LM4911_15 Datasheet, PDF (19/30 Pages) Texas Instruments – Stereo 40mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or OCL Output

LM4911, LM4911Q
www.ti.com
APPLICATION INFORMATION
SNAS152M – MAY 2004 – REVISED JUNE 2011
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4911/LM4911Q 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 selecting the ratio of R_f to R_i.
Consequently, the gain for each channel of the IC is:
A_VD = -(R_f / R_i)
By driving the loads through outputs V_oA and V_oB with V_oC acting as a buffered bias voltage the
LM4911/LM4911Q does not require output coupling capacitors. The classical single-ended amplifier configuration
where one side of the load is connected to ground requires large, expensive output coupling capacitors.
A configuration such as the one used in the LM4911/LM4911Q has a major advantage over single supply, single-
ended amplifiers. Since the outputs V_oA, V_oB, and V_oC are all biased at 1/2 V_DD, no net DC voltage exists across
each load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-
ended amplifier configuration. 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.
OUTPUT CAPACITOR vs. CAPACITOR COUPLED
The LM4911/LM4911Q is an stereo audio power amplifier capable of operating in two distinct output modes:
capacitor coupled (C-CUPL) or output capacitor-less (OCL). The LM4911/LM4911Q may be run in capacitor
coupled mode by using a coupling capacitor on each single-ended output (V_oA and V_oB) and connecting V_oC to
ground. 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 or other single-ended (SE) load. The signal return to
circuit ground is through the headphone jack's sleeve.
The LM4911/LM4911Q can also eliminate these output coupling capacitors by running in OCL mode. Unless
shorted to ground, VoC is internally configured to apply a ½ V_DD bias voltage to a stereo headphone jack's
sleeve. This voltage matches the bias voltage present on V_oA and V_oB outputs that drive the headphones. The
headphones operate in a manner similar to a bridge-tied load (BTL). Because the same DC voltage is applied to
both headphone speaker terminals this results in no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal's output amplitude increases on the speaker's terminal.
The headphone jack's sleeve is not connected to circuit ground when used in OCL mode. Using the headphone
output jack as a line-level output will place the LM4911/LM4911Q's ½ V_DD bias voltage on a plug's sleeve
connection. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the
very small minority of equipment that is DC coupled, the LM4911/LM4911Q monitors the current supplied by the
amplifier that drives the headphone jack's sleeve. If this current exceeds 500mA_PK, the amplifier is shutdown,
protecting the LM4911/LM4911Q and the external equipment.
MODE SELECT DETAIL
The LM4911/LM4911Q may be set up to operate in one of two modes: OCL and cap-coupled. The default state
of the LM4911/LM4911Q at power up is cap-coupled. During initial power up or return from shutdown, the
LM4911/LM4911Q must detect the correct mode of operation (OCL or cap-coupled) by sensing the status of the
V_OC 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 V_OC; and at 80mV, 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. OCL mode will ramp up about 11
times faster than cap-coupled. Shutdown is not a valid command during this time period (T_WU) and should not
enabled to ensure a proper power on reset (POR) signal. In addition, the slew rate of V_DD must be greater than
2.5V/ms to ensure reliable POR. Recommended power up timing is shown in Figure 63 along with proper usage
of Shutdown and Mute. The mode select circuit is suspended during C_B discharge time.
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