AMC6821-Q1 Datasheet, PDF (13/54 Pages) Texas Instruments – INTELLIGENT TEMPERATURE MONITOR AND PWM FAN CONTROLLER

AMC6821-Q1
www.ti.com ...................................................................................................................................................................................................... SBAS475 – JUNE 2009
The remote sensing transistor can be a substrate transistor built within the microprocessor (as in a Pentium-IV),
or a discrete small-signal type transistor. This architecture is shown in Figure 13. The internal bias diode biases
the IN– terminal above ground to prevent the ground noise from interfering with the measurement. An external
capacitor (up to 1000pF) may be placed between IN+ and IN– to further reduce the noise from interfering.
IN+
Substrate
Sensing
IN-
Transistor
uP
I₁
SW1
I₂
SW2
Bias Diode
I_BIAS
LPF and
Signal Conditioning
Remote
Temperature
Registers
ADC and
Signal
Processing
Figure 13. Remote Temperature Sensor
The analog sensing signal is pre-processed by a low-pass filter and signal conditioning circuitry, then digitized by
the ADC. The resulting digital signal is further processed by the digital filter and processing unit. The final result
is stored in the local temperature data register and remote temperature data register, respectively. The eight
MSBs are stored in the corresponding Temp-DATA-HByte register, and the three LSBs are stored in the
Temp-DATA-LByte register. Refer to the Temperature Data Registers section for details.
The format of the final result is in two’s complement; see Table 11. It should be noted that the device measures
the temperature from –40°C to +125°C, although the code represents temperature from –128°C to +127°C.
Series Resistance Cancellation
Parasitic resistance (seen in series with the remote diode) to the IN+ and IN– inputs to the AMC6821 is caused
by a variety of factors, including printed circuit board (PCB) trace resistance and trace length. This series
resistance appears as a temperature offset in the remote sensor temperature measurement, and causes more
than 0.45°C error per ohm. The AMC6821 is implemented with a TI-patented technology to automatically cancel
out the effect of this series resistance, giving a more accurate result without the need for user characterization of
this resistance. With this technology, the AMC6821 is able to reduce the effects of series resistance to typically
less than 0.0025°C per ohm.
Reading Temperature Data
It is important to note that temperature can be read by an 8-bit value (with 1°C of resolution) from the
Temp-DATA-HByte register, or as an 11-bit value (with 0.125°C of resolution) from the Temp-DATA-LByte and
Temp-DATA-HByte registers. If only 1°C of resolution is required, the temperature readings can be read back at
any time and in no particular order. If reading the 11-bit measurement is required, the process involves a
two-register read for each measurement. To get an 11-bit result of the remote sensor, the controller must read
the Temp-DATA-LByte register (0x06) first, and then the Remote-Temp-DATA-HByte register (0x0B) to complete
the reading. However, to get bit 11 of the local sensor only, or to get both local and remote sensors, the
controller must read Temp-DATA-LByte first, Local-Temp-DATA-HByte (0x0A) second, and
Remote-Temp-DATA-HByte third. This method causes all associated temperature data registers to be frozen
until the Remote-Temp-DATA-HByte register has been read. This process also prevents the high byte data from
being updated while the three LSBs are being read, and vice-versa.
Copyright © 2009, Texas Instruments Incorporated
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