ADT7460ARQZ-RL7 ONSEMI [ON Semiconductor], ADT7460ARQZ-RL7 Datasheet - Page 15

• Manufacturer Part Number: ADT7460ARQZ-RL7
• Description: dBCOOL Remote Thermal Monitor and Fan Controller
• Manufacturer: ONSEMI [ON Semiconductor]

applications. The input range of the ADC without the
attenuators is 0 V to 2.25 V.
Single−Channel ADC Conversion
places the ADT7460 into single−channel ADC conversion
mode. In this mode, the ADT7460 can be made to read a
single voltage channel only. If the internal ADT7460 clock is
used, the selected input is read every 711 ms. The appropriate
ADC channel is selected by writing to Bits <7:5> of the
TACH1 Minimum High Byte register (Reg. 0x55).
Temperature Measurement System
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 local
temperature register (Address 0x26). As both positive and
grounded, and it 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 25 and Figure 26 show how to connect the
ADT7460 to an NPN or PNP transistor for temperature
measurement. To prevent ground noise from interfering
with the measurement, the more negative terminal of the
Table 7. Configuration Register 2 (Reg. 0x73)
Table 8. TACH1 Minimum High Byte (Reg. 0x55)
<7:5>
Setting Bit 6 of Configuration Register 2 (Reg. 0x73)
The ADT7460 contains an on−chip band gap temperature
If a discrete transistor is used, the collector is not
<4>
<5>
<6>
Bit
Bit
1: Averaging Off
1: Bypass Input Attenuators
1: Single−Channel Convert Mode
Selects ADC channel for single−channel convert
mode
Value
000
010
Figure 24. Signal Conditioning for Remote Diode Temperature Sensors
TRANSISTOR
Description
Description
SENSING
REMOTE
CPU
Channel Selected
2.5 V
THERMDC
THERMDA
V
CC
D+
D–
I
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ADT7460
DIODE
BIAS
15
N y I I
negative temperatures can be measured, the temperature
data is stored in twos complement format, as shown in
Table 9. Theoretically, the temperature sensor and ADC can
measure temperatures from −128°C to +127°C with a
resolution of 0.25°C. However, this exceeds the operating
temperature range of the device, so local temperature
measurements outside this range are not possible.
Remote Temperature Measurement
diode sensors or diode−connected transistors connected to
Pins 12 and 13, or Pins 10 and 11.
transistor operated at a constant current exhibits a negative
temperature coefficient of about −2 mV/°C. Unfortunately,
the absolute value of V
individual calibration is required to null this out, so the
technique is unsuitable for mass production. The technique
used in the ADT7460 is to measure the change in V
the device is operated at two different currents. This is given
by:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
measure the output of a remote temperature sensor. This
figure shows the external sensor as a substrate transistor
provided
microprocessors. It could equally well be a discrete
transistor, such as a 2N3904.
sensor is not referenced to ground but is biased above ground
by an internal diode at the D− input.
operating currents of I and N × 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,
BIAS
The ADT7460 can measure the temperature of two remote
The forward voltage of a diode or diode−connected
Figure 24 shows the input signal conditioning used to
To measure DV
f
C
where:
= 65kHz
LPF
V
DD
for
temperature
BE
, the sensor is switched between
BE
varies from device to device, and
V
V
OUT+
OUT–
monitoring
TO ADC
BE
. This voltage is
on
BE
when
some