MAX6695/MAX6696
Detailed Description
The MAX6695/MAX6696 are temperature sensors
designed to work in conjunction with a microprocessor
or other intelligence in temperature monitoring, protec-
tion, or control applications. Communication with the
MAX6695/MAX6696 occurs through the SMBus serial
interface and dedicated alert pins. The overtempera-
ture alarms OT1 and OT2 are asserted if the software-
programmed temperature thresholds are exceeded.
OT1 and OT2 can be connected to a fan, system shut-
down, or other thermal-management circuitry.
The MAX6695/MAX6696 convert temperatures to digital
data continuously at a programmed rate or by selecting
a single conversion. At the highest conversion rate,
temperature conversion results are stored in the “main”
temperature data registers (at addresses 00h and 01h)
as 7-bit + sign data with the LSB equal to +1°C. At
slower conversion rates, 3 additional bits are available
at addresses 11h and 10h, providing +0.125°C resolu-
tion. See Tables 2, 3, and 4 for data formats.
ADC and Multiplexer
The MAX6695/MAX6696 averaging ADC (Figure 1) inte-
grates over a 62.5ms or 125ms period (each channel,
typ), depending on the conversion rate (see
Electrical
Characteristics
table). The use of an averaging ADC
attains excellent noise rejection.
The MAX6695/MAX6696 multiplexer (Figure 1) automat-
ically steers bias currents through the remote and local
diodes. The ADC and associated circuitry measure
each diode’s forward voltages and compute the tem-
perature based on these voltages. If a remote channel
is not used, connect DXP_ to DXN. Do not leave DXP_
and DXN unconnected. When a conversion is initiated,
all channels are converted whether they are used or
not. The DXN input is biased at one V
BE
above ground
by an internal diode to set up the ADC inputs for a dif-
ferential measurement. Resistance in series with the
remote diode causes about +1/2°C error per ohm.
A/D Conversion Sequence
A conversion sequence consists of a local temperature
measurement and two remote temperature measure-
ments. Each time a conversion begins, whether initiat-
ed automatically in the free-running autoconvert mode
(RUN/STOP = 0) or by writing a one-shot command, all
three channels are converted, and the results of the
three measurements are available after the end of con-
version. Because it is common to require temperature
measurements to be made at a faster rate on one of the
remote channels than on the other two channels, the
conversion sequence is Remote 1, Local, Remote 1,
Remote 2. Therefore, the Remote 1 conversion rate is
double that of the conversion rate for either of the other
two channels.
A BUSY status bit in status register 1 (see Table 7 and
the
Status Byte Functions
section) shows that the
device is actually performing a new conversion. The
results of the previous conversion sequence are always
available when the ADC is busy.
Remote-Diode Selection
The MAX6695/MAX6696 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 dis-
crete 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 MAX6695/MAX6696 (not the
MAX6695Y/MAX6696Y) are optimized for n = 1.008. A
thermal diode on the substrate of an IC is normally a PNP
with its collector grounded. DXP_ must be connected to
the anode (emitter) and DXN must be connected to the
cathode (base) of this PNP.
If a sense transistor with an ideality factor other than
1.008 is used, the output data will be 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 temperature 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 and
n
NOMIMAL
for the MAX6695/MAX6696 is 1.008.
As an example, assume you want to use the MAX6695
or MAX6696 with a CPU that has an ideality factor of
1.002. If the diode has no series resistance, the mea-
sured data is related to the real temperature as follows:
For a real temperature of +85°C (358.15K), the measured
temperature is +82.87°C (356.02K), an error of -2.13°C.
Effect of Series Resistance
Series resistance (R
S
) with a sensing diode contributes
additional error. For nominal diode currents of 10μA
.
.
