Rainbow Electronics MAX1619 - Manual (Page 8)

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. Accuracy
has been experimentally verified for all the devices list-
ed in Table 1. The MAX1619 can also directly measure
the die temperature of CPUs and other integrated cir-
cuits 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 must be less than 0.95V at 100µA;
check to ensure this is true at the lowest expected
temperature. Large power transistors don’t work. Also,
ensure that the base resistance is less than 100
Ω
. Tight
specifications for forward-current gain (+50 to +150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
V
BE characteristics.
For heatsink 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 MAX1619’s
effective accuracy. The thermal time constant of the
QSOP-16 package is about 4sec in still air. To settle to
within +1°C after a sudden +100°C change, the
MAX1619 junction temperature requires about five time
constants. The use of smaller packages for remote sen-
sors, such as SOT23s, improves the situation. Take
care to account for thermal gradients between the heat
source and the sensor, and ensure that stray air cur-
rents 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 and OVERT outputs. For example, at
an 8Hz rate and with ALERT and OVERT each sinking
1mA, the typical power dissipation is:
(V CC )(450µA) + 2(0.4V)(1mA)
Package
θ
JA is about 120°C/W, so with V CC = 5V and
no copper PC board heatsinking, the resulting tempera-
ture rise is:
∆
T = 3.1mW(120°C/W) = 0.36°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 rejection;
therefore, careful PC board 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. Capacitance higher 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 ).
MAX1619
Remote/Local Temperature Sensor with Dual-
Alarm Outputs and SMBus Serial Interface
8 _______________________________________________________________________________________
CMPT3904 Central Semiconductor (USA)
MMBT3904 Fairchild Semiconductor (USA)
SST3904 Rohm Semiconductor (Japan)
FMMT3904CT-ND Zetex (England)
MANUFACTURER MODEL NUMBER
SMBT3904 Siemens (Germany)
Table 1. Remote-Sensor Transistor
Manufacturers
Note: Transistors must be diode-connected (base shorted to
collector).
MMBT3904 Motorola (USA)