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PDF G781 Data sheet ( Hoja de datos )
| Número de pieza | G781 | |
| Descripción | 1C Remote and Local Temperature Sensor with SMBus Serial Interface | |
| Fabricantes | ETC | |
| Logotipo |  |
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Global Mixed-mode Technology Inc.
G781
±1°C Remote and Local Temperature Sensor with
SMBus Serial Interface
Features
Two Channels: Measures Both Remote and
Local Temperatures
No Calibration Required
SMBus 2-Wire Serial Interface
Programmable Under/Overtemperature Alarms
Supports SMBus Alert Response
Accuracy:
±1°C (+60°C to +100°C, remote)
±3°C (+60°C to + 100°C, local)
320µA (typ) Average Supply Current During
Conversion
+3V to +5.5V Supply Range
Small 8-Lead SO Package
Applications
Desktop and Notebook
Computers
Smart Battery Packs
LAN Servers
Industrial Controllers
Central Office
Telecom Equipment
Test and Measurement
Multi-Chip Modules
Pin Configuration
General Description
The G781 is a precise digital thermometer that reports
the temperature of both a remote sensor and its own
package. The remote sensor is a diode-connected
transistor typically a low-cost, easily mounted 2N3904
NPN type that replace conventional thermistors or
thermocouples. Remote accuracy is ±1°C with no cali-
bration needed. The remote channel can also meas-
ure the die temperature of other ICs, such as micro-
processors, that contain an on-chip, diode-connected
transistor.
The 2-wire serial interface accepts standard System
Management Bus (SMBus) Write Byte, Read Byte,
Send Byte, and Receive Byte commands to program
the alarm thresholds and to read temperature data.
The data format is 11bits plus sign, with each bit cor-
responding to 0.125°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.
The G781 is available in a small, 8-pin SOP sur-
face-mount package.
Ordering Information
PART*
G781
TEMP. RANGE
-20°C to +120°C
PIN-PACKAGE
8-SOP
Typical Operating Circuit
VCC 1
G781
8 SMBCLK
DXP 2
7 SMBDATA
DXN 3
6 ALERT
THERM 4
5 GND
8 Pin SOP
2N3904
0.1µF
3V TO 5.5V
VCC
DXP
SMBCLK
SMBDATA
DXN
2200pF
ALERT
THERM
GND
10kΩ EACH
CLOCK
DATA
INTERRUPT TO µC
Ver: 1.0
Oct 02, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
1
1 page  
Global Mixed-mode Technology Inc.
G781
A/D Conversion Sequence
If a Start command is written (or generated automati-
cally in the free-running auto-convert mode), both
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. The
G781 can also directly measure the die temperature of
CPUs and other integrated circuits having on-board
temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively 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 expected temperature. The
forward voltage must be less than 0.95V at 300µA;
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 manufac-
turer has good process controls and that the devices
have consistent Vbe characteristics.
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the G781’s ef-
fective accuracy. The thermal time constant of the
SOP- package is about 140 in still air. For the G781
junction temperature to settle to within +1°C after a
sudden +100°C change requires about five time con-
stants or 12 minutes. The use of smaller packages for
remote sensors, such as SOT23s, improves the situa-
tion. Take care to account for thermal gradients be-
tween the heat source and the sensor, and ensure that
stray air currents across the sensor package do not
interfere with measurement accuracy. Self-heating
does not significantly affect measurement accuracy.
Remote-sensor self-heating due to the diode 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. For example, at an 8Hz rate and with
ALERT sinking 1mA, the typical power dissipation is
VCC x 320µA plus 0.4V x 1mA. Package theta J-A is
about 120°C /W, so with VCC = 3.3V and no copper PC
board heat-sinking, the resulting temperature rise is:
dT = 1.45mW x 120°C /W = 0.17°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER
MODEL NUMBER
Philips
PMBS3904
Motorola(USA)
MMBT3904
National Semiconductor (USA) MMBT3904
Note:Transistors must be diode-connected (base
shorted to collector).
