ADT7421ARMZ-2RL ON Semiconductor, ADT7421ARMZ-2RL Datasheet - Page 6

• Manufacturer Part Number: ADT7421ARMZ-2RL
• Description: IC TEMP MONITOR 8-MSOP
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of ADT7421ARMZ-2RL

• Input Type: Logic
• Output Type: Logic
• Interface: 2-Wire SMBus
• Current - Supply: 4mA
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Full Temp Accuracy: +/- 2.75 C, +/- 3.25 C
• Digital Output - Bus Interface: Serial (2-Wire)
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Theory of Operation
and over/undertemperature alarm, with the added ability to
automatically cancel the effect of beta variations in
embedded thermal transistors in small geometry CPU’s.
When the ADT7421 is operating normally, the on−board
ADC operates in a free running mode. The analog input
multiplexer
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.
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
transistor 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.
controlled and configured via the serial SMBus. The
contents of any register are also read back via the SMBus.
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.
Beta Variation Cancellation
method which cancels out the effect of varying Beta factors
being observed when different currents are applied to the
embedded thermal transistor in small geometry processes.
This method also ensure consistent and accurate
temperature measurements between CPU’s.
Series Resistance Cancellation
ADT7421, seen in series with the remote transistor, 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 0.5°C offset per
ohm of parasitic resistance in series with the remote
transistor.
series resistance on the temperature reading, giving a more
The ADT7421 is a local and remote temperature sensor
The local and remote measurement results are compared
The limit registers are programmed and the device
Control and configuration functions consist of switching
The ADT7421 includes a new temperature sensing
Parasitic resistance to the D+ and D− inputs to the
The ADT7421 automatically cancels the effect of this
alternately
selects
either
the on−chip
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6
accurate result, without the need for user characterization of
this resistance. The ADT7421 is designed to automatically
cancel typically up to 50 W of resistance. By using an
advanced temperature measurement method, this process is
transparent to the user.
Temperature Measurement Method
the negative temperature coefficient of a transistor,
measuring the base emitter voltage (V
operated at constant current. However, this technique
requires calibration to null the effect of the absolute value of
V
in V
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.
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 transistor
at the D− input. C1 may be added as a noise filter (a
recommended maximum value of 2200 pF).
sensor is switched among three related currents. As shown
in Figure 9, N1 × I and N2 × I are different multiples of the
current, I. The currents through the temperature transistor
are switched between I and N1 × I, giving V
between I and N2 × I, giving DV
calculated using the two DV
also cancels the effect of any series resistance on the
temperature measurement.
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
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 10, 20, and
36 conversions per second, no digital averaging occurs.
temperature sensor are performed in the same manner.
BE
A simple method of measuring temperature is to exploit
The technique used in the ADT7421 measures the change
Figure 9 shows the input signal conditioning used to
To measure DV
The resulting DV
Signal conditioning and measurement of the internal
, which varies from device to device.
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when the device operates at three different currents.
BE
BE
, the operating current through the
waveforms are passed through a
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measurements. This method
BE2
. The temperature is then
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) of a transistor
BE1
; and then
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.