LM4916

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SNAS179E – MAY 2003 – REVISED MAY 2013

APPLICATION INFORMATION

SINGLE ENDED (SE) CONFIGURATION EXPLANATION

As shown in Figure 4, the LM4916 has two operational amplifiers internally, which have externally configurable

gain. The closed loop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri.

Consequently, the gain for each channel of the IC is

(1)

When the LM4916 operates in Single Ended mode, coupling capacitors are used on each output (VoA and VoB)

and the SE/BTL pin (Pin 8) is connected to ground. These output coupling capacitors blocks the half supply

voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones or

other single-ended (SE) loads. The signal return to circuit ground is through the headphone jack's sleeve.

BRIDGED (BTL) CONFIGURATION EXPLANATION

As shown in Figure 5, the LM4916 has two internal operational amplifiers. The first amplifier's gain is externally

configurable, while the second amplifier should be externally fixed in a unity-gain, inverting configuration. The

should be fixed by the two external 20k

Ω resistors. Figure 5 shows that the output of amplifier one serves as the

input to amplifier two which results in both amplifiers producing signals identical in magnitude, but out of phase

by 180°. Consequently, the differential gain for the IC is

(2)

By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as

"bridged mode" is established. Bridged mode operation is different from the classical single-ended amplifier

configuration where one side of the load is connected to ground. A bridge amplifier design has a few distinct

advantages over the single-ended configuration. It provides a differential drive to the load, thus doubling output

swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended

amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not

current limited or clipped. In order to choose an amplifier's closed-loop gain without causing excessive clipping,

please refer to the AUDIO POWER AMPLIFIER DESIGN section.

A bridge configuration, such as the one used in LM4916, also creates a second advantage over single-ended

amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across

the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-

ended amplifier configuration.

MODE SELECT DETAIL

The LM4916 can be configured in either Single Ended or BTL mode (see Figure 4 and Figure 5). The default

state of the LM4916 at power up is single ended. During initial power up or return from shutdown, the LM4916

must detect the correct mode of operation by sensing the status of the SE/BTL pin. When the bias voltage of the

part ramps up to 60mV (as seen on the Bypass pin), an internal comparator detects the status of SE/BTL; and at

10mV, latches that value in place. Ramp up of the bias voltage will proceed at a different rate from this point on

depending upon operating mode. BTL mode will ramp up about 11 times faster than Single Ended mode.

Recommended power up timing is shown in Figure 39 along with proper usage of Shutdown and Mute. The

applications solution to the problem of using different supply voltages with different turn-on times in a system with

the LM4916. This circuit shows the LM4916 with a 25-50k

Ω. Pull-up resistor connected from the shutdown pin to

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