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ADM1021A Datasheet(PDF) 7 Page - ON Semiconductor

Part No. ADM1021A
Description  Low Cost Microprocessor System Temperature Monitor Microcomputer
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADM1021A Datasheet(HTML) 7 Page - ON Semiconductor

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ADM1021A
http://onsemi.com
7
Functional Description
The ADM1021A contains a two-channel A-to-D
converter with special input-signal conditioning to enable
operation with remote and on-chip diode temperature
sensors. When the ADM1021A is operating normally, the
A-to-D converter operates in 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. These signals are digitized by
the ADC and the results stored in the local and remote
temperature value registers as 8-bit, twos complement
words.
The measurement results are compared with local and
remote, high and low temperature limits, stored in four
on-chip registers. Out-of-limit comparisons generate flags
that are stored in the status register, and one or more
out-of-limit results will cause the ALERT output to pull low.
The limit registers can be programmed and the device
controlled and configured via the serial System
Management Bus (SMBus). The contents of any register can
also be read back via the SMBus.
Control and configuration functions consist of:
Switching the Device between Normal Operation and
Standby Mode
Masking or Enabling the ALERT Output
Selecting the Conversion Rate
On initial powerup, the remote and local temperature
values default to –128C. Since the device normally powers
up converting, a measurement of local and remote
temperature is made, and these values are then stored before
a comparison with the stored limits is made. However, if the
part is powered up in standby mode (STBY pin pulled low),
no new values are written to the register before a comparison
is made. As a result, both RLOW and LLOW are tripped in
the status register, thus generating an ALERT output. This
can be cleared in one of two ways.
1. Change both the local and remote lower limits to
–128C and read the status register (which in turn
clears the ALERT output).
2. Take the part out of standby and read the status
register (which in turn clears the ALERT output).
This works only if the measured values are within
the limit values.
Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, or the
base-emitter voltage of a transistor, operated at constant
current. Unfortunately, this technique requires calibration to
null the effect of the absolute value of VBE, which varies
from device to device.
Figure 13. Input Signal Conditioning
LOW-PASS FILTER
fC = 65 kHz
REMOTE
SENSING
TRANSISTOR
BIAS
DIODE
D+
D−
VDD
IBIAS
IN  I
VOUT+
VOUT−
To ADC
C1*
*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 2.2 nF TYP, 3 nF MAX
The technique used in the ADM1021A is to measure the
change in VBE when the device is operated at two different
currents. This is given by:
(eq. 1)
DVBE + kT q 1n (N)
where:
k is Boltzmann’s constant.
q is the charge on the electron (1.6  10–19 Coulombs).
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 13 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 provided for temperature monitoring on some
microprocessors, but it could be a discrete transistor. If a
discrete transistor is used, the collector will not be grounded
and should be 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. If
the sensor is operating in a noisy environment, one can
optionally be added as a noise filter. Its value is typically
2,200 pF, but it should be no more than 3,000 pF. See the
Layout Considerations section for more information.


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