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參數(shù)資料
| 型號： | G771 |
| 廠商： | Global Mixed-mode Technology Inc. |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | Two Remote Temperature Sensors with SMBus Serial Interface and System Reset Circuit |
| 中文描述： | 兩個遠程溫度傳感器，帶有SMBus串行接口和系統(tǒng)復位電路 |
| 文件頁數(shù)： | 13/15頁 |
| 文件大?。?/td> | 268K |
| 代理商： | G771 |
Ver 0.2 Preliminary
Dec 11, 2001
TEL: 886-3-5788833
http://www.gmt.com.tw
7
G771
Global Mixed-mode Technology Inc.
Keep in mind that copper can't be used as an EMI
shield, and only ferrous materials such as steelwork
will. Placing a copper ground plane between the DXP-
DXN traces and traces carrying high-frequency noise
signals do not help reduce EMI.
PC Board Layout Checklist
Place the G771 close to a remote diode.
Keep traces away from high voltages (+12V bus).
Keep traces away from fast data buses and CRTs.
Use recommended trace widths and spacing.
Place a ground plane under the traces
Use guard traces flanking DXP and DXN and con-
necting to GND.
Route two DXPx-DXN pairs independently
Connect the common DXN as close as possible to
the DXN pin on IC.
Place the noise filter and the 0.1F Vcc bypass
capacitors close to the G771.
Fig 2(a) Connect the common DXN as close as
possible to the DXN pin on IC.
Fig 2 (b) Recommended DXP/DXN PC
Twisted Pair and Shielded Cables
For remote-sensor distances longer than 8 in., or in
particularly noisy environments, a twisted pair is rec-
ommended. Its practical length is 6 feet to 12feet (typi-
cal) before noise becomes a problem, as tested in a
noisy electronics laboratory. For longer distances, the
best solution is a shielded twisted pair like that used
for audio microphones. Connect the twisted pair to
DXP and DXN and the shield to GND, and leave the
shield's remote end unterminated.
Excess capacitance at DX_limits practical remote
sensor distances (see Typical Operating Characteris-
tics), For very long cable runs, the cable's parasitic
capacitance often provides noise filtering, so the
2200pF capacitor can often be removed or reduced in
value. Cable resistance also affects remote-sensor
accuracy; 1
series resistance introduces about + 1°C
error.
Low-Power Standby Mode
Standby mode disables the ADC and reduces the sup-
ply-current drain to less than 10A. Enter standby
mode via the RUN/STOP bit in the configuration byte
register. In standby mode, all data is retained in mem-
ory, and the SMB interface is alive and listening for
reads and writes. This is valid for temperature sensor
only.
Standby mode is not a shutdown mode. With activity
on the SMBus, extra supply current is drawn (see
Typical
Operating
Characteristics).
In
software
standby mode, the G771 can be forced to perform
temperature measurement via the one-shot command,
despite the RUN/STOP bit being high.
Supply-current drain during the 125ms conversion
period is always about 500A. Slowing down the con-
version rate reduces the average supply current (see
Typical Operating Characteristics). In between con-
versions, the instantaneous supply current is about
200A due to the current consumed by the system
resetting circuit.
Reset Immunity Negative-Going VCC Transients
In addition to issuing a reset to the microprocessor (P)
during power-up, power-down, and brownout condi-
tions, the G771 is relatively immune to short duration
negative-going VCC transients (glitches).
Typically, for the G771, a VCC transient that goes
100mV below the reset threshold and lasts 20s or
less will not cause a reset pulse. A 0.1F bypass ca-
pacitor mounted as close as possible to the VCC pin
provides additional transient immunity.
Ensuring a Valid Reset Output Down to VCC = 0V
When VCC falls below 1V, the G771 RESET output
no longer sinks current-it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to RESET can drift to undetermined voltages.
This presents no problem in most applications, since
most P and other circuitry is inoperative with VCC be-
low 1V. However, in applications where RESET
must be valid down to 0V, adding a pull-down resistor
to RESET causes any stray leakage currents to flow
to ground, holding RESET low (Figure 3). R1's value
is not critical; 100k
is large enough not to load
RESET and small enough to pull RESET to ground.
DXP1
DXN
DXP2
DXP1
DXN
G771
DXP2
GND
Chip Boundary
GND
DXP1
DXN
DXP2
DXP1
DXN
G771
DXP2
GND
Chip Boundary
GND
DXP
DXN
GND
10 MILS
MINIMUM
10 MILS
GND
DXP
DXN
GND
10 MILS
MINIMUM
10 MILS
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