±1°C, SMBus-Compatible Remote/Local Temperature
Sensors with Overtemperature Alarms
Table 1. Remote-Sensor Transistor
MANUFACTURER
Central Semiconductor (USA)
Fairchild Semiconductor (USA)
On Semiconductor (USA)
Rohm Semiconductor (USA)
Samsung (Korea)
Siemens (Germany)
Zetex (England)
MODEL NUMBER
CMPT3904
2N3904, 2N3906
2N3904, 2N3906
SST3904
KST3904-TF
SMBT3904
FMMT3904CT-ND
that the base resistance is less than 100Ω. Tight speci-
fications for forward current gain (50 <
β
< 150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
V
BE
characteristics.
MAX6657/MAX6658/MAX6659
Thermal Mass and Self-Heating
When sensing local temperature, these devices are
intended to measure the temperature of the PC board
to which they are soldered. The leads provide a good
thermal path between the PC board traces and the die.
Thermal conductivity between the die and the ambient
air is poor by comparison, making air temperature mea-
surements impractical. Because the thermal mass of
the PC board is far greater than that of the MAX6657/
MAX6658/MAX6659, the devices follow temperature
changes on the PC board with little or no perceivable
delay.
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, thermal mass has virtu-
ally no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen-
sors, smaller packages (i.e., a SOT23) yield the best
thermal response times. Take care to account for ther-
mal 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 autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the
ALERT
output. For example, with V
CC
=
+5.0V, a 16Hz conversion rate and
ALERT
sinking
1mA, the typical power dissipation is:
V
CC
x 450µA + 0.4V x 1mA = 2.65mW
θ
J-A
for the 8-pin SO package is about +170°C/W, so
assuming no copper PC board heat sinking, the result-
ing temperature rise is:
∆T
= 2.65mW x +170°C/W = +0.45°C
Even under these engineered circumstances, it is diffi-
cult to introduce significant self-heating errors.
Note:
Transistors must be diode connected (base shorted to
collector).
whether they are used or not. The DXN input is biased
at one V
BE
above ground by an internal diode to set up
the ADC inputs for a differential measurement.
Resistance in series with the remote diode causes
about +1/2°C error per ohm.
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 autoconvert mode
(RUN/STOP = 0) or by writing a “one-shot” command,
both channels are converted, and the results of both
measurements are available after the end of conver-
sion. A BUSY status bit in the Status register shows that
the device is actually performing a new conversion. The
results of the previous conversion sequence are still
available when the ADC is busy.
Remote-Diode Selection
The MAX6657/MAX6658/MAX6659 can directly mea-
sure the die temperature of CPUs and other ICs that
have on-board temperature-sensing diodes (see
Typical Operating Circuit)
or they can measure the tem-
perature of a discrete diode-connected transistor. The
type of remote diode used is set by bit 5 of the
Configuration Byte. If bit 5 is set to zero, the remote
sensor is a diode-connected transistor, and if bit 5 is set
to 1, the remote sensor is a substrate or common collec-
tor PNP transistor. For best accuracy, the discrete tran-
sistor should be a small-signal device with its collector
and base connected together. Accuracy has been
experimentally verified for all the devices listed in Table 1.
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 at
the highest expected temperature must be greater than
0.25V at 10µA, and at the lowest expected tempera-
ture, forward voltage must be less than 0.95V at 100µA.
Large power transistors must not be used. Also, ensure
ADC Noise Filtering
The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-
ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea-
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