LM4663MT National Semiconductor, LM4663MT Datasheet - Page 6

• Manufacturer Part Number: LM4663MT
• Description: 2 Watt Stereo Class D Audio Power Amplifier with Stereo Headphone Amplifier
• Manufacturer: National Semiconductor

www.national.com
Applications Information
Bypass Pin Capacitor
In order to achieve the largest unclipped output voltage
swing, a DC reference voltage for the circuit is derived inter-
nally by a resistor divider from the power supply voltage. To
reduce sensitivity to audio frequency variations on the refer-
ence voltage, a de-coupling capacitor is recommended be-
tween the BYPASS pin and the best available AC ground ref-
erence. This is typically the shield connection from the signal
source. A poor choice is the power ground because consid-
erable current flows from the LM4663 through pins 4,8,17,
and 21 to the power supply return. This current is rich in har-
monic frequencies related to the input audio frequency.
These harmonics can easily couple into the signal path by
sharing power ground conductors with the bypass capacitor
connection. The result is increased THD.
Minimum Load Resistance
As specified in the Absolute Maximum Ratings, the minimum
load resistance supported by the LM4663 at the output of the
filters is 2.5 Ohms. Loads (usually loudspeakers) should be
tested over the audio band to determine whether or not there
are resonant points of diminshed impedance less than this
limit.
Output Filter Components
Table 2 shows suggested standard values of the capacitor
(C) and the inductor (L) used in Figure 1’s output filter for 4
and 8
18kHz.
Short Circuit Tolerance
The LM4663, when used in the recommended application
circuit, is tolerant of limited duration short circuit connections
between the filtered positive and negative outputs. Shorts
from either output to ground or supply voltage, or from the
unfiltered outputs to any other low impedance node can re-
sult in permanent damage to the IC.
Start-up Current
The LM4663 exhibits abnormally high quiescent current
drain when the supply voltage is below the specified operat-
ing range of 4.5 to 5.5V. It is, therefore, recommended that
the IC be initially powered up in the shutdown mode (SD =
V
supplies is not usually a problem unless the current limit of
the supply is set too low (
have a much lower current capability (such as USB applica-
tions). In these cases power must be applied with shutdown
asserted and switched to the operating mode after power is
present and settled. An external circuit that provides this
condition automatically is given in Figure 2, and is recom-
mended for those current-critical applications.
Printed Circuit Board Layout
Figures 3 through 7 show the layout of a recommended
four-layer PC board that is optimized for the LM4663 and as-
sociated external components. This layout emphasizes short
connections between the power reservoir capacitors near
Load Resistance ( ) Capacitor C (µF) Inductor L (µH)
TABLE 2. Output filter component values for different
DD
). Normal application of power from laboratory bench
loads. The bandwidth for each case is typically
4
8
load resistance.
<
1A); however, some applications
2
1
(Continued)
22
44
6
the power amp outputs and their associated pins. Short con-
nections reduce inductance and EMI radiation from the out-
put switching currents. Also, the distance from the switching
outputs to the filter inductors is similarly minimized to reduce
radiated noise. Table 4 is the recommended circuit’s
bill-of-materials (BOM). Figure 8 is a photograph of the rec-
ommended four-layer PC board.
The board is designed for use with an external 5V supply, 4
speakers, and 32 headphones. Apply the supply voltage to
the V
channel speaker between the Lout+ and Lout− pads and the
right channel speaker between the Rout+ and Rout− pads.
Connect the left and right headphone speakers to the HPL
and HPR pads, respectively. The board has two sets of HPL
and HPR pads. Use the set found across the top edge for a
headphone jack like that shown in Figure 1. The other set
found along the right edge can be used for a three terminal
headphone jack. Ensure that a speaker’s + terminal is con-
nected to an amplifier’s + output. This preserves the phase
relationship between the left and right channels.
The board accepts two stereo inputs. Apply channel 1’s right
and left input signals to the Rin1 and Lin1 pads, respectively.
Apply channel 2’s right and left input signals to the Rin2 and
Lin2 pads, respectively.
The board’s bottom edge has seven jumpers. From left to
right, these jumpers select micropower shutdown, input
channel, headphone amplifier, Class D amplifier, and head-
phone amplifier when plugging in headphones. Table 3 is a
guide for selecting the various functions.
Jumper
SD-L
SD-H
INSEL-L
INSEL-H
Class AB
Class D
HP Jack
TABLE 3. These are the functions selected by the
DD
jumpers on the recommended PC board.
pad and ground to the GND pad. Connect the left
Function
Shorting selects normal operation.
SD-H Jumper is open.
Shorting activates shutdown:
micropower operation.
SD-L Jumper is open.
Shorting selects stereo input 1.
INSEL-H Jumper is open.
Shorting selects stereo input 2.
INSEL-L Jumper is open.
Shorting activates the headphone
amplifier and deactivates the Class D
amplifier. Class D and HP jack
jumpers are open
Shorting activates the Class D amplifier
and deactivates the headphone
amplifier. Class AB and HP jack
jumpers are open
Shorting allows the act of plugging in
headphones to activate the headphone
amplifier and deactivate the Class D
amplifier. Removing headphones
restores Class D amplifier operation and
deactivates the headphone amplifier.
Class AB and Class D jumpers are
open. (Requires the use of a headphone
jack like that shown in Figure 1.)