LM4915

SNAS176A – MAY 2003 – REVISED MAY 2013

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POWER SUPPLY BYPASSING

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

rejection ratio (PSRR). The capacitor location on both the bypass and power supply pins should be as close to

the device as possible. A larger half-supply bypass capacitor improves PSRR because it increases half-supply

stability.

Typical applications employ a 5V regulator with 10µF and 0.1µF bypass capacitors that increase supply stability.

This, however, does not eliminate the need for bypassing the supply nodes of the LM4915. A 1µF capacitor is

recommended for CS. A 4.7µF capacitor is recommended for CB. This value coupled with small input capacitors

(0.1µF to 0.47µF) gives virtually zero click and pop with outstanding PSRR performance.

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the LM4915's shutdown function. Activate micro-power

shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4915's micro-power

shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The trigger point is 0.4V for a

logic-low level, and 1.8V for a logic-high level. The low 0.1µA (typ) shutdown current is achieved by applying a

voltage that is as near as ground as possible to the SHUTDOWN pin. 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. pull-up resistor between the SHUTDOWN pin and VDD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch

connects the SHUTDOWN pin to ground, activating micro-power shutdown.

The switch and resistor ensure that the SHUTDOWN pin 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

SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull-up resistor.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications using integrated power amplifiers is critical to optimize

device and system performance. While the LM4915 is tolerant of external component combinations, and requires

minimal external components, consideration to component values must be used to maximize overall system

quality.

The input coupling capacitor, Ci, forms a first order high pass filter which limits low frequency response given by

fc = 1/(2πRiCi). Ri is internally set to 20kΩ. This value should be chosen based on needed frequency response

for a few distinct reasons.

Selection of Input Capacitor Size

Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized

capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to

150Hz. Thus, using a large input capacitor may not increase actual system performance.

In addition to system cost and size, click and pop performance is affected by the size of the input coupling

capacitor, CI. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally

1/2 VDD). This charge comes from the output via the feedback and is apt to create pops upon device enable.

Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be

minimized

Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value.

Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the

LM4915 turns on. The slower the LM4915's outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the

smaller the turn-on pop. Choosing CB equal to 4.7µF along with a small value of CI (in the range of 0.1µF to

0.47µF), should produce a virtually clickless and popless shutdown function. While the device will function

properly, (no oscillations or motorboating), with CB equal to 1.0µF, the device will be much more susceptible to

turn-on clicks and pops. Thus, a value of CB equal to 4.7µF is recommended in all but the most cost sensitive

designs.

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