8
Remote/Local Temperature Sensor
with SMBus Serial Interface
MAX1617
Maxim Integrated
A/D Conversion Sequence
If a Start command is written (or generated automatical-
ly in the free-running auto-convert mode), both channels
are converted, and the results of both measurements
are available after the end of conversion. A BUSY status
bit in the status byte shows that the device 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-qual-
ity, diode-connected small-signal transistor. See Table 1
for a recommended list of diode-connected small-signal
transistors. The MAX1617 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 (V
DXP
- V
DXN
) must be less than 0.95V
at 100µA; additionally, ensure the maximum V
DXP
(DXP
voltage) ≤ (0.78 x V
CC
- 1.1) volts over your expected
range of 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.
For heat-sink mounting, the 500-32BT02-000 thermal
sensor from Fenwal Electronics is a good choice. This
device consists of a diode-connected transistor, an
aluminum plate with screw hole, and twisted-pair cable
(Fenwal Inc., Milford, MA, 508-478-6000).
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the MAX1617’s
effective accuracy. The thermal time constant of the
QSOP-16 package is about 140sec in still air. For the
MAX1617 junction temperature to settle to within +1°C
after a sudden +100°C change requires about five time
constants or 12 minutes. The use of smaller packages
for remote sensors, such as SOT23s, improves the situ-
ation. Take care to account for thermal gradients
between 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 cur-
rent at the ALERT output. For example, at an 8Hz rate
and with ALERT sinking 1mA, the typical power dissi-
pation is V
CC
x 450µA plus 0.4V x 1mA. Package theta
J-A is about 150°C/W, so with V
CC
= 5V and no copper
PCB heat-sinking, the resulting temperature rise is:
dT = 2.7mW x 150°C/W = 0.4°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals such
as 60Hz/120Hz power-supply hum. Micropower opera-
tion places constraints on high-frequency noise rejec-
tion; therefore, careful PCB layout and proper external
noise filtering are required for high-accuracy remote
measurements 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
capacitance. Higher capacitance than 3300pF intro-
duces errors due to the rise time of the switched cur-
rent source.
Nearly all noise sources tested cause the ADC measure-
ments to be higher than the actual temperature, typically
by +1°C to +10°C, depending on the frequency and
amplitude (see Typical Operating Characteristics).
CMPT3904Central Semiconductor (USA)
MMBT3904Motorola (USA)
MMBT3904
SST3904Rohm Semiconductor (Japan)
KST3904-TFSamsung (Korea)
FMMT3904CT-NDZetex (England)
MANUFACTURER MODEL NUMBER
SMBT3904Siemens (Germany)
Table 1. Remote-Sensor Transistor
Manufacturers
Note: Transistors must be diode-connected (base shorted to
collector).
National Semiconductor (USA)
