LM4910 datasheet(10/22 Pages) NSC

10 / 22 page

Application Information

ELIMINATING OUTPUT COUPLING CAPACITORS

Typical single-supply audio amplifiers that drive single-

ended (SE) headphones use a coupling capacitor on each

SE output. This output coupling capacitor blocks the half-

supply voltage to which the output amplifiers are typically

biased and couples the audio signal to the headphones. The

signal return to circuit ground is through the headphone

jack’s sleeve.

The LM4910 eliminates these output coupling capacitors.

Amp3 is internally configured to apply a bandgap referenced

voltage (V

REF = 1.58V) to a stereo headphone jack’s sleeve.

This voltage matches the quiescent voltage present on the

Amp1 and Amp2 outputs that drive the headphones. The

headphones operate in a manner similar to a bridge-tied-

load (BTL). The same DC voltage is applied to both head-

phone speaker terminals. This results in no net DC current

flow through the speaker. AC current flows through a head-

phone speaker as an audio signal’s output amplitude in-

creases on the speaker’s terminal.

The headphone jack’s sleeve is not connected to circuit

ground. Using the headphone output jack as a line-level

output will place the LM4910’s bandgap referenced voltage

on a plug’s sleeve connection. This presents no difficulty

when the external equipment uses capacitively coupled in-

puts. For the very small minority of equipment that is DC-

coupled, the LM4910 monitors the current supplied by the

amplifier that drives the headphone jack’s sleeve. If this

current exceeds 500mA

PK, the amplifier is shutdown, pro-

tecting the LM4910 and the external equipment.

ELIMINATING THE HALF-SUPPLY BYPASS CAPACITOR

Typical single-supply audio amplifers are normally biased to

1/2V

DD in order to maximize the output swing of the audio

signal. This is usually achieved with a simple resistor divider

network from V

DD to ground that provides the proper bias

voltage to the amplifier. However, this scheme requires the

use of a half-supply bypass capacitor to improve the bias

voltage’s stability and the amplifier’s PSRR performance.

The LM4910 utilizes an internally generated, buffered band-

gap reference voltage as the amplifier’s bias voltage. This

bandgap reference voltage is not a direct function of V

and therefore is less susceptible to noise or ripple on the

power supply line. This allows for the LM4910 to have a

stable bias voltage and excellent PSRR performance even

without a half-supply bypass capacitor.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4910 contains advanced circuitry that virtually elimi-

nates output transients (’clicks and pops’). This circuitry

prevents all traces of transients when the supply voltage is

first applied or when the part resumes operation after coming

out of shutdown mode. The LM4910 remains in a muted

condition until there is sufficient input signal magnitude

(>5mV

RMS, typ) to mask any remaining transient that may

occur. Figure 2 shows the LM4910’s lack of transients in the

differential signal (Trace B) across a 320 load. The LM4910’s

active-low SHUTDOWN pin is driven by the logic signal

shown in Trace A. Trace C is the V

O1 output signal and Trace

D is the V

O3 output signal.

To ensure optimal click and pop performance under low gain

configurations (less than 0dB), it is critical to minimize the

RC combination of the feedback resistor R

F and stray input

capacitance at the amplifier inputs. A more reliable way to

lower gain or reduce power delivered to the load is to place

a current limiting resistor in series with the load as explained

in the Minimizing Output Noise / Reducing Output Power

section.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4910 has three operational

amplifiers internally. Two of the amplifier’s have externally

configurable gain while the other amplifier is internally fixed

at the bias point acting as a unity-gain buffer. The closed-

loop gain of the two configurable amplifiers is set by select-

ing the ratio of R

f to Ri. Consequently, the gain for each

channel of the IC is

V = -(Rf/Ri)

By driving the loads through outputs V

O1 and VO2 with VO3

acting as a buffered bias voltage the LM4910 does not

require output coupling capacitors. The typical single-ended

amplifier configuration where one side of the load is con-

nected to ground requires large, expensive output coupling

capacitors.

A configuration such as the one used in the LM4910 has a

major advantage over single supply, single-ended amplifiers.

Since the outputs V

O1,VO2, and VO3 are all biased at VREF

= 1.58V, no net DC voltage exists across each load. This

eliminates the need for output coupling capacitors that are

required in a single-supply, single-ended amplifier configura-

tion. Without output coupling capacitors in a typical single-

supply, single-ended amplifier, the bias voltage is placed

across the load resulting in both increased internal IC power

dissipation and possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier. A direct consequence of the increased

power delivered to the load by a bridge amplifier is an

increase in internal power dissipation. The maximum power

dissipation for a given application can be derived from the

power dissipation graphs or from Equation 1.

DMAX = 4(VDD)

2 /(

π2R

(1)

It is critical that the maximum junction temperature T

JMAX of

150˚C is not exceeded. Since the typical application is for

20030592

FIGURE 2.

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