Rainbow Electronics MAX6642 - Manual (Page 9)

command byte) that occurs immediately after POR
returns the current local temperature data.
Single-Shot
The single-shot command immediately forces a new
conversion cycle to begin. If the single-shot command
is received while the MAX6642 is in standby mode
(RUN bit = 1), a new conversion begins, after which the
device returns to standby mode. If a single-shot con-
version is in progress when a single-shot command is
received, the command is ignored. If a single-shot
command is received in autonomous mode (RUN bit =
0), the command is ignored.
Configuration Byte Functions
The configuration byte register (Table 4) is a read-write
register with several functions. Bit 7 is used to mask
(disable) interrupts. Bit 6 puts the MAX6642 into stand-
by mode (STOP) or autonomous (RUN) mode. Bit 5 dis-
ables local temperature conversions for faster (8Hz)
remote temperature monitoring. Bit 4 prevents setting
the ALERT output until two consecutive measurements
result in fault conditions.
Status Byte Functions
The status byte register (Table 5) indicates which (if
any) temperature thresholds have been exceeded. This
byte also indicates whether the ADC is converting and
whether there is an open-circuit fault detected on the
external sense junction. After POR, the normal state of
all flag bits is zero, assuming no alarm conditions are
present. The status byte is cleared by any successful
read of the status byte after the overtemperature fault
condition no longer exists.
Slave Addresses
The MAX6642 has eight fixed addresses available.
These are shown in Table 6.
The MAX6642 also responds to the SMBus alert
response slave address (see the Alert Response
Address section).
POR and UVLO
To prevent ambiguous power-supply conditions from
corrupting the data in memory and causing erratic
behavior, a POR voltage detector monitors V
CC
and
clears the memory if V CC falls below 2.1 (typ). When
power is first applied and V CC rises above 2.1 (typ),
the logic blocks begin operating, although reads and
writes at V
CC levels below 3V are not recommended. A
second V CC comparator, the ADC undervoltage lockout
(UVLO) comparator prevents the ADC from converting
until there is sufficient headroom (V
CC = +2.7V typ).
Power-Up Defaults
Power-up defaults include:
• ALERT output is cleared.
• ADC begins autoconverting at a 4Hz rate.
• Command byte is set to 00h to facilitate quick
local Receive Byte queries.
• Local (internal) T
HIGH
limit set to +70°C.
• Remote (external) T HIGH limit set to +120°C.
Applications Information
Remote-Diode Selection
The MAX6642 can directly measure the die temperature
of CPUs and other ICs that have on-board temperature-
sensing diodes (see the Typical Operating Circuit) or
they can measure the temperature of a discrete diode-
connected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6642 is optimized for n
= 1.008, which is the typical value for the Intel Pentium
III. A thermal diode on the substrate of an IC is normally
a PNP with its collector grounded. DXP should be con-
nected to the anode (emitter) and the cathode should
be connected at GND of the MAX6642.
If a sense transistor with an ideality factor other than
1.008 is used, the output data is different from the data
obtained with the optimum ideality factor. Fortunately,
the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL is used to measure the tem-
perature of a diode with a different ideality factor n 1 .
The measured temperature T M can be corrected using:
T T
n
n M ACTUAL
NOMINAL
=





 1
MAX6642
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
_______________________________________________________________________________________ 9
PART NO. SUFFIX ADDRESS
MAX6642ATT90 1001 000
MAX6642ATT92 1001 001
MAX6642ATT94 1001 010
MAX6642ATT96 1001 011
MAX6642ATT98 1001 100
MAX6642ATT9A 1001 101
MAX6642ATT9C 1001 110
MAX6642ATT9E 1001 111
Table 6. Slave Address