1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
Detailed Description
Table 1. Remote-Sensor Transistor
Manufacturers
The MAX6654 is a temperature sensor that communi-
cates through an SMBus/I2C-compatible interface with a
µP in thermal-management applications. Essentially an
11-bit serial analog-to-digital converter (ADC) with a
sophisticated front end, the MAX6654 measures the
change in diode voltage at different current levels to cal-
culate temperature. It contains a current source, a multi-
plexer, an ADC, an SMBus interface, and associated
control logic (Figure 1). Temperature data from the ADC
is loaded into data registers, where it is automatically
compared with data previously stored in four
over/undertemperature alarm registers.
MANUFACTURER
Central Semiconductor (USA)
Fairchild Semiconductor (USA)
ON Semiconductor (USA)
Rohm Semiconductor (USA)
Samsung (Korea)
MODEL NUMBER
CMPT3904
2N3904, 2N3906
2N3904, 2N3906
SST3904
KST3904-TF
Siemens (Germany)
SMBT3904
Zetex (England)
FMMT3904CT-ND
Note: Transistors must be diode connected (base shorted to
collector).
ADC and Multiplexer
The ADC is an averaging type that integrates over a
60ms period (each channel, typically, in the 8-bit “lega-
cy” mode), with excellent noise rejection.
Remote-Diode Selection
The MAX6654 can directly measure the die tempera-
ture of CPUs and other ICs having on-board tempera-
ture-sensing diodes as shown in the Typical Operating
Circuit, or it can measure the temperature of a discrete
diode-connected transistor. For best accuracy, the dis-
crete transistor should be a small-signal device with its
collector and base connected together. Accuracy has
been experimentally verified for all of the devices listed
in Table 1.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure their forward voltages and
compute their temperatures. Both channels are auto-
matically converted once the conversion process has
started, either in free-running or single-shot mode. If
one of the two channels is not used, the device still per-
forms both measurements, and the user can ignore the
results of the unused channel. If the remote-diode
channel is unused, connect DXP to DXN rather than
leave the pins open.
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 >0.28V at 10µA; check to ensure this is true at
the highest expected temperature. The forward voltage
must be <0.9V at 100µA; check to ensure this is true at
the lowest expected temperature. Large power transis-
tors don’t work at all. Also, ensure that the base resis-
tance is <100Ω. Tight specifications for forward-current
gain (+50 to +150, for example) indicate that the manu-
facturer has good process controls and that the
devices have consistent VBE characteristics.
The DXN input is biased at 1V
above ground by an
BE
internal diode to set up the ADC inputs for a differential
measurement. The worst-case DXP-DXN differential
input voltage range is 0.28V to 0.9V.
Excess resistance in series with the remote diode caus-
es about +1/21C error per ohm when the parasitic resis-
tance cancellation mode is not being used. When the
parasitic resistance cancellation mode is being used,
excess resistance of up to 100Ω will not cause any dis-
cernable error. A 200µV offset voltage forced on DXP-
DXN causes about 11C error.
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 alu-
minum plate with screw hole, and twisted-pair cable
(Fenwal Inc., Milford, MA, 508-478-6000).
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 byte shows that the
device is actually performing a new conversion; howev-
er, even if the ADC is busy, the results of the previous
conversion are always available.
Thermal Mass and Self-Heating
Thermal mass can significantly affect the time required
for a temperature sensor to respond to a sudden
change in temperature. The thermal time constant of
the 16-pin QSOP package is about 140s in still air. For
the junction temperature of a MAX6654 in still air to set-
tle to within +11C after a sudden +1001C change in air
temperature, about five time constants or 12 minutes
are required. However, the MAX6654 is not intended to
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