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APPLICATION INFORMATION
FULLY DIFFERENTIAL AMPLIFIER
Advantages of Fully Differential Amplifiers
BOOST CONVERTER
Boost Terms
SLOS520 – AUGUST 2007
The TPA2013D1 is a fully differential amplifier with differential inputs and outputs. The fully differential amplifier
consists of a differential amplifier with common-mode feedback. The differential amplifier ensures that the
amplifier outputs a differential voltage on the output that is equal to the differential input times the gain. The
common-mode feedback ensures that the common-mode voltage at the output is biased around VCC/2 regardless
of the common-mode voltage at the input. The fully differential TPA2013D1 can still be used with a single-ended
input; however, the TPA2013D1 should be used with differential inputs when in a noisy environment, like a
wireless handset, to ensure maximum noise rejection.
Input-coupling capacitors not required:
–
The fully differential amplifier allows the inputs to be biased at voltage other than mid-supply. The inputs of
the TPA2013D1 can be biased anywhere within the common mode input voltage range listed in the
Recommended Operating Conditions table. If the inputs are biased outside of that range, input-coupling
capacitors are required.
Midsupply bypass capacitor, C
(BYPASS), not required:
–
The fully differential amplifier does not require a bypass capacitor. Any shift in the midsupply affects both
positive and negative channels equally and cancels at the differential output.
Better RF-immunity:
–
GSM handsets save power by turning on and shutting off the RF transmitter at a rate of 217 Hz. The
transmitted signal is picked-up on input and output traces. The fully differential amplifier cancels the signal
better than the typical audio amplifier.
The TPA2013D1 consists of a boost converter and a Class-D amplifier. The boost converter takes a low supply
voltage, VDD, and increases it to a higher output voltage, VCC. VCC is the power supply for the Class-D amplifier.
The two main passive components necessary for the boost converter are the boost inductor and the boost
capacitor. The boost inductor stores current, and the boost capacitor stores charge. As the Class-D amplifier
depletes the charge in the boost capacitor, the boost inductor charges it back up with the stored current. The
cycle of charge/discharge occurs at a frequency of fboost.
The TPA2013D1 allows a range of VCC voltages, including setting VCC lower than VDD.
The following is a list of terms and definitions used in the boost equations found later in this document.
C
Minimum boost capacitance required for a given ripple voltage on VCC.
L
Boost inductor
fboost
Switching frequency of the boost converter.
ICC
Current pulled by the Class-D amplifier from the boost converter.
IL
Average current through the boost inductor.
R1 and R2
Resistors used to set the boost voltage.
VCC
Boost voltage. Generated by the boost converter. Voltage supply for the Class-D
amplifier.
VDD
Supply voltage to the IC.
ΔIL
Ripple current through the inductor.
ΔV
Ripple voltage on VCC due to capacitance.
Copyright 2007, Texas Instruments Incorporated
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