AMC7834 Datasheet, PDF (35/93 Pages) Texas Instruments – AMC7834 12-Bit Integrated Power-Amplifier Monitor and Control System with Temperature, Current and Voltage Supervision Capabilities

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AMC7834
SLAS972B – NOVEMBER 2014 – REVISED MARCH 2016
7.3.3.2 Remote Temperature Sensors
The AMC7834 device includes two remote junction-temperature sensors. The remote sensing transistors can be
a discrete, small-signal type transistor or a substrate transistor built within the power amplifier. These transistors
are typically low-cost NPN- or PNP-type transistors such as the 2N3904 and 2N3906. Figure 49 shows the
recommended connection for NPN and PNP transistors in diode configuration.
The AMC7834 device also allows PNP transistor configuration as shown in Figure 50. PNP transistor
configuration for both remote temperature sensors is enabled by setting the RMT-GND-COLL bit to 1 in register
0x11.
NOTE
Pins D1– and D2– are internally shorted. Total parasitic capacitance to AGND on these
pins must be less than 800 pF.
2N3904
NPN
AMC7834
D+
D±
2N3906
PNP
AMC7834
D+
D±
D+
D+
2N3906
PNP
D±
2N3906
PNP
D±
Figure 49. NPN and PNP Diode Configuration
Figure 50. PNP Transistor Configuration
Errors in remote temperature sensor readings are typically the consequence of misalignment in the ideality factor
and current excitation used by the AMC7834 versus the manufacturer-specified operating current for a given
transistor. Some manufacturers specify a low-level (I_LOW) and high-level (I_HIGH) current for the temperature-
sensing substrate transistors. The AMC7834 uses an I_LOW of 7 µA and I_HIGH of 112 µA and is designed to work
with discrete transistors, such as the 2N3904 and SN3906. If an alternative transistor is used, the following
conditions should be met:
1. Base-emitter voltage (V_BE) > 0.25 V at 7 µA for the highest sensed temperature
2. Base-emitter voltage (V_BE) < 1.20 V at 112 µA for the lowest sensed temperature
3. Base resistance < 100 Ω
4. Tight control of V_BE characteristics indicated by small variations in h_FE (50 to 150)
The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an
ideal one. The AMC7834 is trimmed for η = 1.008. If the selected remote sensing transistor's ideality factor is
different, the effective η-factor should be adjusted at the system level.
Remote junction-temperature sensors are usually implemented in a noisy environment. Noise is most often
created by fast digital signals and can corrupt measurements. A bypass capacitor placed differentially across the
inputs of the remote temperature sensors can make the application more robust against unwanted coupled
signals. If filtering is required, its time constant, including any routing resistance, should be limited to 5 µs or less.
The combined series resistance on the remote temperature sensor pins must be less than 1 kΩ.
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