Datasheet

MAX6653/MAX6663/MAX6664
Temperature Monitors and
PWM Fan Controllers
14 ______________________________________________________________________________________
Table 15. Power-On Default Conditions
MAX6653 MAX6663 MAX6664
Temperature Monitoring Monitoring at 4Hz Monitoring at 4Hz Monitoring at 4Hz
PWM Output Low High High
PWM Mode PWM duty cycle control mode PWM duty cycle control mode Automatic fan speed control mode
Duty cycle setting (not
enabled until a 1 is written
to Bit 0 of Register 00h)
33% 100% Automatic
PWM Polarity
Inverted
(100% duty cycle = output high)
Not Inverted
(100% duty cycle = output low)
Not Inverted
(100% duty cycle = output low)
Alarm Speed
For the MAX6663, the alarm speed bit, bit 0 of status
register 1 (02h), indicates that the PWM duty cycle is
100%, excluding the case of fan spin-up. For the
MAX6653/MAX6664, this bit indicates that the THERM
output is low. Once this bit is set, the only way to clear it
is by reading status register 1. However, the bit does
not reassert on the next monitoring cycle if the condi-
tion still exists. It does assert if the condition is discon-
tinued and then returns.
Power-On Default Conditions
At power-up, the MAX6653/MAX6663/MAX6664 are
monitoring temperature to protect the system against
thermal damage. The PWM outputs are in known states.
Note that although the "Monitoring" bit (Configuration
register 1, Bit 0) is enabled, automatic fan speed control
does not begin until a 1 is rewritten to Bit 0.
Other default conditions as listed in the Register Summary
section.
After applying power to the MAX6653/MAX6663/
MAX6664, set the desired operating characteristics (fan
configuration, alarm thresholds, etc.). Write to
Configuration register 1 last. When a 1 is first written to
Bit 0 of this register, fan control will commence as
determined by the register contents.
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6653/MAX6663/MAX6664 as close
as is practical to the remote diode. In noisy environ-
ments, such as a computer motherboard, this dis-
tance 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 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 1°C error. If high-voltage traces
are unavoidable, connect guard traces to GND on
either side of the DXP-DXN traces (Figure 6).
4) The 10-mil widths and spacing recommended in
Figure 6 are not absolutely necessary, as they offer
only a minor improvement in leakage and noise over
narrow traces. Use wider traces when practical.
5) Add a 200Ω resistor in series with VCC for best
noise filtering (see Typical Operating Circuits).
MINIMUM
10 MILS
10 MILS
10 MILS
10 MILS
GND
DXN
DXP
GND
Figure 6. Recommended DXP/DXN PC Traces