1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
minor improvement in leakage and noise), but try to
Hardware and software standby modes behave almost
identically; all data is retained in memory, and the SMB
interface is alive and listening for reads and writes. The
only difference is that in hardware standby mode, the
one-shot command does not initiate a conversion.
use them where practical.
8) Keep in mind that copper can’t be used as an EMI
shield, and only ferrous materials such as steel
work well. Placing a copper ground plane between
the DXP-DXN traces and traces carrying high-fre-
quency noise signals does not help reduce EMI.
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 MAX6654 can be forced to perform A/D conver-
sions through the one-shot command, despite the
RUN/STOP bit being high.
PC Board Layout Checklist
Place the MAX6654 close to the remote-sense junc-
tion.
•
•
•
•
•
•
Keep traces away from high voltages (+12V bus).
Keep traces away from fast data buses and CRTs.
Use recommended trace widths and spacings.
Place a ground plane under the traces.
Activate hardware standby mode by forcing the STBY
pin low. In a notebook computer, this line may be con-
nected to the system SUSTAT# suspend-state signal.
The STBY pin low state overrides any software conver-
sion command. If a hardware or software standby com-
mand is received while a conversion is in progress, the
conversion cycle is truncated, and the data from that
conversion is not latched into either temperature read-
ing register. The previous data is not changed and
remains available.
Use guard traces flanking DXP and DXN and con-
necting to GND.
•
•
Place the noise filter and the 0.1µF V
capacitors close to the MAX6654.
bypass
CC
Add a 200Ω resistor in series with V
for best
CC
Supply-current drain during the 125ms conversion peri-
od is always about 550µA. Slowing down the conver-
sion rate reduces the average supply current (see
Typical Operating Characteristics). In between conver-
sions, the supply current is about 25µA due to the cur-
rent consumed by the conversion rate timer. In standby
mode, supply current drops to about 3µA. At very low
supply voltages (under the power-on-reset threshold),
the supply current is higher due to the address pin bias
currents. It can be as high as 100µA, depending on
ADD0 and ADD1 settings.
noise filtering (see Typical Operating Circuit).
Twisted-Pair and Shielded Cables
For remote-sensor distances longer than 8in, or in partic-
ularly noisy environments, a twisted pair is recommend-
ed. Its practical length is 6ft to 12ft (typ) before noise
becomes a problem, as tested in a noisy electronics lab-
oratory. For longer distances, the best solution is a
shielded twisted pair like that used for audio micro-
phones. For example, Belden #8451 works well for dis-
tances up to 100ft in a noisy environment. Connect the
twisted pair to DXP and DXN and the shield to GND, and
leave the shield’s remote end unterminated.
SMBus Digital Interface
From a software perspective, the MAX6654 appears as
a set of byte-wide registers that contain temperature
data, alarm threshold values, or control bits. A standard
SMBus 2-wire serial interface is used to read tempera-
ture data and write control bits and alarm threshold
data. The device responds to the same SMBus slave
address for access to all functions.
Excess capacitance at DXN and DXP limits practical
remote-sensor distances (see Typical Operating
Characteristics). 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;
1Ω series resistance introduces about +1/21C error.
Setting bit 4 of the configuration register to 1 invokes
the parasitic resistance cancellation mode. This rejects
external resistance in excess of 100Ω while maintaining
conversion accuracy.
The MAX6654 employs four standard SMBus protocols:
Write Byte, Read Byte, Send Byte, and Receive Byte
(Figures 3, 4, 5). The shorter Receive Byte protocol
allows quicker transfers, provided that the correct data
register was previously selected by a Read Byte
instruction. Use caution with the shorter protocols in
multimaster systems, since a second master could
overwrite the command byte without informing the first
master.
Low-Power Standby Mode
Standby mode disables the ADC and reduces the sup-
ply-current drain to less than 10µA. Enter standby
mode by forcing the STBY/pin low or through the
RUN/STOP bit in the configuration byte register.
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