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G768D

Two Remote Temperature Sensors and One Fan Controller with SMBus Serial Interface and System Reset Circuit

Global Mixed-mode Technology Inc 

Global Mixed-mode Technology Inc.

G768D

FANVCC

FG

VCC

VCC

CLK

FAN CONTROL

CONTROL

LOGIC

THERMAL SHUTDOWN

LOGIC

SMBUS

REGISTERS

TH_SHUT

SMBCLK

SMBDATA

ALERT

DXP1

DXP2

DXN

+

+ MUX

+

ADC

RESET

CIRCUIT

RESET

INTERNAL GROUND

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), both two

channels are converted, and the results of both 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 G768D can also

directly 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 G768D's

effective accuracy. The thermal time constant of the

SSOP-16 package is about 140sec in still air. For the

G768D 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 sen-

sor ,and ensure that stray air current across the sen-

sor package do not interfere with measurement accu-

racy.

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: 1.2

Apr 03, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

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