LM4911MM datasheet(16/22 Pages) NSC

16 / 22 page

Application Information

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4911 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

VD = -(Rf /Ri)

By driving the loads through outputs V

oA and VoB with VoC

acting as a buffered bias voltage the LM4911 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 cou-

pling capacitors.

A configuration such as the one used in the LM4911 has a

major advantage over single supply, single-ended amplifiers.

Since the outputs V

oA, VoB, and VoC are all biased at 1/2

DD, no net DC voltage exists across each load. This elimi-

nates the need for output coupling capacitors which 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.

OUTPUT CAPACITOR vs. CAPACITOR COUPLED

The LM4911 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 may

be run in capacitor coupled mode by using a coupling ca-

pacitor on each single-ended output (V

oA and VoB) 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 can also eliminate these output coupling ca-

pacitors by running in OCL mode. Unless shorted to ground,

VoC is internally configured to apply a 1⁄2 V

DD bias voltage to

a stereo headphone jack’s sleeve. This voltage matches the

bias voltage present on V

oA and VoB 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’s 1⁄2

DD bias voltage on a plug’s sleeve connection. This pre-

sents no difficulty when the external equipment uses capaci-

tively coupled inputs. For the very small minority of equip-

ment that is DC coupled, the LM4911 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 and the external equip-

ment.

MODE SELECT DETAIL

The LM4911 may be set up to operate in one of two modes:

OCL and cap-coupled. The default state of the LM4911 at

power up is cap-coupled. During initial power up or return

from shutdown, the LM4911 must detect the correct mode of

operation (OCL or cap-coupled) by sensing the status of the

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

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. Recom-

mended power up timing is shown in Figure 5 along with

proper usage of Shutdown and Mute. The mode select cir-

cuit is suspended during C

B discharge time.

The circuit shown in Figure 4 presents an applications solu-

tion to the problem of using different supply voltages with

different turn-on times in a system with the LM4911. This

circuit shows the LM4911 with a 25-50k

Ω pull-up resistor

connected from the shutdown pin to V

DD. The shutdown pin

of the LM4911 is also being driven by an open drain output of

an external microcontroller on a separate supply. This circuit

ensures that shutdown is disabled when powering up the

LM4911 by either allowing shutdown to be high before the

LM4911 powers on (the microcontroller powers up first) or

allows shutdown to ramp up with V

DD (the LM4911 powers

up first). This will ensure the LM4911 powers up properly and

enters the correct mode of operation (cap-coupled or OCL).

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