Alarm Threshold Registers
Seventeen registers store ALARM and OVERT thresh-
old data. The MAX6655/MAX6656 contain three regis-
ters for high-temperature (T
HIGH
), three for low-
temperature (T
LOW
), four for high-voltage (V
HIGH
), four
for low-voltage (V
LOW
) thresholds, and three more reg-
isters store OVERT data. If a measured temperature or
voltage exceeds the corresponding alarm threshold
value, an ALARM interrupt is asserted. OVERT asserts
when temperature exceeds the corresponding alarm
threshold value. The POR state of the T
HIGH
register is
full scale (0111 1111 or +127°C). The POR state of the
T
LOW
register is 1100 1001 or -55°C.
Configuration Byte Functions
Configuration Bytes 1 and 2 (Tables 7 and 8) are used
to mask (disable) interrupts, disable temperature and
voltage measurements, and put the device in software
standby mode. The serial interface can read back the
contents of these registers.
Status Byte Functions
The two Status Byte registers (Tables 9 and 10) indi-
cate which (if any) temperature or voltage thresholds
have been exceeded. Status Byte 1 also indicates
whether the ADC is converting and whether there is a
fault in the remote-diode DXP-DXN path. After POR, the
normal state of all the flag bits is zero, except the MSB,
assuming none of the alarm conditions are present. The
MSB toggles between 1 and 0 indicating whether the
ADC is converting or not. A Status Byte is cleared by
any successful read of that Status Byte. Note that the
ALERT interrupt latch clears when the status flag bit is
read, but immediately asserts after the next conversion
if the fault condition persists.
High and low alarm conditions can exist at the same time
in the Status Byte because the MAX6655/MAX6656 are
correctly reporting environmental changes.
Applications Information
Remote-Diode Selection
Remote temperature accuracy depends on having a
good-quality, diode-connected transistor. See Table 11
for appropriate discrete transistors. The MAX6655/
MAX6656 can directly measure the die temperature of
CPUs and other ICs with on-board temperature-sensing
transistors.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the ADC input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process controls and that the devices
have consistent V
BE
characteristics. Do not use power
transistors.
Self-Heating
Thermal mass can significantly affect the time required
for a temperature sensor to respond to a sudden
change in temperature. The thermal time constant of
the 16-pin QSOP package is about 140s in still air.
When measuring local temperature, it senses the tem-
perature of the PC board to which it is soldered. The
leads provide a good thermal path between the PC
board traces and the MAX6655/MAX6656 die. Thermal
conductivity between the MAX6655/MAX6656 die and
the ambient air is poor by comparison. Because the
thermal mass of the PC board is far greater than that of
the MAX6655/MAX6656, the device follows temperature
changes on the PC board with little or no perceivable
delay.
When measuring temperature with discrete remote sen-
sors, the use of smaller packages, such as a SOT23,
yields the best thermal response time. Take care to
account for thermal gradients between the heat source
and the sensor, and ensure that stray air currents
across the sensor package do not interfere with mea-
surement accuracy. When measuring the temperature
of a CPU or other IC with an on-chip sense junction,
thermal mass has virtually no effect; the measured tem-
perature of the junction tracks the actual temperature
within a conversion cycle.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, at the minimum
delay between conversions, and with ALERT sinking
1mA, the typical power dissipation is V
CC
x 550µA +
0.4V x 1mA. Package θ
JA
is about 150°C/W, so with
V
CC
= +5V and no copper PC board heat sinking, the
resulting temperature rise is:
∆T = 3.1mW x 150°C/W = +0.46°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
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
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
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