Rainbow Electronics MAX6660 - Manual (Page 7)

MAX6660
Remote-Junction Temperature-Controlled
Fan-Speed Regulator with SMBus Interface
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ly measure the die temperature of CPUs and other ICs
that have 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
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 100
Ω. Tight specifications for
forward current gain (50 <
β <150, for example) indi-
cate that the manufacturer has good process controls
and that the devices have consistent VBE 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 individually calibrate the MAX6660 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, 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 (e.g., 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.
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 operation
places constraints on high-frequency noise rejection;
therefore, careful PC board layout and proper external
noise filtering are required for high-accuracy remote mea-
surements 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 rise time of the switched current
source. Nearly all noise sources tested cause the ADC
measurements to be higher than the actual tempera-
ture, typically by +1°C to +10°C, depending on the fre-
quency and amplitude.
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6660 as close as is 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 +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 high-
er voltage traces, such as +12VDC. Leakage cur-
rents from PC board contamination must be dealt
with carefully since a 20M
Ω leakage path from
DXP to ground causes about +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.
5) When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. A copper-solder thermocouple
exhibits 3µV/°C, and it takes about 200µV of voltage
error at DXP-DXN to cause a +1°C measurement
error. Adding a few thermocouples causes a negligi-
ble error.
6) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10mil widths
and spacings that are recommended in Figure 2 are
not absolutely necessary, as they offer only a minor
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 Transistor
Note: Transistors must be diode connected (base shorted to
collector).