ADT7475ARQZ ON Semiconductor, ADT7475ARQZ Datasheet - Page 13

• Manufacturer Part Number: ADT7475ARQZ
• Description: IC REMOTE THERMAL CTRLR 16-QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7475ARQZ

• 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: 16-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

Temperature Measurement Method
Local Temperature Measurement
sensor whose output is digitized by the on−chip, 10−bit
ADC. The 8−bit MSB temperature data is stored in the
temperature registers (0x25, 0x26, and 0x27). Because both
positive and negative temperatures can be measured, the
temperature data is stored in Offset 64 format or twos
complement format, as shown in Table 3 and Table 4.
measure temperatures from −128°C to +127°C (or −64°C to
+191°C in the extended temperature range) with a
resolution of 0.25°C.
change in V
currents.
This is given by:
measure the output of a remote temperature sensor. This
figure shows the external sensor as a substrate transistor,
provided
microprocessors. It could also be a discrete transistor such
as a 2N3904/2N3906.
and should be linked to the base. If a PNP transistor is used,
the base is connected to the D input and the emitter to the
D+ input. If an NPN transistor is used, the emitter is
connected to the D input and the base to the D+ input.
Figure 21 and Figure 22 show how to connect the ADT7475
to an NPN or PNP transistor for temperature measurement.
To prevent ground noise from 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.
The ADT7475 contains an on−chip band gap temperature
Theoretically, the temperature sensor and ADC can
The technique used in the ADT7475 is to measure the
Figure 20 shows the input signal conditioning used to
If a discrete transistor is used, the collector is not grounded
where:
DV
k is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
BE
BE
for
+ kT q
when the device is operated at two different
temperature
TRANSISTOR
SENSING
Figure 20. Signal Conditioning for Remote Diode Temperature Sensors
REMOTE
In(N)
CPU
monitoring
THERMDA
THERMDC
D+
D–
I
on
DIODE
BIAS
(eq. 1)
http://onsemi.com
N y I
some
I
BIAS
LOW−PASS FILTER
13
f
V
C
DD
= 65kHz
the device, so local temperature measurements outside the
ADT7475 operating temperature range are not possible.
Remote Temperature Measurement
diode sensors or diode−connected transistors connected to
Pin 10 and Pin 11 or to Pin 12 and Pin 13.
transistor operated at a constant current exhibits a negative
temperature coefficient of about –2 mV/°C. Because the
absolute value of V
individual calibration is required to null this out, the
technique is unsuitable for mass production.
operating currents of I and N x I. The resulting waveform is
passed through a 65 kHz low−pass filter to remove noise and
to a chopper stabilized amplifier that performs the functions
of amplification and rectification of the waveform to
produce a dc voltage proportional to DV
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 shown in
Table 3. 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 was to place a capacitor across the D+ pin
and D− pin to help combat the effects of noise. However,
large capacitance’s affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1000 pF.
Sometimes, this sensor noise is a problem in a very noisy
environment. In most cases, a capacitor is not required
However, this exceeds the operating temperature range of
The ADT7475 can measure the temperature of two remote
The forward voltage of a diode or diode−connected
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.
BE
BE
, the sensor is switched between
varies from device to device and
V
V
OUT+
OUT–
TO ADC
BE
. This voltage is