CONTROL
FAN CONTROL
CONTROL
LOGIC
SMBUS
REGISTERS
MUX
ADC
RESET
CIRCUIT
OUT1
FG1
OUT2
FG2
SMBCLK
SMBDATA
ALERT
RESET
V
CC
+
+
+
DXP1
DXN
+
INTERNAL GROUND
CLK


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