LM48312

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SNAS494D – JANUARY 2010 – REVISED MAY 2013

Table 1. Gain Setting

GAIN

GAIN SETTING

FLOAT

0dB

VDD

3dB

GND

6dB

20k

Ω to GND

9dB

20k

Ω to VDD

12dB

For proper gain selection:

1. Use 20k

Ω resistors with 10% tolerance or better for the 9dB and 12dB gain settings.

2. Short GAIN to either VDD or GND through 100Ω or less for the 3dB and 6dB gain settings.

3. FLOAT = 20M

Ω or more for the 0dB gain setting.

SHUTDOWN FUNCTION

The LM48312 features a low current shutdown mode. Set SD = GND to disable the amplifier and reduce supply

current to 0.01µA.

Switch SD between GND and VDD for minimum current consumption is shutdown. The LM48312 may be disabled

with shutdown voltages in between GND and VDD, the idle current will be greater than the typical 0.1µA value.

Increased THD+N may also be observed when a voltage of less than VDD is applied to SD.

The LM48312 shutdown input has and internal pulldown resistor. The purpose of this resistor is to eliminate any

unwanted state changes when SD is floating. To minimize shutdown current, SD should be driven to GND or left

floating. If SD is not driven to GND or floating, an increase in shutdown supply current will be noticed.

PROPER SELECTION OF EXTERNAL COMPONENTS

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. Typical applications employ a voltage regulator with 10µF and

0.1µF bypass capacitors that increase supply stability. These capacitors do not eliminate the need for bypassing

of the LM48312 supply pins. A 1µF capacitor is recommended.

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 LM48312. 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

(1)

Where RIN is the value of the input resistor given in the Electrical Characteristics table.

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 LM48312 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 LM48312 is compatible with single-ended sources. When configured for single-ended inputs, input

capacitors must be used to block and DC component at the input of the device. Figure 39 shows the typical

single-ended applications circuit.

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