TMP401
SBOS371A AUGUST 2006 REVISED OCTOBER 2007
www.ti.com
7
SERIES RESISTANCE CANCELLATION
Series resistance in an application circuit that typically
results from printed circuit board (PCB) trace resistance
and remote line length (see Figure 11) is automatically
cancelled by the TMP401, preventing what would
otherwise result in a temperature offset. When using a 5V
supply voltage, a total of up to 3k& of series line resistance
is cancelled by the TMP401, eliminating the need for
additional   characterization   and   temperature   offset
correction. Series line resistance should be limited to
500& total when using a 3.3V supply voltage. See typical
characteristics curves (Figure 4 and Figure 5) for details
on the effect of series resistance and power-supply
voltage on sensed remote temperature error.
DIFFERENTIAL INPUT CAPACITANCE
The TMP401 tolerates differential input capacitance of up
to 1000pF with minimal change in temperature error. The
effect of capacitance on sensed remote temperature error
is shown in Figure 6, Remote Temperature Error vs
Differential Capacitance.
TEMPERATURE MEASUREMENT DATA
Temperature measurement data is taken over a default
range of 0癈 to +127癈 for both local and remote locations.
Measurements from 55癈 to +150癈 can be made both
locally and remotely by reconfiguring the TMP401 for the
extended temperature range. To change the TMP401
configuration   from   the   standard   to   the   extended
temperature range, switch bit 2 (RANGE) of the
Configuration Register from low to high.
Temperature data resulting from conversions within the
default measurement range is represented in binary form,
as shown in Table 1, Standard Binary column. Note that
any temperature below 0癈 results in a data value of zero
(00h). Likewise, temperatures above +127癈 result in a
value of 127 (7Fh). The device can be set to measure over
an extended temperature range by changing bit 2 of the
Configuration Register from low to high. The change in
measurement range and data format from standard binary
to extended binary occurs at the next temperature
conversion.   For   data   captured   in   the   extended
temperature range configuration, an offset of 64 (40h) is
added to the standard binary value, as shown in Table 1,
Extended Binary column. This configuration allows
measurement of temperatures below 0癈. Note that binary
values corresponding to temperatures as low as 64癈,
and as high as +191癈 are possible; however, most
temperature sensing diodes only measure with the range
of 55癈 to +150癈. Additionally, the TMP401 is rated only
for ambient temperatures ranging from 40癈 to +125癈.
Parameters in the Absolute Maximum Ratings table must
be observed.
Table 1. Temperature Data Format
(Local and Remote Temperature High Bytes)
LOCAL/REMOTE TEMPERATURE REGISTER
HIGH BYTE VALUE (+15C RESOLUTION)
STANDARD BINARY
EXTENDED BINARY
TEMP
(5C)
BINARY
HEX
BINARY
HEX
64
0000 0000
00
0000 0000
00
50
0000 0000
00
0000 1110
0E
25
0000 0000
00
0010 0111
27
0
0000 0000
00
0100 0000
40
1
0000 0001
01
0100 0001
41
5
0000 0101
05
0100 0101
45
10
0000 1010
0A
0100 1010
4A
25
0001 1001
19
0101 1001
59
50
0011 0010
32
0111 0010
72
75
0100 1011
4B
1000 1011
8B
100
0110 0100
64
1010 0100
A4
125
0111 1101
7D
1011 1101
BD
127
0111 1111
7F
1011 1111
BF
150
0111 1111
7F
1101 0110
D6
175
0111 1111
7F
1110 1111
EF
191
0111 1111
7F
1111 1111
FF
NOTE: Whenever changing between standard and
extended temperature ranges, be aware that the
temperatures stored in the temperature limit registers are
NOT automatically reformatted to correspond to the new
temperature range format. These temperature limit values
must be reprogrammed in the appropriate binary or
extended binary format.
Both local and remote temperature data use two bytes for
data storage. The high byte stores the temperature with
1癈 resolution. The second or low byte stores the decimal
fraction value of the temperature and allows a higher
measurement resolution; see Table 2. The measurement
resolution for the remote channel is 0.0625癈, and is not
adjustable. The measurement resolution for the local
channel is adjustable; it can be set for 0.5癈, 0.25癈,
0.125癈, or 0.0625癈 by setting the RES1 and RES0 bits
of the Resolution Register; see the Resolution Register
section.