G768 View Datasheet(PDF) - Global Mixed-mode Technology Inc

Part Name

Description

Manufacturer

G768

Remote/Local Temperature Sensor, 2 Fan Controllers with SMBus Serial Interface and System Reset Circuit

Global Mixed-mode Technology Inc 

Global Mixed-mode Technology Inc.

OUT1

FG1

FAN CONTROL

VCC

CONTROL

LOGIC

FAN CONTROL

SMBUS

REGISTERS

G768B

CLK

OUT2

FG2

SMBCLK

SMBDATA

ALERT

DXP1

DXP2

DXN

INTERNAL GROUND

+

+

MUX

+

+

ADC

Fig 1. Functional Diagram

RESET

CIRCUIT

RESET

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 100Ω. 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).

Ver 1.3

Oct 28, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

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