LM48860

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SNAS398D – JANUARY 2008 – REVISED MAY 2013

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply

rejection. Applications that employ a 3V power supply typically use a 4.7µF capacitor in parallel with a 0.1µF

ceramic filter capacitor to stabilize the power supply's output, reduce noise on the supply line, and improve the

supply's transient response. Keep the length of leads and traces that connect capacitors between the LM48860's

power supply pin and ground as short as possible.

MICRO POWER SHUTDOWN

The voltage applied to the SD_LC (shutdown left channel) pin and the SD_RC (shutdown right channel) pin

controls the LM48860’s shutdown function. When active, the LM48860’s micropower shutdown feature turns off

the amplifiers’ bias circuitry, reducing the supply current. The trigger point is 0.45V for a logic-low level, and 1.2V

for logic-high level. The low 0.01µA (typ) shutdown current is achieved by applying a voltage that is as near as

ground a possible to the SD_LC/SD_RC pins. A voltage that is higher than ground may increase the shutdown

current. Do not let SD_LC/SD_RC float, connect either to high or low.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM48860's performance requires properly selecting external components. Though the LM48860

operates well when using external components with wide tolerances, best performance is achieved by optimizing

component values.

Charge Pump Capacitor Selection

Use low ESR (equivalent series resistance) (<100m

Ω) ceramic capacitors with an X7R dielectric for best

performance. Low ESR capacitors keep the charge pump output impedance to a minimum, extending the

headroom on the negative supply. Higher ESR capacitors result in reduced output power from the audio

amplifiers.

Charge pump load regulation and output impedance are affected by the value of the flying capacitor (C4). A

larger valued C4 (up to 3.3uF) improves load regulation and minimizes charge pump output resistance. Beyond

3.3uF, the switch-on resistance dominates the output impedance.

The output ripple is affected by the value and ESR of the output capacitor (C3). Larger capacitors reduce output

ripple on the negative power supply. Lower ESR capacitors minimize the output ripple and reduce the output

impedance of the charge pump.

The LM48860 charge pump design is optimized for 2.2uF, low ESR, ceramic, flying and output capacitors.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value input coupling capacitors (C1 and C2 in Figure 1). A

high value capacitor can be expensive and may compromise space efficiency in portable designs. In many

cases, however, the speakers used in portable systems, whether internal or external, have little ability to

reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little

improvement by using high value input and output capacitors.

As shown in Figure 1, the internal input resistor, Ri and the input capacitors, C1 and C2, produce a -3dB high-

pass filter cutoff frequency that is found using Equation 2.

fi-3dB = 1 / 2πRINC

(Hz)

(2)

The value of RIN can be found in the Electrical Characteristics tables.

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