Rainbow Electronics MAX6649 - Manual (Page 6)

MAX6646/MAX6647/MAX6649
The DXP-DXN differential input voltage range is 0.25V to
0.95V. Excess resistance in series with the remote diode
causes +0.5°C (typ) error per ohm.
Remote Temperature Measurement
Range
The MAX6646/MAX6647/MAX6649 measure remote
temperatures significantly above the +120°C limit of
many temperature sensors. External diode-connected
transistors work well as temperature sensors up to
approximately +145°C, where accuracy begins to
degrade. Thermal diodes on some CPUs have charac-
teristics that produce “apparent temperatures” far
above actual operating temperatures. The MAX6646/
MAX6647/MAX6649 measure apparent temperatures as
high as +170°C, as long as the actual temperature is
less than +145°C.
A/D Conversion Sequence
A conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated auto-
matically in the free-running autonomous mode (RUN =
0) or by writing a one-shot command, both channels are
converted, and the results of both measurements are
available after the end of a conversion. A BUSY status bit
in the status byte indicates that the device is performing a
new conversion. The results of the previous conversion
are always available, even if the ADC is busy.
Low-Power Standby Mode
Standby mode reduces the supply current to less than
12µA by disabling the ADC and timing circuitry. Enter
standby mode by setting the RUN bit to 1 in the configu-
ration byte register (Table 6). All data is retained in mem-
ory, and the SMBus interface is active and listening for
SMBus commands. Standby mode is not a shutdown
mode. With activity on the SMBus, the device draws more
supply current (see Typical Operating Characteristics). In
standby mode, the MAX6646/MAX6647/MAX6649 can be
forced to perform A/D conversions through the one-shot
command, regardless of the RUN bit status.
If a standby command is received while a conversion is
in progress, the conversion cycle is truncated, and the
data from that conversion is not latched into a tempera-
ture register. The previous data is not changed and
remains available.
Supply-current drain during the 125ms conversion period
is 250µA (typ). Slowing down the conversion rate reduces
the average supply current (see Typical Operating
Characteristics). Between conversions, the conversion
rate timer consumes 25µA (typ) of supply current. In
standby mode, supply current drops to 3µA (typ).
SMBus Digital Interface
From a software perspective, the MAX6646/MAX6647/
MAX6649 appear as a set of byte-wide registers that
contain temperature data, alarm threshold values, and
control bits. A standard SMBus-compatible 2-wire serial
interface is used to read temperature data and write
control bits and alarm threshold data.
The MAX6646/MAX6647/MAX6649 employ four standard
SMBus protocols: write byte, read byte, send byte, and
receive byte (Figures 1, 2, and 3). The shorter receive
byte protocol allows quicker transfers, provided that the
correct data register was previously selected by a read
byte instruction. Use caution when using the shorter pro-
tocols in multimaster systems, as a second master could
overwrite the command byte without informing the first
master.
Temperature data can be read from the read internal
temperature (00h) and read external temperature (01h)
registers. The temperature data format for these regis-
ters is 8 bits for each channel, with the LSB representing
1°C (Table 1). The MSB is transmitted first.
An additional 3 bits can be read from the read external
extended temperature register (10h), which extends
the data to 11 bits and the resolution to 0.125°C per
LSB. An additional 3 bits can be read from the read
internal extended temperature register (11h), which
extends the data to 11 bits and the resolution to
0.125°C per LSB (Table 2).
When a conversion is complete, the main temperature
register and the extended temperature register are
updated simultaneously. Ensure that no conversions
are completed between reading the main register and
the extended register, so that both registers contain the
result of the same conversion.
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
6 _______________________________________________________________________________________
FRACTIONAL TEMP (°C) DIGITAL OUTPUT
0.000 000X XXXX
0.125 001X XXXX
0.250 010X XXXX
0.375 011X XXXX
0.500 100X XXXX
0.625 101X XXXX
0.750 110X XXXX
0.875 111X XXXX
Table 2. Extended Resolution Temperature
Register Data Format (10h, 11h)