LM4549B

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SNAS598B – JULY 2012 – REVISED JULY 2015

System Example (continued)

The bandgap reference and the anti-pop slow turnon circuit were improved in the LM4549B. A pullup resistor is

temperature coefficient will improve with no resistor on this pin. In addition, the THD will improve by 0.2–0.5 dB.

The external capacitor is charged by an internal current source, ramping the voltage slowly. This results in slow

turn-on of the audio stages, eliminating “pops and clicks”. Thus, turn-on performance is also improved. The

pullup resistor, in conjunction with the internal impedance and the external capacitor, form a frequency

dependent divider from the analog supply. Noise on the analog supply will be coupled into the audio path, with

approximately 30 dB. of attenuation. Although this is not a large amount if the noise on the supply is tens of

millivolts, it will prevent SNR from exceeding 80 dB.

In Figure 21 and Figure 22, the input coupling capacitors are shown as 1-µF capacitors. This is only necessary

for extending the response down to 20 Hz. for music applications. For telematics or voice applications, the lower

3-dB point can be much higher. Using a specified input resistance of 10 k

Ω, (40 kΩ typical), a 0.1-µF capacitor

may be used. The lower 3-dB point will still be below 300 Hz. By using a smaller capacitor, the package size may

be reduced, leading to a lower system cost.

10 Power Supply Recommendations

The LM4549B is designed to operate from an analog input voltage supply range between 4.2 V and 5.5 V.

The digital input voltage supply range is between 3 V and 5.5 V.

TI recommends connecting 1-µF and 0.1-µF decoupling capacitors in series on both the analog and digital

supply pins.

11 Layout

11.1 Layout Guidelines

•

The LM4549B must be initialized by using RESET# to perform a Power-On Reset.

•

Don't leave unused Analog inputs floating. Tie all unused inputs together and connect to Analog Ground

through a capacitor (that is, 0.1-µF).

•

Do not leave CD_GND floating when using the CD stereo input. CD_GND is the AC signal reference for the

CD channels and should be connected to the CD source ground (Analog Ground may also be acceptable)

through a 1-µF capacitor.

•

If using a non-standard AC Link controller take care to keep the SYNC and SDATA_IN signals low during

Cold Reset to avoid entering the ATE or Vendor test modes by mistake.

•

The PC_Beep input should be muted if not used since it defaults to 0-dB gain on reset, unlike the mute

default of the other analog inputs.

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Product Folder Links: LM4549B