ADT7421ARMZ-2RL ON Semiconductor, ADT7421ARMZ-2RL Datasheet - Page 15

• 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

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cause THERM and THERM2 to operate as described.
Application Information
Remote Sensing Transistor
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types are either
PNP or NPN transistors connected as transistors
(base−shorted to collector). If an NPN transistor is used, the
collector and base are connected to D+ and the emitter to D−.
If a PNP transistor is used, the collector and base are
connected to D− and the emitter to D+. Note that Beta
Cancellation should be turned OFF when using a discrete
transistor. This is done by setting Bit 4 of the Configuration
Register to 1.
discrete transistors, consider several factors:
•
•
Both the external and internal temperature measurements
The ADT7421 is designed to work with substrate
To reduce the error due to variations in both substrate and
When the THERM2 limit is exceeded, the THERM2
signal asserts low.
If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
The THERM output de−asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 15, there is no hysteresis value shown.
As the system cools further, and the temperature falls
below the THERM2 limit, the THERM2 signal resets.
Again, no hysteresis value is shown for THERM2.
The ideally factor, n
the deviation of the thermal transistor from ideal
behavior. The ADT7421 is trimmed for an n
1.008. The following equation may be used to calculate
the error introduced at a temperature, T (°C), when
using a transistor whose n
Consult the processor data sheet for the n
To factor this in, the user writes the DT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7421, I
low level current, I
current levels do not match the current levels specified
by the CPU manufacturer, it may become necessary to
remove an offset. The CPU data sheet should advise
whether this offset needs to be removed and how to
calculate it. This offset is programmed to the offset
register. It is important to note that if more than one
offset must be, considered, the algebraic sum of these
offsets must be programmed to the offset register.
DT + (n
r
* 1.008) 1.008
LOW
F
, of the transistor is a measure of
, is 13.5 mA. If the ADT7421
F
does not equal 1.008.
(273.15 Kelvin ) T)
HIGH
, is 220 mA and the
F
values.
F
value of
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15
accuracy is obtained by choosing devices according to the
following criteria:
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in SOT−23 packages are suitable devices to use.
Thermal Inertia and Self−Heating
sensing transistor and/or the internal temperature sensor
being at the same temperature as that being measured. Many
factors can affect this. Ideally, place the sensor in good
thermal contact with the part of the system being measured.
If it is not, the thermal inertia caused by the sensor’s mass
causes a lag in the response of the sensor to a temperature
change. In the case of the remote sensor, this should not be
a problem since it is either a substrate transistor in the
processor or a small package device, such as the SOT−23,
placed in close proximity to it.
processor and only monitors the general ambient
temperature around the package. How accurately the
temperature of the board and/or the forced airflow reflects
the temperature to be measured dictates the accuracy of the
measurement. Self−heating due to the power dissipated in
the ADT7421 or the remote sensor causes the chip
temperature of the device or remote sensor to rise above
ambient. However, the current forced through the remote
sensor is so small that self−heating is negligible. In the case
of the ADT7421, the worst−case condition occurs when the
device is converting at 36 conversions per second while
sinking the maximum current of 1 mA at the ALERT and
THERM output. In this case, the total power dissipation in
the device is about 4.5 mW. The thermal resistance, D
the 8−lead MSOP is approximately 142°C/W.
Layout Considerations
the ADT7421 is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. Take the following precautions:
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If a discrete transistor is used with the ADT7421, the best
Transistors, such as the 2N3904, 2N3906, or equivalents
Accuracy depends on the temperature of the remote
The on−chip sensor, however, is often remote from the
Digital boards can be electrically noisy environments, and
Base−emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature
Base−emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature
Base resistance less than 100 W
Small variation in h
control of V
Place the ADT7421 as close as possible to the remote
sensing transistor. Provided that, the worst noise sources,
that is, clock generators, data/address buses, and CRT’s
are avoided, this distance can be 4 to 8 inches.
Route the D+ and D− tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
BE
characteristics
FE
(50 to 150) that indicates tight
JA
, of