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G768B Schematic ( PDF Datasheet ) - ETC

Teilenummer G768B
Beschreibung Remote/Local Temperature Sensor/ 2 Fan Controllers with SMBus Serial Interface and System Reset Circuit
Hersteller ETC
Logo ETC Logo 




Gesamt 15 Seiten
G768B Datasheet, Funktion
Global Mixed-mode Technology Inc.
G768B
Remote/Local Temperature Sensor, 2 Fan Controllers
with SMBus Serial Interface and System Reset Circuit
Features
„Measures Two Remote and One Local
Temperatures
„No Calibration Required
„SMBus 2-Wire Serial Interface
„Programmable Under/Over-temperature
Alarms
„Supports SMBus Alert Response
„Accuracy: ±5°C (-40°C to + 125°C, remote)
±3°C (+60°C to + 100°C, remote)
„+4.5V to +5.5V Supply Range
„Constant Fan Speed Control
„Built-in MOSFET switch
„Internal short-circuit protection
„PWM control for stable operation
„Watchdog for fan control
„Precision Monitoring of 5V Power-Supply
Voltage
„140ms Min Power-On Reset Pulse Width
RESET Output
„Guaranteed RESET Valid to VCC=1V
„Power Supply Transient Immunity
„No External Components needed for reset
function
„Small, 16-Pin SSOP Package
Applications
„Desktop and Notebook
„Central Office Computers
„Telecom Equipment
„Smart Battery Packs
„Test and Measurement
„LAN Servers
„Multi-Chip Modules
„Industrial Controls
Pin Configuration
G768B
OUT1 1
Vcc 2
DXP1 3
DXN 4
DXP2 5
RESET 6
GND 7
GND 8
16 OUT2
15 Vcc
14 SMBCLK
13 FG2
12 SMBDATA
11 ALERT
10 FG1
9 CLK
16Pin SSOP
Ver 1.3
Oct 28, 2002
General Description
The G768B contains a precise digital thermometer, 2
fan controllers, and a system-reset circuit.
The G768B is backward compatible with G768, Except
that there is an additional watchdog function. This
function prevents fan from being out of control when
system fails.
The thermometer reports the temperature of 2 remote
sensors and its own package. The remote sensors are
diode-connected transistors typically a low-cost, easily
mounted 2N3904 NPN type that replace conventional
thermistors or thermocouples. Remote accuracy is ±5°C
for multiple transistor manufacturers, with no calibration
needed. The remote channel can also measure the die
temperature of other ICs, such as microprocessors, that
contain an on-chip, diode-connected transistor.
The 2-wire serial interface accepts standard System
Management Bus (SMBusTM) Write Byte, Read Byte,
Send Byte, and Receive Byte commands to program
the alarm thresholds and to read temperature data.
The data format is 7 bits plus sign, with each bit cor-
responding to 1°C, in two’s-complement format.
Measurements can be done automatically and
autonomously, with the conversion rate programmed
by the user or programmed to operate in a single-shot
mode. The adjustable rate allows the user to control
the supply-current drain.
G768B also contains a 2-channel fan speed controller.
It connects directly to the fans and performs
closed-loop control of the fan speed independently.
The only external component required is a 10µF ca-
pacitor per channel. It determines the current fan
speed based on the fan rotation pulses and an exter-
nally supplied clock. (To be continued)
Ordering Information
PART*
G768B
TEMP. RANGE
-10°C to +85°C
PIN-PACKAGE
16SSOP
TEL: 886-3-5788833
http://www.gmt.com.tw
1






G768B Datasheet, Funktion
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 100. 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
6

6 Page









G768B pdf, datenblatt
Global Mixed-mode Technology Inc.
G768B
Command Byte Functions
The 8-bit command byte register (Table 4) is the mas-
ter index that points to the various other registers
within the G768B. The register's POR state is 0000
0000, so that a Receive Byte transmission (a protocol
that lacks the command byte) that occurs immediately
after POR returns the current local temperature data.
The one-shot command immediately forces a new
conversion cycle to begin. In software standby mode
( RUN /STOP bit = high), a new conversion is begun,
after which the device returns to standby mode. If a
conversion is in progress when a one-shot command
is received in auto-convert mode (RUN/STOP bit = low)
between conversions, a new conversion begins, the
conversion rate timer is reset, and the next automatic
conversion takes place after a full delay elapses.
Thermal Configuration Byte Functions
The configuration byte register (Table 5) is used to
mask (disable) interrupts and to put the device in
software standby mode. The lower six bits are inter-
nally set to (XX1111), making them "don't care" bits.
Write
zeros to these bits. This register's contents can be
read back over the serial interface.
Thermal Status Byte Functions
The thermal status byte register (02h) (Table 6) indi-
cates which (if any) temperature thresholds have been
exceeded. This byte also indicates whether or not the
ADC is converting and whether there is an open circuit
in the remote diode DXPx-DXN path. After POR, the
normal state of all the flag bits is zero, assuming none
of the alarm conditions are present. The status byte is
cleared by any successful read of the status, unless
the fault persists. Note that the ALERT interrupt latch
is not automatically cleared when the status flag bit is
cleared.
When reading the status byte, you must check for in-
ternal bus collisions caused by asynchronous ADC
timing, or else disable the ADC prior to reading the
status byte (via the RUN/STOP bit in the configuration
byte). In one-shot mode, read the status byte only af-
ter the conversion is complete, which is 150ms max
after the one-shot conversion is commanded.
Table 5. Configuration-Byte Bit Assignments
BIT NAME POR STATE
FUNCTION
7 (MSB) MASK
0 Masks all ALERT interrupts when high.
6 RUN / STOP
0
Standby mode control bit. If high, the device immediately stops converting and enters
standby mode. If low, the device converts in either one-shot or timer mode.
5-0 RFU
0 Reserved for future use
Table 6. Status-Byte Bit Assignments
BIT NAME
FUNCTION
7(MSB)
BUSY
A high indicates that the ADC is busy converting.
6
RHIGH2*
A high indicates that the 2nd diode high-temperature alarm has activated.
5
RLOW2*
A high indicates that the 2nd diode low-temperature alarm has activated.
4
RHIGH1*
A high indicates that the 1st diode high-temperature alarm has activated.
3
RLOW1*
A high indicates that the 1st diode low-temperature alarm has activated.
2
OPEN*
A high indicates a remote-diode continuity (open-circuit) fault.
1
RFU
Reserved for future use (returns 0)
0(LSB)
RFU
Reserved for future use (returns 0)
*These flags stay high until cleared by POR, or until the status byte register is read.
Table 7. Conversion-Rate Control Byte
DATA
00h
01h
02h
03h
04h
05h
06h
07h
08h to FFh
CONVERSION
RATE (Hz)
0.0625
0.125
0.25
0.5
1
2
4
8
RFU
Ver: 1.3
Oct 28, 2002
Temperature Sensor Average
Supply Current (µA TYP, at Vcc = 5V)
30
33
35
48
70
128
225
425
-
TEL: 886-3-5788833
http://www.gmt.com.tw
12

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