LM4931ITLSLASHNOPB datasheet(43/50 Pages) TI2

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LM4931

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SNAS251E – APRIL 2004 – REVISED MAY 2013

PLL Loop Filter

The LM4931 demoboard features an onboard second and third order PLL loop filter. Jumpers (S6-S9) configure

the demoboard to select between the second and third order PLL loop filters. Reference values for the loop filters

are given in the Table 25 section. For a more detailed discussion on how to optimize a second and third order

PLL loop filter.

ANALOG INPUTS AND OUTPUTS

The LM4931 features a high-efficiency class D mono BTL output for connection to an 8

Ω external speaker. This

output can provide up to 1.1W of power into an 8 ohms load with a 5V analog supply. A single-ended stereo

headphone output is also featured, providing up to 26mW of power per channel into 32

Ω with a 5V analog

supply. The MIC Jack input (JP1) provides for a low level analog input. Pin 3 provides the power to the MIC and

the positive input of the LM4931. Gain for the MIC preamp is set in the MIC_GAIN (02h) register.

HIGH EFFICIENCY CLASS D AMPLIFIER FUNCTION

The class D mono output signals generated by the LM4931 consist of two, BTL connected, output signals that

pulse momentarily from near ground potential to VDD. The two outputs can pulse independently with the

exception that they both may never pulse simultaneously as this would result in zero volts across the BTL load.

The minimum width of each pulse is approximately 160ns. However, pulses on the same output can occur

sequentially, in which case they are concatenated and appear as a single wider pulse to achieve an effective

100% duty cycle. This results in maximum audio output power for a given supply voltage and load impedance.

The LM4931 can achieve much higher efficiencies than class AB amplifiers while maintaining acceptable THD

performance. The short (160ns) drive pulses emitted at the LM4931 outputs means that good efficiency can be

obtained with minimal load inductance. The typical transducer load on an audio amplifier is quite reactive

(inductive). For this reason, the load can act as it’s own filter, so to speak. This "filter-less" switching

amplifier/transducer load combination is much more attractive economically due to savings in board space and

external component cost by eliminating the need for a filter.

CLASS D POWER DISSIPATION AND EFFICIENCY

In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required

to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For

audio systems, the energy delivered in the audible bands is considered useful including the distortion products of

the input signal. Sub-sonic (DC) and super-sonic components (>22kHz) are not useful. The difference between

the power flowing from the power supply and the audio band power being transduced is dissipated in the

LM4931 and in the transducer load. The amount of power dissipation in the LM4931 is very low. This is because

the ON resistance of the switches used to form the output waveforms is typically less than 0.25

Ω. This leaves

only the transducer load as a potential "sink" for the small excess of input power over audio band output power.

The LM4931 dissipates only a fraction of the excess power requiring no additional PCB area or copper plane to

act as a heat sink.

DUAL MICROPHONE SUPPORT

The LM4931 can be configured to accept two separate microphone inputs when used in conjunction with the

LMS4684. The LMS4684 is a dual SPDT analog switch that will allow the MIC_P and MIC_N inputs of the

LM4931 to switch between a differential handset microphone and a single-ended handsfree microphone. The

MIC DETECT block shown in Figure 67 can be implemented with a microphone jack’s mechanical control pin to

set the voltage at the IN1 and IN2 pins of the LMS4684. The voltage applied at the IN1 and IN2 pins sets the

position of the switch. For a more detailed discussion on the operation of the analog switch, please refer to the

LMS4684 datasheet.

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