MAX6643/MAX6644/MAX6645
Automatic PWM Fan-Speed Controllers with
Overtemperature Output
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Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3Ω. The series resistance contributes an
offset of:
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.008
and series resistance of 3Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 0.66°C = 0.7°C
for a diode temperature of +60.7°C.
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
For best accuracy, the discrete transistor should be a
small-signal device with its collector connected to
base, and emitter connected to GND. Table 5 lists
examples of discrete transistors that are appropriate for
use with the MAX6643/MAX6644/MAX6645.
The transistor must have a relatively high forward volt-
age; otherwise, the ADC input voltage range can be vio-
lated. The forward voltage at the highest expected
temperature must be greater than 0.25V at 10µA, and at
the lowest expected temperature, the forward voltage
must be less than 0.95V at 100µA. Large power transis-
tors must not be used. Also, ensure that the base resis-
tance is less than 100Ω. Tight specifications for forward
current gain (50 < ß <150, for example) indicate that the
manufacturer has good process controls and that the
devices have consistent V
BE
characteristics.
ADC Noise Filtering
The integrating ADC has inherently good noise rejec-
tion, especially of low-frequency signals such as
60Hz/120Hz power-supply hum. Micropower operation
places constraints on high-frequency noise rejection.
Lay out the PCB carefully with proper external noise fil-
tering for high-accuracy remote measurements in elec-
trically noisy environments.
Filter high-frequency electromagnetic interference
(EMI) at the DXP pins with an external 2200pF capaci-
tor connected between DXP, DXP1, or DXP2 and
ground. This capacitor can be increased to about
3300pF (max), including cable capacitance. A capaci-
tance higher than 3300pF introduces errors due to the
rise time of the switched-current source.
Twisted Pairs and Shielded Cables
For remote-sensor distances longer than 8in, or in par-
ticularly noisy environments, a twisted pair is recom-
mended. Its practical length is 6ft to 12ft (typ) before
noise becomes a problem, as tested in a noisy electron-
ics laboratory. For longer distances, the best solution is
a shielded twisted pair like that used for audio micro-
phones. For example, Belden 8451 works well for dis-
tances up to 100ft in a noisy environment. Connect the
twisted pair to DXP and GND and the shield to ground,
and leave the shield’s remote end unterminated. Excess
capacitance at DXP limits practical remote-sensor dis-
tances (see the Typical Operating Characteristics).
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the recommend-
ed 2200pF capacitor can often be removed or reduced
in value. Cable resistance also affects remote-sensor
accuracy. A 1Ω series resistance introduces about
+1/2°C error.
PCB Layout Checklist
1) Place the MAX6643/MAX6644/MAX6645 as close as
practical to the remote diode. In a noisy environment,
such as a computer motherboard, this distance can
be 4in to 8in or more, as long as the worst noise
sources (such as CRTs, clock generators, memory
buses, and ISA/PCI buses) are avoided.
2) Do not route the DXP lines next to the deflection coils
of a CRT. Also, do not route the traces across a fast
memory bus, which can easily introduce +30°C error,
even with good filtering. Otherwise, most noise
sources are fairly benign.
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