View detail for AT73C240 - Audio Power Amplifier Filtering and Gain

AT73C240 - Audio Power Amplifier Filtering and
Gain
1. Scope
This document decribes how to determine the capacitance of the Input Capacitors of
the Audio Power Amplifier for the product AT73C240.
AT73C240 has an Audio Power Amplifier, bridge-tied loaded, designed in CMOS
technology, capable of driving an 8 Ohm Loudspeaker, making it suitable as a handsfree speaker driver in Wireless Handset Application.
High quality mono output is provided. The DAC output can be connected trough a buffer stage to the input of the Audio Power Amplifier, using two coupling capacitors.
This document defines how to calculate the required capacitance of these two coupling capacitors.
Caution: the Audio power amplifier is not internally protected against short-circuit.
The user should avoid any short-circuit on the load.
Power
Management
and Analog
Companions
(PMAAC)
Application Note
2. Operating Conditions
Table 2-1.
Operating Condition Table For AT73C240 Product
Parameter/Function
Pads
Min
Max
Unit
Storage Temperature
--
-55
150
°C
Operating Temperature
--
-40
85
°C
Audio Power Input
Voltage
VBAT
3.0
5.5
V
Digital Input Voltage
VDIG
2.4
3.3
V
Analog Input Voltage
AVDD, AVDDHS
2.7
3.3
V
3. Glossary
BW -- Bandwith
CMRR -- Common Mode Rejection Ratio
DAC -- Digital to Analog Converter
PA -- Audio Power Amplifier
6465A–PMAAC–04-Jun-09
4. AT73C240 Block Diagram
-36 to +12dB / 3dB step
AT73C240
GNDB
VREF
PAINN
VBAT
CBP
HPP
Audio PA
HPN
AT73C240 Block Diagram
PAINP
Figure 4-1.
VDIG
AVDD
Voltage
Reference
AVDDHS
SPI_DOUT
LINER
PGA
SPI_DIN / TWD
Status
Registers
INGND
-36 to +12dB / 3dB step
SPI
SPI_CLK / TWCK
LINEL
SPI_CSB / TW_ADD
PGA
-46.5dB to 0dB / 1.5dB step -34.5dB to +12dB / 1.5dB step
MCLK
-6 to +6dB / 3dB step
32
driver
+
DAC
Volume
Control
+
Volume
Control
RSTB
Digital Filter
VCM
-6 to +6dB / 3dB step
HSR
32
driver
+
DAC
Volume
Control
+
Volume
Control
Digital Filter
Serial Audio I/F
HSL
SMODE
SDIN
LRFS
BCLK
GNDA
-46.5dB to 0dB / 1.5dB step -34.5dB to +12dB / 1.5dB step
MONOP
MONO
+
GNDD
MONON
2
Application Note
6465A–PMAAC–04-Jun-09
Application Note
5. Power Amplifier Inupt Capacitor Selection
5.1
Use Needs
The two input capacitors, C8 and C15 in Figure 5-1, connected between MONON-PAINN and
MONOP-PAINP, affect two parameters:
• The low frequency cut-off
• The common mode rejection at low frequencies.
The low frequency cut-off depends on those capacitor values and the PA input impedance.
This impedance also depends on the gain selected (the higher the gain, the lower the input
impedance).
The low frequency cut-off is generally chosen to allow full bandwidth reproduction in all cases.
The worst case is the highest low frequency cut-off available.
Power Amplifier Apllication Diagram
Figure 5-1.
AT73C240 Power Amplifier Application Diagram
PAINN
AT73C240
C16
3.6 V
VBAT
CBP
C7
HPP
Battery (Li-Ion or 3 x NiMh or NiCd)
5.2
Audio PA
Loudspeaker
HPN
C15
PAINP
MONOP
C8
MONON
5.3
Calculation
Since minimum input impedance value is 10kOhms at maximum gain, the input capacitance
regarding the lower frequency to reproduce (Fcl) is calculated at follows:
5
Cin = 10 ⁄ ( 6283 × Fcl )
• Fcl is expressed in Hz
• Cin is expressed in µF
For a 50Hz low frequency cut-off, the calculated value is 0.318µF, or a 330nF as a standard
capacitor value.
Another parameter is the CMRR: to ensure a 40dB minimum rejection at low frequency, the
matching between the two input capacitors must be better than 1%.
3
6465A–PMAAC–04-Jun-09
6. Power Amplifier Input Impedance Versus Gain
The input impedance depends on the power amplifier gain. The minimum input impedance
(15kOhms typ, 10kOhms min) occurs with maximum gain value selected (+20dB), and at the
opposite the maximum input impedance (150kOhms typ, 100kOhms min) is obtained with the
minimum gain value (-22dB).
The following table gives all typical process input impedance values versus the gain selected.
Table 6-1.
4
Input Impedance vs Power Amplifier Gain
Input Impedance
(Ohms)
Gain (dB)
Input Impedance
(Ohms)
Gain (dB)
15k
+20
100k
-4
20k
+17
113k
-7
27k
+14
124k
-10
36k
+11
133k
-13
46k
+8
140k
-16
59k
+5
145k
-19
71k
+2
150k
-22
86k
-1
Application Note
6465A–PMAAC–04-Jun-09
Application Note
Revision History
Doc. Rev
Date
Comments
6465A
04-Jun-09
First issue
Change
Request Ref.
5
6465A–PMAAC–04-Jun-09
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6465A–PMAAC–04-Jun-09