fc
IN +
1
2p R
IN CIN
C
IN +
1
2p fc
IN RIN
or
(7)
Charge Pump Flying Capacitor and CPVSS Capacitor
Decoupling Capacitors
Midrail Bypass Capacitor, CBYPASS
Low Dropout Regulator (LDO) Description
Layout Recommendations
www.ti.com.............................................................................................................................................................................................. SLOS601 – OCTOBER 2008
If a high-pass filter with a –3-dB point of no more than 20 Hz is desired over all gain settings, the minimum
impedance would be used in the
Equation 7. The minimum input impedance for TPA6047A4 is 20 k
. The
F. However, this is C
IN,
and the desired value is for
C(DCINPUT-BLOCKING). Multiplying CIN by 2 yields 0.80 F, which is close to the standard capacitor value of 1 F.
Place 1-
F capacitors at each input terminal of the TPA6047A4 to complete the filter.
The charge pump flying capacitor serves to transfer charge during the generation of the negative supply voltage.
The CPVSS capacitor must be at least equal to the flying capacitor in order to allow maximum charge transfer.
Low ESR capacitors are an ideal selection, and a value of 1 F is typical. Use X5R or better cermaic material.
The TPA6047A4 is a DirectPath headphone amplifier that requires adequate power supply decoupling to
ensure that the noise and total harmonic distortion (THD) are as low as possible. To filter high-frequency
transients, spikes, and digital hash on the power line, use good low equivalent-series-resistance (ESR) ceramic
capacitors, typically 1 F. Find the smallest package possible, and place as close as possible to the device VDD
lead. Placing the decoupling capacitors close to the TPA6047A4 is important for the performance of the amplifier.
Use a 10
F or greater capacitor near the TPA6047A4 to filter lower frequency noise signals; however, the high
PSRR of the TPA6047A4 makes the 10-
F capacitor unnecessary in most applications.
The midrail bypass capacitor, C(BYPASS), has several important functions. During start-up or recovery from
shutdown mode, CBYPASS determines the rate at which the amplifier starts up. A 1-F capacitor yields a start-up
time of approximately 25 ms. CBYPASS also reduces the noise coupled into the output signal by the power supply.
This improves the power supply ripple rejection (PSRR) of the amplifier. Ceramic or polyester capacitors with low
ESR and values in the range of 0.47
F to 1 F are recommended.
The TPA6047A4 contains a 4.75-V output low dropout regulator (LDO) capable of providing 120 mA with a drop
of less than 150 mV from the 5-V supply. This can be used to power an external CODEC. A 10-
F decoupling
capacitor is recommended at the output of the LDO as well as 0.1-
F capacitor to filter high-frequency noise from
the supply line.
Solder the exposed thermal pad (metal pad on the bottom of the part) on the TPA6047A4 QFN package to a
ground pad on the PCB. Fore more information, see the land pattern drawing.
It is important to keep the TPA6047A4 external components close to the body of the amplifier to limit noise
pickup. One should lay out the differential input leads symmetrical and close together to take advantage of the
inherent common mode rejection of the TPA6047A4. The layout of the TPA6047A4 evaluation module (EVM) is a
good example of component placement and the layout files are available at
www.ti.com.Copyright 2008, Texas Instruments Incorporated
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