LM4992SDBD/NOPB National Semiconductor, LM4992SDBD/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM4992SDBD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4992SDBD/NOPB

• Lead Free Status / Rohs Status: Compliant

Application Information
typical value of 0.2µA. 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 to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor. This scheme guarantees 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 LM4992 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4992 is unity-gain stable which gives the designer
maximum system flexibility. The LM4992 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 com-
plete 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 band-
width 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
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
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 1/2 V
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 LM4992 turns
on. The slower the LM4992’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
Choosing C
the range of 0.1µF to 0.39µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
i.
A larger input coupling capacitor requires more charge to
B
B
, is the most critical component to minimize
equal to 1.0µF along with a small value of C
DD
), the smaller the turn-on pop.
(Continued)
i
DD
, forms a
). This
i
(in
11
C
to turn-on clicks and pops. Thus, a value of C
1.0µF is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
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.
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4992 to reproduce peaks in excess of 1W
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 2.
From Equation 2, the minimum A
Since the desired input impedance was 20 kΩ, and with a
A
allocation of R
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required
As stated in the External Components section, R
junction with C
The high frequency pole is determined by the product of the
desired frequency pole, f
With a A
300kHz which is much smaller than the LM4992 GBWP of
1.5MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4992 can still be used without running into bandwidth
limitations.
The LM4992 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor (C4) may be
needed as shown in Figure 2 to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that elimi-
Given:
VD
B
equal to 0.1µF, the device will be much more susceptible
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
f
f
C
impedance of 2, a ratio of 1.5:1 of R
L
H
i
= 100 Hz/5 = 20 Hz
= 20 kHz * 5 = 100 kHz
≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
VD
= 3 and f
i
i
= 20 kΩ and R
create a highpass filter.
R
H
f
/R
= 100 kHz, the resulting GBWP =
i
H
= A
, and the differential gain, A
f
VD
= 30 kΩ. The final design step
100 Hz–20 kHz
/2
VD
±
0.25 dB specified.
is 2.83; use A
f
to R
i
results in an
±
www.national.com
B
0.25 dB
1 Wrms
VD
1 Vrms
equal to
i
20 kΩ
in con-
= 3.
8Ω
VD
(2)
.