Rainbow Electronics MAX6622 - Manual (Page 15)

MAX6622
5-Channel Precision Temperature Monitor
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Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
its collector and base must be connected together.
Table 10 lists examples of discrete transistors that are
appropriate for use with the MAX6622. The transistor
must be a small-signal type with a relatively high for-
ward 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 temperature, the for-
ward voltage must be less than 0.95V at 100µA. Large
power transistors must not be used. Also, ensure that
the base resistance is less than 100Ω. Tight specifica-
tions for forward current gain (50 < ß < 150, for exam-
ple) indicate that the manufacturer has good process
controls and that the devices have consistent V
BE
char-
acteristics. Manufacturers of discrete transistors do not
normally specify or guarantee ideality factor. This is
normally not a problem since good-quality discrete
transistors tend to have ideality factors that fall within a
relatively narrow range. We have observed variations in
remote temperature readings of less than ±2°C with a
variety of discrete transistors. Still, it is good design
practice to verify good consistency of temperature
readings with several discrete transistors from any
manufacturer under consideration.
Unused Diode Channels
If one or more of the remote diode channels is not
needed, the DXP and DXN inputs for that channel
should either be unconnected, or the DXP input should
be connected to V
CC . The status register indicates a
diode "fault" for this channel and the channel is ignored
during the temperature-measurement sequence. It is
also good practice to mask any unused channels
immediately upon power-up by setting the appropriate
bits in the configuration 2 and configuration 3 registers.
This will prevent unused channels from causing ALERT
or OVERT to assert.
Thermal Mass and Self-Heating
When sensing local temperature, the MAX6622 mea-
sures the temperature of the printed-circuit board (PCB)
to which it is soldered. The leads provide a good ther-
mal path between the PCB traces and the die. As with
all IC temperature sensors, thermal conductivity
between the die and the ambient air is poor by compari-
son, making air temperature measurements impractical.
Because the thermal mass of the PCB is far greater than
that of the MAX6622, the device follows temperature
changes on the PCB 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 virtual-
ly no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote tran-
sistors, the best thermal response times are obtained
with transistors in small packages (i.e., SOT23 or
SC70). 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.
ADC Noise Filtering
The integrating ADC has good noise rejection for low-
frequency signals, such as power-supply hum. In envi-
ronments with significant high-frequency EMI, connect
an external 2200pF capacitor between DXP_ and
DXN_. Larger capacitor values can be used for added
filtering, but do not exceed 3300pF because it can
introduce errors due to the rise time of the switched
current source. High-frequency noise reduction is
needed for high-accuracy remote measurements.
Noise can be reduced with careful PCB layout as dis-
cussed in the
PCB Layout section.
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 10. Remote-Sensors Transistor
Manufacturers
Note: Discrete transistors must be diode connected (base
shorted to collector).