lm4902ld National Semiconductor Corporation, lm4902ld Datasheet - Page 14

• Manufacturer Part Number: lm4902ld
• Description: 265mw At 3.3v Supply Audio Power Amplifier With Shutdown Mode
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Typical applications employ a 5V regulator with 10µF and a
0.1µF bypass capacitors which aid in supply stability, but do
not eliminate the need for bypassing the supply nodes of the
LM4902. The selection of bypass capacitors, especially C
is thus dependent upon desired PSRR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4902 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic low is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to GND, the LM4902 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages greater than GND, the
idle current may be greater than the typical value of 0.1µA. In
either case, the shutdown pin should be tied to a definite
voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another so-
lution is to use a single-pole, single-throw switch in conjunc-
tion with an external pull-up resistor. When the switch is
closed, the shutdown pin is connected to ground and dis-
ables the amplifier. If the switch is open, then the external
pull-up resistor will enable the LM4902. This scheme guar-
antees that the shutdown pin will not float, thus preventing
unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4902 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall
system quality.
The LM4902 is unity-gain stable, giving a designer maximum
system flexibility. The LM4902 should be used in low gain
configurations to minimize THD+N values, and maximize the
signal to noise ratio. Low gain configurations require large
input signals to obtain a given output power. Input signals
equal to or greater than 1 Vrms are available from sources
such as audio codecs. Please refer to the section, Audio
Power Amplifier Design, for a more complete explanation
of proper gain selection.
Besides gain, one of the major considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in Figure 1 . The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Selection of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
(Continued)
i
, forms a
B
,
14
systems, whether internal or external, have little ability to
reproduce signals below 150Hz. In this case using a large
input capacitor may not increase system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (nominally
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
turn-on pops since it determines how fast the LM4902 turns
on. The slower the LM4902’s outputs ramp to their quiescent
DC voltage (nominally
Choosing C
(in the range of 0.1µF to 0.39µF), should produce a clickless
and popless shutdown function. While the device will func-
tion properly, (no oscillations or motorboating), with C
to 0.1µF, the device will be much more susceptible to turn-on
clicks and pops. Thus, a value of C
is recommended in all but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a 300 mW/8
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
and add the dropout voltage. Using this method, the mini-
mum supply voltage would be (V
V
Voltage curve in the Typical Performance Characteristics
section.
Using the Output Power vs Supply Voltage graph for an 8
load, the minimum supply rail is 3.5V. But since 5V is a
standard supply voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4902 to reproduce peaks in excess of 700 mW
without producing audible distortion. At this time, the de-
signer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 5.
Given:
OD
i
. A larger input coupling capacitor requires more charge to
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
is extrapolated from the Dropout Voltage vs Supply
B
B
, is the most critical component to minimize
equal to 1.0 µF along with a small value of C
1
Audio Amplifier
⁄
2
V
DD
), the smaller the turn-on pop.
100Hz–20 kHz
B
opeak
equal to 1.0µF or larger
opeak
+ (2
using Equation 4
*
1
V
⁄
2
300mWrms
OD
V
±
DD
)), where
0.25dB
B
1Vrms
). This
20k
equal
8
(4)
(5)
i