LM4890IBP National Semiconductor, LM4890IBP Datasheet - Page 19

• Manufacturer Part Number: LM4890IBP
• Description: IC AMP AUDIO PWR 1W MONO 8USMD
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4890IBP

• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1W x 1 @ 8 Ohm
• Voltage - Supply: 2.2 V ~ 5.5 V
• Features: Depop, Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 8-MicroSMD
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LM4890IBPTR

Application Information
SHUTDOWN OUTPUT IMPEDANCE
For R
8.3kΩ
11.7kΩ
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 LM4890 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4890 is unity-gain stable which gives the designer
maximum system flexibility. The LM4890 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 1Vrms 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
to 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
mize turn-on pops since it determines how fast the LM4890
turns on. The slower the LM4890’s outputs ramp to their
quiescent DC voltage (nominally 1/2V
turn-on pop. Choosing C
small value of C
IN
Z
Z
Z
The -3dB roll off for these measurements is 600kHz
. A larger input coupling capacitor requires more charge
OUT1
OUT2
OUT1-2
f
= 20k ohms:
(between Out1 and GND) = 10k||50k||R
(between Out2 and GND) = 10k||(40k+(10k||R
(between Out1 and Out2) = 40k||(10k+(10k||R
BYPASS
IN
, (in the range of 0.1µF to 0.39µF), should
, is the most critical component to mini-
BYPASS
equal to 1.0µF along with a
DD
), the smaller the
(Continued)
f
IN
= 6kΩ
DD
, forms a
). This
f
f
)) =
)) =
19
produce a virtually clickless and popless shutdown function.
While the device will function properly, (no oscillations or
motorboating), with C
be much more susceptible to turn-on clicks and pops. Thus,
a value of C
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.
5V is a standard voltage which in most applications is cho-
sen for the supply rail. Extra supply voltage creates head-
room that allows the LM4890 to reproduce peaks in excess
of 1W without producing audible distortion. At this time, the
designer 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 is 20 kΩ, and with an A
gain of 3, a ratio of 1.5:1 of R
of R
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
Given:
OD BOT
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
IN
f
f
L
H
= 100Hz/5 = 20Hz
= 20 kΩ and R
= 20kHz * 5 = 100kHz
and V
BYPASS
OD TOP
equal to 1.0µF is recommended in all but
are extrapolated from the Dropout Volt-
R
BYPASS
f
f
/R
= 30 kΩ. The final design step is to
IN
opeak
±
= A
0.25 dB specified.
equal to 0.1µF, the device will
f
VD
to R
100 Hz–20 kHz
+ (V
/2
VD
IN
OD TOP
is 2.83; use A
opeak
results in an allocation
+ V
using Equation 2
OD BOT
±
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0.25 dB
1 Wrms
VD
1 Vrms
)), where
20 kΩ
= 3.
8Ω
(2)
(3)
VD