Rainbow Electronics MAX6649 - Manual (Page 11)

MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
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writes at V CC levels below 3V are not recommended. A
second V
CC
comparator, the ADC UVLO comparator,
prevents the ADC from converting until there is suffi-
cient headroom (V
CC = 2.8V typ).
Power-Up Defaults
Power-up defaults include:
• Interrupt latch is cleared.
• ADC begins autoconverting at a 4Hz rate.
• Command byte is set to 00h to facilitate quick local
temperature receive byte queries.
• Local (internal) T HIGH limit is set to +85°C.
• Local (internal) T LOW limit is set to 0°C.
• Remote (external) T
HIGH
limit is set to +85°C.
• Remote (external) T LOW limit is set to 0°C.
• OVERT limit is set to +85°C.
Applications Information
Remote-Diode Selection
The MAX6649 can directly measure the die tempera-
ture of CPUs and other ICs that have on-board temper-
ature-sensing diodes (see Typical Operating Circuit ),
or it 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 MAX6649 is optimized for n =
1.008, which is the typical value for the Intel
® Pentium ®
III and the AMD Athlon MP model 6. If a sense transistor
with a different ideality factor is used, the output data is
different. 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:
Where temperature is measured in Kelvin.
T T
n
n
M ACTUAL
NOMINAL
=






1
DATA
CONVERSION
RATE (Hz)
00h 0.0625
01h 0.125
02h 0.25
03h 0.5
04h 1
05h 2
06h 4
07h 4
08h-FFh Reserved
Table 8. Conversion-Rate Control Byte
(04h)
BIT NAME
POR
STATE
FUNCTION
7 (MSB) BUSY 0 A/D is busy converting when 1.
6 LHIGH 0
Local (internal) high-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
5 LLOW 0
Local (internal) low-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
4 RHIGH 0
Remote (external) high-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
3 RLOW 0
Remote (external) low-temperature alarm has tripped when 1; cleared by POR or readout of the
status byte if the fault condition no longer exists.
2 FAULT 0
A 1 indicates DXN and DXP are either shorted or open; cleared by POR or readout of the status
byte if the fault condition no longer exists.
1 EOT 0 A 1 indicates the remote (external) junction temperature exceeds the external OVERT threshold.
0 IOT 0 A 1 indicates the local (internal) junction temperature exceeds the internal OVERT threshold.
Table 7. Status Register Bit Assignments (02h)
Intel and Pentium are registered trademarks of Intel Corp.