LM49120, LM49120TLEVAL

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SNAS431C – JUNE 2008 – REVISED MAY 2013

The OCL architecture eliminates the bulky, expensive output coupling capacitors required by traditional

headphone amplifiers. In OCL mode, the LM49120 headphone section uses three amplifiers. Two amplifiers

drive the headphones, while the third (VOC) is set to the internally generated bias voltage (typically VDD/2). The

third amplifier is connected to the return terminal of the headphone jack (Figure 1). In this configuration, the

signal side of the headphone is biased to VDD/2, the return is biased to VDD/2, thus there is no net DC voltage

across the headphone, eliminating the need for an output coupling capacitor. Removing the output coupling

capacitors from the headphone signal path reduces component count, reducing system cost and board space

consumption, as well as improving low frequency performance.

In OCL mode, the headphone return sleeve is biased to VDD/2. When driving headphones, the voltage on the

return sleeve is not an issue. However, if the headphone output is used as a line out, the VDD/2 can conflict with

the GND potential that the line-in would expect on the return sleeve. When the return of the headphone jack is

connected to GND the VOC amplifier of the LM49120 detects an output short circuit condition and is disabled,

preventing damage to the LM49120, and allowing the headphone return to be biased at GND.

Single-Ended, Capacitor Coupled Mode

In single-ended mode, the VOC amplifier is disabled, and the headphone outputs are coupled to the jack through

series capacitors, allowing the headphone return to be connected to GND (Figure 2). In SE mode, the LM49120

requires output coupling capacitors to block the DC component of the amplifier output, preventing DC current

from flowing to the load. The output capacitor and speaker impedance form a high pass filter with a -3dB roll-off

determined by:

f–3dB = 1 / 2πRLCO (Hz)

(1)

Where RL is the headphone impedance, and CO is the value of the output coupling capacitor. Choose CO such

that f-3dB is well below the lowest frequency of interest. Setting f-3dB too high results in poor low frequency

performance. Select capacitor dielectric types with low ESR to minimize signal loss due to capacitor series

resistance and maximize power transfer to the load.

Headphone Amplifier BTL Mode

The LM49120 headphone amplifiers feature a BTL mode where the two headphone outputs, LOUT and ROUT are

configured to drive a mono speaker differentially. In BTL mode, the amplifier accepts audio signals from either

the differential MONO inputs, or the single-ended stereo inputs, and converts them to a mono BTL output.

However, if the stereo inputs are 180° out of phase, no audio will be present at the amplifier outputs. Bit B3

(HP/BTL) in the Shutdown Control Register determines the headphone output mode. Set HP/BTL = 0 for stereo

headphone mode, set HP/BTL = 1 for BTL mode.

Input Mixer/Multiplexer

The LM49120 includes a comprehensive mixer multiplexer controlled through the I2C interface. The

mixer/multiplexer allows any input combination to appear on any output of the LM49120. Multiple input paths can

be selected simultaneously. Under these conditions, the selected inputs are mixed together and output on the

selected channel. Table 4 and Table 5 show how the input signals are mixed together for each possible input

selection.

Audio Amplifier Gain Setting

Each channel of the LM49120 has two separate gain stages. Each input stage features a 32-step volume control

with a range of -46dB to +18dB (Table 6). The loudspeaker output stage has two additional gain settings: 0dB

and +6dB (Table 8) when the differential MONO input is selected, and +6dB and +12dB when the single-ended

stereo inputs are selected. The headphone gain is not affected by the input mode. Each headphone output stage

has 8 gain settings (Table 7). This allows for a maximum separation of 22dB between the speaker and

headphone outputs when both are active.

Calculate the total gain of the given signal path as follows:

AVOL + AVOS = AVTOTAL (dB)

where

•

AVOL is the volume control level, AVOS is the output stage gain setting

•

AVTOTAL is the total gain for the signal path.

(2)

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