MAX6648_08 Datasheet, PDF (14/16 Pages) Maxim Integrated Products – Precision SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms

Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
Functional Diagram
V_CC
MAX6648
MAX6692
DXP
DXN
ALERT
OVERT
MUX
REMOTE
LOCAL
DIODE
FAULT
S
Q
R
S
Q
R
2
ADC
CONTROL
LOGIC
8
8
REGISTER BANK
COMMAND BYTE
REMOTE TEMPERATURE
LOCAL TEMPERATURE
ALERT THRESHOLD
ALERT RESPONSE ADDRESS
OVERT THRESHOLD
SMBus
READ
WRITE
7
ADDRESS
DECODER
SMBDATA
SMBCLK
Thermal Mass and Self-Heating
When sensing local temperature, these devices are
intended to measure the temperature of the PCB to
which they are soldered. The leads provide a good ther-
mal path between the PCB traces and the die. Thermal
conductivity between the die and the ambient air is poor
by comparison, making air temperature measurements
impractical. Because the thermal mass of the PCB is far
greater than that of the MAX6648/MAX6692, the devices
follow temperature changes on the PCB with little or no
perceivable delay.
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, thermal mass has virtu-
ally no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen-
sors, smaller packages, such as SOT23s, yield the best
thermal response times. Take care to account for ther-
mal gradients between the heat source and the sensor,
and ensure that stray air currents across the sensor
package do not interfere with measurement accuracy.
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 current
at the ALERT output. For example, with V_CC = 5.0V, at a
4Hz conversion rate and with ALERT sinking 1mA, the
typical power dissipation is:
5.0V x 500µA + 0.4V x 1mA = 2.9mW
θ_J-A for the 8-pin µMAX package is about +221°C/W,
so assuming no copper PCB heat sinking, the resulting
temperature rise is:
ΔT = 2.9mW x (+221°C/W) = +0.6409°C
Even under nearly worst-case conditions, it is difficult to
introduce a significant self-heating error.
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