LM4917

SNAS238G – AUGUST 2004 – REVISED MAY 2013

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For package TSSOP,

supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the

3V power supply, with a 16

Ω load, the maximum ambient temperature possible without violating the maximum

junction temperature is approximately 119.9°C provided that device operation is around the maximum power

dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical

Performance Characteristics curves for power dissipation information for lower output powers.

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 in parallel with a 0.1µF ceramic filter

capacitors to stabilize the power supply's output, reduce noise on the supply line, and improve the supply's

transient response. However, their presence does not eliminate the need for a local 0.1µF supply bypass

that connect capacitors between the LM4917'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 LM4917’s shutdown function. When active, the LM4917’s micropower shutdown feature turns off the

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

shutdown current.

There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw

switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100k

Ω pull-up resistor

Select normal amplifier operation by opening the switch. Closing the switch connects the SD_LC/SD_RC pins to

ground, activating micro-power shutdown. The switch and resistor ensure that the SD_LC/SD_RC pins will not

float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital

output to apply the control voltage to the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins with active

circuitry eliminates the pull-up resistor.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4917's performance requires properly selecting external components. Though the LM4917

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

component values.

The LM4917 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more

than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-

noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands

input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources

DESIGN section for more information on selecting the proper gain.

Charge Pump Capacitor Selection

Choose low ESR (<100m

Ω) ceramic capacitors for optimum performance. Low ESR capacitors keep the charge

pump output impedance to a minimum, extending the headroom on the negative supply. Choose capacitors with

an X7R dielectric for best performance over temperature.

Charge pump load regulation and output resistance is affected by the value of the flying capacitor (C1). A larger

valued C1 improves load regulation and minimizes charge pump output resistance. The switch on-resistance and

capacitor ESR dominates the output resistance for capacitor values above 2.2µF.

The output ripple is affected by the value and ESR of the output capacitor (C2). Larger valued capacitors reduce

output ripple on the negative power supply. Lower ESR capacitors minimizes the output ripple and reduces the

output resistance of the charge pump.

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