LMV1099

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SNAS490D – JULY 2010 – REVISED MAY 2013

Earpiece Control Registers

OUTPUT POWER LIMIT (PLEV)

While TI has done extensive ambient SPL analysis, there will always be unusual circumstances that may cause

the amplifier to be at its maximum 18dB setting. LMV1099 features an Output Voltage Limit function to limit the

output power delivered to a speaker. When the SNR enhancer is active, the Output Voltage Limit works to

protect the loudspeaker in conditions where a large downlink input signal is present. The Output Voltage Limit

can be set to a selectable (3.6VP-P or 4.1VP-P) output level to avoid violating the maximum power limitation of the

transducer.

SNR ENHANCER BYPASS (EP_BYPASS_AGC)

The SNR enhancer can be bypassed by setting B4 of the Earpiece Control Register to 1. When the SNR

enhancer is bypassed, the earpiece amplifier has a fixed 0dB gain.

EP_RI (INPUT IMPEDANCE)

The earpiece input of the LMV1099 features three input impedance options, this impedance in conjunction with

the input capacitor creates a high-pass filter. The three options provide various cutoff frequencies for the high-

pass filter. Table 7 shows the respective cutoff frequencies for each of the input impedance options when using a

68nF input capacitor.

Table 7. Input Impedance options

Input Impedance

fC

60k

Ω

40Hz

9k

Ω

260Hz

6k

Ω

390Hz

Changing the input coupling capacitor will affect the filters –3dB point through the simple RC equation shown

below:

f = 1 / 2

πRC

(1)

Microphone Placement

Because the LMV1099 is a microphone array Far Field Noise Reduction solution, proper microphone placement

is critical for optimum performance. Two things need to be considered: The spacing between the two

microphones and the position of the two microphones relative to near field source.

If the spacing between the two microphones is too small near field speech will be canceled along with the far

field noise. Conversely, if the spacing between the two microphones is large, the far field noise reduction

performance will be degraded. The optimum spacing between mic1 and mic2 is 1.5-2.5cm. This range provides a

balance of minimal near field speech loss and maximum far field noise reduction. The microphones should be in

line with the desired sound source 'near speech' and configured in an endfire array (see Figure 27) orientation

from the sound source. If the 'near speech' (desired sound source) is equidistant to the source like a broadside

array (see Figure 26) the result will be a great deal of near field speech loss.

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