Rainbow Electronics MAX6649 - Manual (Page 5)

MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
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Detailed Description
The MAX6649 is a temperature sensor designed to
work in conjunction with a microprocessor or other intel-
ligence in thermostatic, process-control, or monitoring
applications. Communication with the MAX6649 occurs
through the SMBus-compatible serial interface and
dedicated alert pins. ALERT asserts if the measured
local or remote temperature is greater than the soft-
ware-programmed ALERT high limit or less than the
ALERT low limit. ALERT also asserts if the remote-sens-
ing diode pins are shorted or unconnected. The
overtemperature alarm, OVERT, asserts if the software-
programmed OVERT limit is exceeded. OVERT can be
connected to fans, a system shutdown, a clock throttle
control, or other thermal-management circuitry.
The MAX6649 converts temperatures to digital data
either at a programmed rate or in single conversions.
Temperature data is represented as 11 bits, with the
LSB equal to 0.125°C. The “main” temperature data reg-
isters (at addresses 00h and 01h) are 8-bit registers that
represent the data as 8 bits with the full-scale reading
indicating the diode fault status (Table 1). The remaining
3 bits of temperature data are available in the “extend-
ed” registers at addresses 11h and 10h (Table 2).
ADC and Multiplexer
The averaging ADC integrates over a 60ms period
(each channel, typically), with excellent noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure each diode’s forward volt-
age and compute the temperature based on this volt-
age. Both channels are automatically converted once
the conversion process has started, either in free-run-
ning or single-shot mode. If one of the two channels is
not used, the device still performs both measurements,
and the user can ignore the results of the unused chan-
nel. If the remote-diode channel is unused, connect
DXP to DXN rather than leaving the pins open.
Pin Description
PIN NAME FUNCTION
1 V
CC
Supply Voltage Input, 3V to 5.5V. Bypass V
CC
to GND with a 0.1µF capacitor. A 200 Ω series
resistor is recommended but not required for additional noise filtering.
2 DXP
Combined Remote-Diode Current Source and A/D Positive Input for Remote-Diode Channel. DO
NOT LEAVE DXP FLOATING; tie DXP to DXN if no remote diode is used. Place a 2200pF
capacitor between DXP and DXN for noise filtering.
3 DXN
Combined Remote-Diode Current Sink and A/D Negative Input. DXN is internally connected to
ground.
4 OVERT
Overtemperature Alert/Interrupt Output, Open Drain. OVERT is logic low when the temperature is
above the software-programmed threshold.
5 GND Ground
6 ALERT
SMBus Alert (Interrupt) Output, Open Drain. ALERT asserts when temperature exceeds user-set
limits (high or low temperature). ALERT stays asserted until acknowledged by either reading the
status register or by successfully responding to an alert response address, provided that the fault
condition no longer exists. See the
ALERT Interrupts section.
7 SDA SMBus Serial-Data Input/Output, Open Drain
8 SCLK SMBus Serial-Clock Input
TEMP ( °C) DIGITAL OUTPUT
145 1001 0001
130 1000 0010
128 1000 0000
25 0001 1001
0 0000 0000
<0 0000 0000
Diode fault
(short or open)
1111 1111
Table 1. Main Temperature Data Register
Format (00h, 01h)