Datasheet

SW2
SW1
I (10 A)
2
m I (750 A)
1
m
DiodeTemperature
Sensor
MUX
ADC
CH8(TEMP)
AMC7823
SLAS453F –APRIL 2005–REVISED MARCH 2012 www.ti.com
ON-CHIP TEMPERATURE SENSOR
The AMC7823 has an integrated temperature sensor to measure the on-chip temperature. This measurement
relies on the characteristics of a semiconductor p-n junction operating at a known current level. The forward
voltage of the diode (V
BE
) depends on the current passing through it and the junction temperature.
Channel CH8 is dedicated to the on-chip temperature sensor. When CH8 is converted, the on-chip temperature
measurement process is activated. Two measurements, and therefore two ADC conversions, are required to
capture the temperature data. First, the diode is driven by current I
1
(750μA) and the forward voltage V
BE1
is
measured. Next, the diode is driven by current I
2
(10μA) and V
BE2
is measured. The difference of these two
voltages, ΔV
BE
, is stored in the ADC-8 Data Register as straight binary code. See Figure 46.
NOTE: When CH8 is converted, two conversions are performed. During the first conversion, SW1 is turned on, SW2 is off,
and the diode is driven by I
1
(750μA). During the second conversion, SW2 is turned on, SW1 is off, and the diode is
driven by I
2
(10μA).
Figure 46. Local Temperature Sensor Operation
In direct-mode operation, the temperature can be measured by converting Channel CH8 only, or by converting
several channels including Channel CH8. In auto-mode, the temperature must be measured by converting at
least two channels including Channel CH8.
The following equations illustrate the corresponding temperature calculation process:
ΔV
BE
(mV) = decimal code (convert from ADC-8 Data Register binary code) × 1.22mV for V
REF
= 2.5V
or
ΔV
BE
(mV) = decimal code (convert from ADC-8 Data Register binary code) × 0.61mV for V
REF
= 1.25V.
Temp (K) = 2.6 × ΔV
BE
(mV)
or
Temp (°C) = 2.6 × ΔV
BE
(mV) – 273K.
The resolution of this calculation is 3.2°C/LSB when the reference voltage is 2.5V and 1.6°C/LSB for a reference
voltage of 1.25V. The temperature sensor is powered by the reference. When the internal reference is used, a
filter capacitor (for example, 10μF) from pin 21 (EXT_REF_IN) to AGND is recommended to improve
temperature reading; refer to the Reference section for more information.
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