Rainbow Electronics MAX6635 - Manual (Page 11)

Temperature Alert
(MAX6634/MAX6635)
ALERT has programmable polarity and two modes:
comparator and interrupt. Polarity and mode are select-
ed through the Configuration register (Table 4). The
ALERT output is open drain.
Interrupt Mode
With
ALERT in interrupt mode, the MAX6634/MAX6635
look for a T HIGH or a T LOW fault. The ALERT pin asserts
an alarm for an undertemperature fault, as well as for
an overtemperature fault. Once either fault has
occurred, it remains active until deasserted by a read
of any register. The device then begins to look for a
temperature change crossing the hysteresis level. The
activation of
ALERT is subject to the depth of the fault
queue.
For example: If T
HIGH
is set to 100°C, T
HYST
is set to
20°C, and the fault queue depth is set to 4, ALERT
does not assert until four consecutive conversions
exceed 100°C. If the temperature is then read through
the I
2 C-compatible interface, ALERT deasserts. ALERT
asserts again when four consecutive conversions are
less than 80°C.
Comparator Mode
In comparator mode,
ALERT is asserted when the num-
ber of consecutive conversions exceeding the value in
the T
HIGH register, or lower than the value in the T LOW
register, is equal to the depth of the fault queue.
ALERT
deasserts when the number of consecutive conversions
less than T HIGH - T HYST or greater than T LOW + T HYST
is equal to the depth of the fault queue.
For example: If T HIGH is set to 100°C, T LOW is set to
80°C, and the fault queue depth is set to four,
ALERT
does not assert until four consecutive conversions
exceed 100°C, or are below 80°C.
ALERT only
deasserts if four consecutive conversions are less than
T
HIGH
- T
HYST
or greater than T
LOW
+ T
HYST
.
Comparator mode allows autonomous clearing of an
ALERT fault without the intervention of a master and is
ideal to use for driving a thermostat (Figure 10).
Overtemperature Alarm (MAX6635)
The MAX6635 also includes an overtemperature output
that is always in comparator mode. Whenever the tem-
perature exceeds a value in the programmable T
MAX
register, OVERT is asserted. OVERT only deasserts
after the temperature drops below T MAX - T HYST . When
the fault queue is activated,
OVERT is subject to that
queue, which sets the number of faults that must occur
before
OVERT asserts or deasserts. This helps prevent
spurious alarms in noisy environments.
Comparator mode also allows autonomous clearing of
an
OVERT fault without the intervention of a master and
thus is ideal to use for driving a cooling fan (Figure 11).
In this application, the polarity of
OVERT is active high.
Shutdown
The MAX6633/MAX6634/MAX6635 feature a shutdown
mode, accessible through the serial interface that
saves power by turning off everything except the POR
MAX6633/MAX6634/MAX6635
12-Bit Plus Sign Temperature Sensors with
SMBus/I 2 C-Compatible Serial Interface
______________________________________________________________________________________ 11
3
8
VCC
+3V TO +5.5V
MAX6634
RELAY
5VDC, 20mA
125VAC, 1A
2N3904
HEATER
SUPPLY
HEATER
4
4k Ω
ALERT
Figure 10. Simple Thermostat
3
10k Ω
8
V CC
+3V TO +5.5V +12V
MAX6635
4
12V 300mA
MOTOR FAN
LOGIC-LEVEL
MOSFET
OVERT
Figure 11. Fan Controller