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection. Micropower operation places con-
straints on high-frequency noise rejection; therefore,
careful PC board layout and proper external noise fil-
tering are required for high-accuracy remote meas-
urements in electrically noisy environments.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. This value can be
increased to about 3300pF(max), including cable ca-
pacitance. Higher capacitance than 3300pF introduces
errors due to the rise time of the switched current
source.
Nearly all noise sources tested cause the ADC meas-
urements to be higher than the actual temperature,
typically by +1°C to 10°C, depending on the frequency
and amplitude.
PC Board Layout
Place the G781 as close as practical to the remote
diode. In a noisy environment, such as a computer
motherboard, this distance can be 4 in. to 8 in. (typical)
or more as long as the worst noise sources (such as
CRTs, clock generators, memory buses, and ISA/PCI
buses) are avoided.
Ver: 1.0
Oct 02, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
5
5 Page 
Global Mixed-mode Technology Inc.
Table 6. Configuration-Byte Bit Assignments
G781
BIT
7 (MSB)
6
5-0
NAME
MASK
RUN /
STOP
RFU
POR STATE
0
0
0
FUNCTION
Masks all ALERT interrupts when high.
Standby mode control bit. If high, the device immediately stops converting and en-
ters standby mode. If low, the device converts in either one-shot or timer mode.
Reserved for future use
Table 7. Status-Byte Bit Assignments
BIT NAME
FUNCTION
7 (MSB)
BUSY
A high indicates that the ADC is busy converting.
6
LHIGH*
A high indicates that the local high-temperature alarm has activated.
5
LLOW*
A high indicates that the local low-temperature alarm has activated.
4
RHIGH*
A high indicates that the remote high-temperature alarm has activated.
3
RLOW*
A high indicates that the remote low-temperature alarm has activated.
2
OPEN*
A high indicates a remote-diode continuity (open-circuit) fault.
1
RTHRM
A high indicates a remote temperature THERM alarm has activated.
0 (LSB)
LTHRM
A high indicates a local temperature THERM alarm has activated.
*These flags stay high until cleared by POR, or until the status byte register is read.
Table 8. Conversion-Rate Control Byte
DATA
00h
01h
02h
03h
04h
05h
06h
07h
08h
09h to FFh
CONVERSION RATE (Hz)
0.0625
0.125
0.25
0.5
1
2
4
8
16
RFU
To check for internal bus collisions, read the status
byte. If the least significant seven bits are ones, dis-
card the data and read the status byte again. The
status bits LHIGH, LLOW, RHIGH, and RLOW are
refreshed on the SMBus clock edge immediately fol-
lowing the stop condition, so there is no danger of los-
ing temperature-related status data as a result of an
internal bus collision. The OPEN status bit (diode con-
tinuity fault) is only refreshed at the beginning of a
conversion, so OPEN data is lost. The ALERT inter-
rupt latch is independent of the status byte register, so
no false alerts are generated by an internal bus colli-
sion.
When auto-converting, if the THIGH and TLOW limits
are close together, it’s possible for both high-temp and
low-temp status bits to be set, depending on the
amount of time between status read operations (espe-
cially when converting at the fastest rate). In these
circumstances, it’s best not to rely on the status bits to
indicate reversals in long-term temperature changes
and instead use a current temperature reading to es-
tablish the trend direction.
For bit 1 and bit 0, a high indicates a temperature
alarm happened for remote and local diode respec-
tively. THERM pin also asserts. These two bits
wouldn’t be cleared when reading status byte.
Conversion Rate Byte
The conversion rate register (Table 8) programs the
time interval between conversions in free-running
auto-convert mode. This variable rate control reduces
the supply current in portable-equipment applications.
The conversion rate byte’s POR state is 08h (16Hz).
The G781 looks only at the 4 LSB bits of this register,
so the upper 4 bits are “don’t care” bits, which should
be set to zero. The conversion rate tolerance is ±25%
at any rate setting.
Valid A/D conversion results for both channels are
available one total conversion time (125ms,typical)
after initiating a conversion, whether conversion is
initiated via the RUN /STOP bit, one-shot command,
or initial power-up.
Ver: 1.0
Oct 02, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
11
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet G781.PDF ] | |
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