LM4888 NSC [National Semiconductor], LM4888 Datasheet - Page 16

• Manufacturer Part Number: LM4888
• Description: Dual 2.1W Audio Amplifier Plus Stereo Headphone & 3D Enhancement
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
HEADPHONE SENSE AND HEADPHONE LOGIC IN
FUNCTIONS
Applying a logic level to the LM4888’s HP Sense headphone
control pin turns off Amp A (+out) and Amp B (+out) muting a
bridged-connected load. Quiescent current consumption is
reduced when the IC is in this single-ended mode.
Figure 2 shows the implementation of the LM4888’s head-
phone control function. With no headphones connected to
the headphone jack, the R11-R13 voltage divider sets the
voltage applied to the HP Sense pin (pin 20) at approxi-
mately 50mV. This 50mV enables Amp A (+out) and Amp B
(+out) placing the LM4888 in bridged mode operation.
While the LM4888 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Therefore, even in an
ideal situation, the output swing cannot cause a false single-
ended trigger. Connecting headphones to the headphone
jack disconnects the headphone jack contact pin from
−OUTA and allows R13 to pull the HP Sense pin up to V
This enables the headphone function, turns off Amp A (+out)
and Amp B (+out) which mutes the bridged speaker. The
amplifier then drives the headphones, whose impedance is
in parallel with resistors R10 and R11. These resistors have
negligible effect on the LM4888’s output drive capability
since the typical impedance of headphones is 32Ω.
Figure 2 also shows the suggested headphone jack electri-
cal connections. The jack is designed to mate with a three-
wire plug. The plug’s tip and ring should each carry one of
the two stereo output signals, whereas the sleeve should
carry the ground return. A headphone jack with one control
pin contact is sufficient to drive the HP Sense pin when
connecting headphones.
There is also a second input circuit that can control the
choice of either BTL or SE modes. This input control pin is
called the HP (Headphone) Logic Input. When the HP Logic
input is high, LM4888 operates in SE mode. When HP Logic
is low (& the HP Sense pin is low), the LM4888 operates in
the BTL mode. In the BTL mode (HP Logic low and HP
Sense Low) if the Headphones are connected directly to the
Single Ended outputs (not using the HP Sense pin on the HP
Jack) then both the Speaker (BTL) and Headphone (SE) will
be functional. In this case the inverted op amp outputs drive
the Speaker as well as the HP load, i.e. 8 ohms in parallel
with 32 ohms. As the LM4888 is capable of driving up to a 3
ohm load driving the Speakers and the Headphones at the
same time will not be a problem as long as the parallel
resistance of each Speaker and each Headphone driver are
more than 3 ohms.
As outlined above driving the Speaker (BTL) and Head-
phone (SE) loads simultaneously using LM4888 is simple
and easy. However this configuration will only work if the HP
Logic pin is used to control the BTL/SE operation and HP
Sense pin is connected to GND.
SHUTDOWN
Logic High
Logic High
Logic High
Logic Low
PIN
HEADPHONE
LOGIC PIN
Don’t Care
Don’t Care
(Continued)
High
Low
TABLE 1. Logic Level Truth Table
DD
.
Low (HP not plugged in)
16
High (HP plugged in)
JACK SENSE PIN
HEADPHONE
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4888’s performance requires properly se-
lecting external components. Though the LM4888 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4888 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
CODECs have outputs of 1V
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C1 and C2) in Figure 1. A high
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals below
Don’t Care
Don’t Care
FIGURE 2. Headphone Circuit
OPERATIONAL OUTPUT
Micro-Power Shutdown
RMS
SINGLE ENDED
SINGLE ENDED
BRIDGED/BTL
(2.83V
MODE
P-P
). Please refer to
20111624