User guide

The transistor must be a small-signal type with a rela-
tively high forward voltage. Otherwise, the A/D input
range could 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 volt-
age must be less than 0.95V at 100µA. Check to ensure
that this is true at the lowest expected temperature.
Large power transistors, power diodes, or small-signal
diodes must not be used. Also, ensure that the base
resistance is less than 100Ω. Tight specifications for
forward-current gain (50 < ß <150, for example) indi-
cate that the manufacturer has good process controls
and that the devices have consistent V
BE
characteris-
tics. Bits 5–2 of the mode register can be used to
adjust the ADC gain to achieve accurate temperature
measurements with diodes not included in the recom-
mended list or to calibrate individually the MAX6661 for
use in specific control systems.
Thermal Mass and Self-Heating
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, the thermal mass of the
sensor has virtually no effect; the measured tempera-
ture of the junction tracks the actual temperature within
a conversion cycle. When measuring temperature with
discrete remote sensors, smaller packages (e.g., a
SOT23) yield the best thermal response times. 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. Sensor self-heating, caused
by the diode current source, is negligible.
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency noise such
as 60Hz line interference. Micropower operation places
constraints on high-frequency noise rejection; there-
fore, careful PC board layout and proper external noise
filtering are required for high-accuracy remote mea-
surements in electrically noisy environments. High-fre-
quency 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 introduces errors due
to the rise time of the switched current source. Nearly
all noise sources tested cause the ADC measurements
to be higher than the actual temperature, typically by
1°C to 10°C, depending on the frequency and ampli-
tude (see Typical Operating Characteristics).
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6661 as close as practical to the
remote diode. In noisy environments, such as a
computer motherboard, this distance can be 4in to
8in (typ). This length can be increased if the worst
noise sources are avoided. Noise sources include
CRTs, clock generators, memory buses, and
ISA/PCI buses.
2) Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce a 30°C error, even with good filtering.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any higher
voltage traces, such as 12VDC. Leakage currents
from PC board contamination must be dealt with
carefully since a 20MΩ leakage path from DXP to
ground causes about a 1°C error. If high-voltage
traces are unavoidable, connect guard traces to GND
on either side of the DXP-DXN traces (Figure 2).
4) Route through as few vias and crossunders as pos-
sible to minimize copper/solder thermocouple
effects.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
_______________________________________________________________________________________ 7
Note: Transistors must be diode connected (base shorted to
collector).
MANUFACTURER MODEL NO.
Central Semiconductor (USA) 2N3904, 2N3906
Fairchild Semiconductor (USA) 2N3904, 2N3906
Rohm Semiconductor (Japan) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 1. Remote-Sensor Transistors
MINIMUM
10mils
10mils
10mils
10mils
GND
DXN
DXP
GND
Figure 2. Recommended DXP-DXN PC Trace