LM4892 NSC [National Semiconductor], LM4892 Datasheet - Page 12

• Manufacturer Part Number: LM4892
• Description: 1 Watt Audio Power Amplifier with Headphone Sense
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
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 LM4892 turns
on. The slower the LM4892’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
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. A second way to determine the minimum sup-
ply rail is to calculate the required V
and add the output voltage. Using this method, the minimum
supply voltage would be (V
V
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
B
Given:
OD BOT
equal to 0.1µF, the device will be much more susceptible
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
and V
B
B
, is the most critical component to minimize
equal to 1.0µF along with a small value of C
OD TOP
are extrapolated from the Dropout Volt-
opeak
DD
), the smaller the turn-on pop.
100 Hz–20 kHz
+ (V
OD TOP
opeak
+ V
(Continued)
using Equation 2
OD BOT
±
B
1 Wrms
0.25 dB
1 Vrms
)), where
equal to
20 kΩ
8Ω
i
(in
12
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4892 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 3.
From Equation 3, the minimum A
Since the desired input impedance was 20kΩ, and with a
A
R
the bandwidth requirements which must be stated as a pair
of −3dB frequency points. Five times away from a −3dB point
is 0.17dB 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
150kHz which is much smaller than the LM4892 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4892 can still be used without running into bandwidth
limitations.
VD
i
= 20kΩ and R
of 3, a ratio of 1.5:1 of R
f
f
C
L
H
i
= 100Hz/5 = 20Hz
= 20kHz * 5 = 100kHz
≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
VD
= 3 and f
i
create a highpass filter.
f
= 30kΩ. The final design step is to address
±
0.25dB specified.
R
f
H
/R
i
= 100kHz, the resulting GBWP =
H
= A
, and the differential gain, A
f
to R
VD
/2
VD
i
results in an allocation of
is 2.83; use A
VD
i
in con-
= 3.
VD
(2)
(3)
.