ADT7476AARQZ-R ON Semiconductor, ADT7476AARQZ-R Datasheet - Page 18

• Manufacturer Part Number: ADT7476AARQZ-R
• Description: IC REMOTE THERMAL CTLR 24-QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7476AARQZ-R

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Fan Speed Counter, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 125°C, External Sensor
• Output Type: SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 24-QSOP
• Full Temp Accuracy: +/- 0.5 C
• Digital Output - Bus Interface: Serial (3-Wire, 4-Wire)
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

currents of I and N x I. The resulting waveform passes
through a 65 kHz low-pass filter to remove noise and
through a chopper-stabilized amplifier. The amplifier
performs the amplification and rectification of the
waveform to produce a dc voltage proportional to DV
This voltage is measured by the ADC to give a temperature
output in 10-bit, twos complement format. To further reduce
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles.
38 ms. The results of remote temperature measurements are
stored in 10-bit, twos complement format, as illustrated in
Table 10. The extra resolution for the temperature
measurements is held in the Extended Resolution Register 2
(0x77). This gives temperature readings with a resolution of
0.25°C.
Noise Filtering
previous practice placed a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However,
large capacitances affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1000 pF.
which makes using the sensor difficult in a very noisy
environment. In most cases, a capacitor is not required
because differential inputs by their very nature have a high
immunity to noise.
Factors Affecting Diode Accuracy
Remote Sensing Diode
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode
To measure DV
A remote temperature measurement takes nominally
For temperature sensors operating in noisy environments,
This capacitor reduces the noise but does not eliminate it,
The ADT7476A is designed to work with substrate
Figure 26. Measuring Temperature Using an
Figure 27. Measuring Temperature Using an
2N3904
2N3906
NPN
PNP
BE
, the sensor switches between operating
NPN Transistor
PNP Transistor
ADT7476A
ADT7476A
D+
D–
D+
D–
http://onsemi.com
BE
.
18
(base-shorted to the 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 is connected to D+.
discrete transistors, a number of factors should be taken into
consideration:
•
•
best accuracy is obtained by choosing devices according to
the following criteria:
•
•
•
•
SOT-23 packages, are suitable devices to use.
Nulling Out Temperature Errors
frequency clocks when routing the D+/D– traces around a
system board. Even when recommended layout guidelines
are followed, some temperature errors can still be
attributable to noise coupled onto the D+/D– lines. Constant
high frequency noise usually attenuates, or increases,
temperature measurements by a linear, constant value.
To reduce the error due to variations in both substrate and
If a discrete transistor is used with the ADT7476A, the
Transistors, such as 2N3904, 2N3906, or equivalents in
As CPUs run faster, it is more difficult to avoid high
To factor this in, the user can write the DT value to the
offset register. The ADT7476A then automatically
adds it to or subtracts it from the temperature
measurement.
The ideality factor, n
the deviation of the thermal diode from ideal behavior.
The ADT7476A is trimmed for an n
Use the following equation to calculate the error
introduced at a temperature T (°C), when using a
transistor whose n
processor’s data sheet for the n
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7476A, I
the low level current, I
current levels do not match the current levels specified
by the CPU manufacturer, it could be necessary to
remove an offset. The CPU’s data sheet advises
whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that if more than one
offset must be considered, then the algebraic sum of
these offsets must be programmed to the offset register.
Base-emitter voltage greater than 0.25 V at 11 mA, at
the highest operating temperature.
Base-emitter voltage less than 0.95 V at 180 mA,
at the lowest operating temperature.
Base resistance less than 100 W.
Small variation in the current gain, h
50 to 150) that indicates tight control of V
characteristics.
DT + ( nf * 1.008 )
f
does not equal 1.008 (see the
f
, of the transistor is a measure of
LOW
273.15 K ) T
, is 11 mA. If the ADT7476A
f
values):
HIGH
f
FE
value of 1.008.
, is 180 mA, and
, (approximately
BE
(eq. 2)