NCT72
http://onsemi.com
7
Theory of Operation
The NCT72 is a local and remote temperature sensor and
over/under temperature alarm, with the added ability to
automatically cancel the effect of 1.5 kW (typical) of
resistance in series with the temperature monitoring diode.
When the NCT72 is operating normally, the on-board ADC
operates in a free running mode. The analog input
multiplexer alternately selects either the on-chip
temperature sensor to measure its local temperature or the
remote temperature sensor. The ADC digitizes these signals
and the results are stored in the local and remote temperature
value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM
temperature
limits, stored in eight on-chip registers. Out-of-limit
comparisons generate flags that are stored in the status
register. A result that exceeds the high temperature limit or
the low temperature limit causes the ALERT
output to
assert. The ALERT
output also asserts if an external diode
fault is detected. Exceeding the THERM
temperature limits
causes the THERM
output to assert low. The ALERT output
can be reprogrammed as a second THERM
output.
The limit registers are programmed and the device
controlled and configured via the serial SMBus. The
contents of any register are also read back via the SMBus.
Control and configuration functions consist of switching
the device between normal operation and standby mode,
selecting the temperature measurement range, masking or
enabling the ALERT
output, switching Pin 6 between
ALERT
and THERM2, and selecting the conversion rate.
Series Resistance Cancellation
Parasitic resistance to the D+ and D− inputs to the NCT72,
seen in series with the remote diode, is caused by a variety
of factors, including PCB track resistance and track length.
This series resistance appears as a temperature offset in the
remote sensor’s temperature measurement. This error
typically causes a 0.5C offset per ohm of parasitic
resistance in series with the remote diode.
The NCT72 automatically cancels the effect of this series
resistance on the temperature reading, giving a more
accurate result, without the need for user characterization of
this resistance. The NCT72 is designed to automatically
cancel typically up to 1.5 kW of resistance. By using an
advanced temperature measurement method, this process is
transparent to the user. This feature permits resistances to be
added to the sensor path to produce a filter, allowing the part
to be used in noisy environments. See the section on Noise
Filtering for more details.
Temperature Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, measuring
the base emitter voltage (V
BE
) of a transistor operated at
constant current. However, this technique requires
calibration to null the effect of the absolute value of V
BE
,
which varies from device to device.
The technique used in the NCT72 measures the change in
V
BE
when the device operates at three different currents.
Previous devices used only two operating currents, but it is
the use of a third current that allows automatic cancellation
of resistances in series with the external temperature sensor.
Figure 14 shows the input signal conditioning used to
measure the output of an external temperature sensor. This
figure shows the external sensor as a substrate transistor, but
it can equally be a discrete transistor. If a discrete transistor
is used, the collector is not grounded but is linked to the base.
To prevent ground noise interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at
the D− input. C1 may be added as a noise filter
(a recommended maximum value of 1,000 pF). However, a
better option in noisy environments is to add a filter, as
described in the Noise Filtering section. See the Layout
Considerations section for more information on C1.
To measure DV
BE
, the operating current through the
sensor is switched among three related currents. As shown
in Figure 14, N1 I and N2 I are different multiples of the
current, I. The currents through the temperature diode are
switched between I and N1 I, giving DV
BE1
; and then
between I and N2 I, giving DV
BE2
. The temperature is
then calculated using the two DV
BE
measurements. This
method also cancels the effect of any series resistance on the
temperature measurement.
The resulting DV
BE
waveforms are passed through a
65 kHz low-pass filter to remove noise and then to a
chopper-stabilized amplifier. This amplifies and rectifies the
waveform to produce a dc voltage proportional to DV
BE
.
The ADC digitizes this voltage producing a temperature
measurement. To reduce the effects of noise, digital filtering
is performed by averaging the results of 16 measurement
cycles for low conversion rates. At rates of 16-, 32-, and
64-conversions/second, no digital averaging occurs.
Signal conditioning and measurement of the internal
temperature sensor are performed in the same manner.
