LM48512

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SNAS497A – OCTOBER 2010 – REVISED APRIL 2012

POWER DISSIPATION AND EFFICIENCY

The major benefit of a Class D amplifier is increased efficiency versus a Class AB. The efficiency of the

LM48512 is attributed to the region of operation of the transistors in the output stage. The Class D output stage

acts as current steering switches, consuming negligible amounts of power compared to their Class AB

counterparts. Most of the power loss associated with the output stage is due to the IR loss of the MOSFET on-

resistance, along with switching losses due to gate charge.

SHUTDOWN FUNCTION

The LM48512 features a low current shutdown mode. Set SDREG = SDAMP = GND to disable the amplifier and

reduce supply current to 0.04

μA.

the typical 0.1

SDREG and SDAMP.

PROPER SELECTION OF EXTERNAL COMPONENTS

Inductor Selection

The LM48512 is designed to use a 2.2

μH inductor. When the boost converter is boosting, the inductor will

typically be the biggest area of efficiency loss in the boost converter circuitry, therefore, choosing an inductor with

the lowest possible series resistance is important. In addition to the series resistance, the saturation rating of the

inductor should also be greater than the maximum operating peak current.

Boost Output Capacitor Selection

The boost converter in the LM48512 is designed to operate with a 22

μF ceramic output capacitor. When the

boost converter is running, the output capacitor supplies the load current during the boost converter on-time.

When the NMOS switch turns off, the inductor energy is discharged through the internal PMOS switch, supplying

power to the load and restoring charge to the output capacitor. This causes a sag in the output voltage (PVOUT)

during the on-time and a rise in the output voltage during the off-time. The output capacitor is chosen to limit this

output ripple and to ensure the converter remains stable.

AUDIO AMPLIFIER 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 device as possible. A 10

μF and a 1μF bypass capacitors are recommended to

increase supply stability.

AUDIO AMPLIFIER 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 LM48512. The input capacitors create a high-pass filter with the

f = 1 / 2

(1)

The input capacitors can also be used to remove low frequency content from the audio signal. Small speakers

cannot reproduce, and may even be damaged by low frequencies. High pass filtering the audio signal helps

protect the speakers. When the LM48512 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 (for example,

217Hz in a GSM phone), 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.

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