LM4950

SNAS174E – JULY 2003 – REVISED MAY 2013

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the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's PSRR.

The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases

turn-on time and can compromise the amplifier's click and pop performance. The selection of bypass capacitor

values, especially CBYPASS, depends on desired PSRR requirements, click and pop performance (as explained in

SELECTING EXTERNAL COMPONENTS), system cost, and size constraints.

MICRO-POWER SHUTDOWN

The LM4950 features an active-low micro-power shutdown mode. When active, the LM4950's micro-power

shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The low 40µA typical

shutdown current is achieved by applying a voltage to the SHUTDOWN pin that is as near to GND as possible. A

voltage that is greater than GND may increase the shutdown current.

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

switch (SPST), a microprocessor, or a microcontroller. When using a switch, connect a 100k

Ω pull-up resistor

between the SHUTDOWN pin and VDD and a second 100kΩ resistor in parallel with the SPST switch connected

between the SHUTDOWN pin and GND. The two resistors form a voltage divider that ensures that the voltage

applied to the SHUTDOWN pin does not exceed VDD/2. Select normal amplifier operation by opening the switch.

Closing the switch applies GND to the SHUTDOWN pin, 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 a microcontroller, use a digital output to apply the active-state voltage to the SHUTDOWN pin.

Again, ensure that the microcontroller or microprocessor logic-high signal does not exceed the LM4950's VDD/2

SHUTDOWN signal limit.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires

amplification and desired output transient suppression.

As shown in Figure 65, the input resistor (RIN) and the input capacitor (CIN) produce a high pass filter cutoff

frequency that is found using Equation 7.

fc = 1/2πRiCi

(7)

As an example when using a speaker with a low frequency limit of 50Hz, Ci, using Equation 7 is 0.159µF. The

0.39µF CINA shown in Figure 65 allows the LM4950 to drive high efficiency, full range speaker whose response

extends below 30Hz.

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consideration should be paid to value of CBYPASS, the

capacitor connected to the BYPASS pin. Since CBYPASS determines how fast the LM4950 settles to quiescent

operation, its value is critical when minimizing turn-on pops. The slower the LM4950's outputs ramp to their

quiescent DC voltage (nominally VDD/2), the smaller the turn-on pop. Choosing CBYPASS equal to 10µF along with

a small value of CIN (in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As

discussed above, choosing CIN no larger than necessary for the desired bandwidth helps minimize clicks and

pops.

OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE

The LM4950 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this

discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode

is deactivated.

As the VDD/2 voltage present at the BYPASS pin ramps to its final value, the LM4950's internal amplifiers are

configured as unity gain buffers and are disconnected from the AMPA and AMPB pins. An internal current source

charges the capacitor connected between the BYPASS pin and GND in a controlled manner. Ideally, the input

and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until

the voltage applied to the BYPASS pin.

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