Rainbow Electronics MAX6670 - Manual (Page 5)

Hysteresis Input
The temperature comparator has hysteresis to prevent
small temperature changes near the threshold temper-
ature from causing the fan to turn on and off repeatedly
over short periods of time. The
FANOUT pin goes
active and powers the fan when the external P-N junc-
tion’s temperature exceeds the factory-programmed
trip temperature. As the cooling fan operates, the cir-
cuit board temperature should decrease, which causes
the external P-N junction’s temperature to decrease.
When the P-N junction’s temperature is equal to the trip
threshold minus the hysteresis, the
FANOUT pin turns
the fan off, removing power from the fan. For the
MAX6670, HYST is a three-level logic input for control-
ling the fan-drive comparator’s hysteresis. Connect
HYST to GND to select 4°C hysteresis, to V
DD to select
12°C hysteresis, or leave floating to select 8°C hystere-
sis. The MAX6668 has a built-in hysteresis of 8°C. This
allows the amount of hysteresis to be matched to the
cooling and noise requirements of the system. Figure 1
shows the temperature trip threshold hysteresis.
Applications Information
Remote-Diode Selection
The MAX6668/MAX6670 directly measure the die tem-
perature of CPUs and other ICs that have on-board
temperature-sensing diodes (see Typical Operating
Circuit) or they can measure the temperature of a dis-
crete diode-connected transistor. For best accuracy,
the discrete transistor should be a small-signal device
with its collector and base connected together. Several
satisfactory discrete sensing transistors are shown in
Table 1.
The sensing transistor must be a small-signal type with
a relatively high forward voltage. Otherwise, the DXP
input voltage range may be violated. The forward volt-
age at the highest expected temperature must be
greater than 0.25V at 10µA, and at the lowest expected
temperature, forward voltage must be less than 0.95V
at 100µA. Do not use large power transistors. Also,
ensure that the base resistance is less than 100
Ω. Tight
specifications for forward current gain (50 < B F < 150,
for example) indicate that the manufacturer has good
process controls and that the transistors have consis-
tent V
BE characteristics.
Noise-Filtering Capacitor
In noisy environments, high-frequency noise can be
attenuated using an external 2200pF capacitor located
at the DXP and DXN pins. Larger capacitor values may
be used for additional filtering, but do not exceed
3300pF; excessive capacitance increases error. Figure
2 shows the recommended DXP/DXN PC traces.
Bypassing and Layout
The location of the remote-sensing junction in the sys-
tem affects the MAX6668/MAX6670s’ operation. When
using a discrete temperature-sensing transistor, place
the sensing junction close to major heat-generating
components, such as a high-speed CPU or a power
device.
To minimize noise and other errors, follow the guide-
lines below:
1) Place the MAX6668/MAX6670 as close as possible
to the remote diode. In a noisy environment, such as
a computer motherboard, this distance can be 10cm
to 20cm (typ) or more as long as the worst noise
sources (such as CRTs, clock generators, memory
buses, and ISA/PCI buses) are avoided. In general,
minimize the distance to the remote-sensing junc-
tion.
2) Do not route the DXP/DXN traces next to the deflec-
tion coils of a CRT. Also, do not route the traces
across a fast memory bus, which can introduce
+30°C error or more, even with good filtering.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any high-
voltage traces, such as +12VDC. Avoid leakage cur-
rents from PC board contamination, since a 20M
Ω
leakage path from DXP to GND causes about +1°C
error.
4) Connect guard traces to GND on either side of the
DXP/DXN traces (Figure 2). With guard traces in
place, routing near high-voltage traces is no longer
an issue.
5) Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects.
6) Use wide traces where possible. Narrow traces are
more inductive and tend to pick up radiated noise.
7) Do not use copper as an EMI shield. Only ferrous
materials such as steel work well. Placing a copper
ground plane between the DXP/DXN traces and
MAX6668/MAX6670
Remote Temperature Switches with Integrated
Fan Controller/Driver
_______________________________________________________________________________________ 5
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
ON Semiconductor (USA) 2N3904, 2N3906
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 1. Remote-Sensor Transistor
Manufacturers