LM48822TLEVAL datasheet(14/22 Pages) TI1

14 / 22 page

LM48822, LM48822TLEVAL

SNAS456C – MAY 2008 – REVISED MAY 2013

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SD_BIAS FUNCTION

The LM48822 BIAS is controlled through the I2C interface. Set bit B5 (SD_BIAS) of the MODE CONTROL

pump. When enabled, VBIAS will track VDD for VDD < 3V. Once VDD exceeds 3V, VBIAS remains fixed at 3V, limiting

the output swing of the device the 6VP-P. Set SD_BIAS = 0 to disable BIAS. Disabling BIAS allows the amplifier

and charge pump to track VDD, increasing output swing; however, a slight degradation in PSSR will occur. Limit

VDD to 4.2V or less when BIAS is disabled.

PROPER SELECTION OF EXTERNAL COMPONENTS

Power Supply Bypassing/Filtering

Proper power supply bypassing is critical for low noise performance and high PSRR. Place the supply bypass

capacitors as close to the supply pins as possible. Place a 1

μF ceramic capacitors from VDD to GND. Additional

bulk capacitance may be added as required.

Charge Pump Capacitor Selection

Use low ESR ceramic capacitors (less than 100m

Ω) for optimum performance.

Charge Pump Flying Capacitor (C1)

The flying capacitor (C1) affects the load regulation and output impedance of the charge pump. A C1 value that

is too low results in a loss of current drive, leading to a loss of amplifier headroom. A higher valued C1 improves

load regulation and lowers charge pump output impedance to an extent. Above 2.2

μF, the RDS(ON) of the charge

pump switches and the ESR of C1 and C2 dominate the output impedance. A lower value capacitor can be used

in systems with low maximum output power requirements.

Charge Pump Flying Capacitor (C2)

The value and ESR of the hold capacitor (C2) directly affects the ripple on CPVSS. Increasing the value of C2

reduces output ripple. Decreasing the ESR of C2 reduces both output ripple and charge pump output impedance.

A lower value capacitor can be used in systems with low maximum output power requirements.

Input Capacitor Selection

Input capacitors may be required for some applications, or when the audio source is single-ended. Input

capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of

the audio source and the bias voltage of the LM48822. The input capacitors create a high-pass filter with the

input resistors RIN. The -3dB point of the high pass filter is found using Equation 1 below.

f = 1 / 2

πRINCIN (Hz)

(1)

where:

the value of RIN is given in the Electrical Characteristics Table.

High pass filtering the audio signal helps protect the speakers. When the LM48822 is using a single-ended

source, power supply noise on the ground is seen as an input signal. Setting the high-pass filter point above the

power supply noise frequencies, 217Hz in a GSM phone, for example, filters out the noise such that it is not

amplified and heard on the output. Capacitors with a tolerance of 10% or better are recommended for impedance

matching and improved CMRR and PSRR.

SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION

The LM48822 is compatible with single-ended sources. Figure 26 shows the typical single-ended applications

circuit.

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