Rainbow Electronics MAX6622 - Manual (Page 14)

MAX6622
5-Channel Precision Temperature Monitor
14 ______________________________________________________________________________________
BIT NAME
POR
STATE
FUNCTION
7 (MSB) Reserved 0 —
6 Reserved 0 Not Used. 0 at POR, then 1.
5 Reserved 0 Not Used. 0 at POR, then 1.
4 Diode fault 4 0
Channel 4 Remote-Diode Fault Bit. This bit is set to 1 when DXP4 and DXN4
are open circuit or when DXP4 is connected to V
CC .
3 Diode fault 3 0
Channel 3 Remote-Diode Fault Bit. This bit is set to 1 when DXP3 and DXN3
are open circuit or when DXP3 is connected to V
CC
.
2 Diode fault 2 0
Channel 2 Remote-Diode Fault Bit. This bit is set to 1 when DXP2 and DXN2
are open circuit or when DXP2 is connected to V
CC .
1 Diode fault 1 0
Channel 1 Remote-Diode Fault Bit. This bit is set to 1 when DXP1 and DXN1
are open circuit or when DXP1 is connected to V
CC .
0 Reserved 0 —
Table 9. Status 3 Register
Applications Information
Remote-Diode Selection
The MAX6622 directly measures the die temperature of
CPUs and other ICs that have on-chip temperature-
sensing diodes (see the
Typical Application Circuit ) or
it can measure the temperature of a discrete diode-
connected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measure-
ments depends on the ideality factor (n) of the remote
“diode” (actually a transistor). The MAX6622 is opti-
mized for n = 1.015. A thermal diode on the substrate
of an IC is normally a pnp with the base and emitter
brought out the collector (diode connection) 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 1.015 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 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
NOMINAL
for the MAX6622 is 1.015. As an example,
assume you want to use the MAX6622 with a CPU that
has an ideality factor of 1.002. If the diode has no
series resistance, the measured data is related to the
real temperature as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +80.41°C (353.56K), an error of
-4.587°C.
Series Resistance Cancellation
Some thermal diodes on high-power ICs can have
excessive series resistance, which can cause tempera-
ture measurement errors with conventional remote tem-
perature sensors. Channel 1 of the MAX6622 has a
series resistance cancellation feature (enabled by bit 3
of the configuration 1 register) that eliminates the effect
of diode series resistance. Set bit 3 to 1 if the series
resistance is large enough to affect the accuracy of
channel 1. The series resistance cancellation function
increases the conversion time for channel 1 by 125ms.
This feature cancels the bulk resistance of the sensor
and any other resistance in series (wire, contact resis-
tance, etc.). The cancellation range is from 0 to 100Ω.
T T
n
n
T T
ACTUAL M
NOMINAL
M M
= Ч
⎛
⎝⎜ ⎞
⎠⎟
= Ч ⎛
⎝⎜
⎞
⎠⎟
=
1
1 015
1 002
1 01297
.
.
( . )
T T
n
n
M ACTUAL
NOMINAL
=
⎛
⎝⎜ ⎞
⎠⎟
1