LM4884MHX/NOPB National Semiconductor, LM4884MHX/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM4884MHX/NOPB
• Description: IC AMP AUDIO 2.6W STER 20TSSOP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4884MHX/NOPB

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 2.6W x 2 @ 3 Ohm
• Voltage - Supply: 3 V ~ 5.5 V
• Features: Depop, Differential Inputs, Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 2.6x2@3OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.3@4Ohm@2W%
• Single Supply Voltage (typ): 5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 62dB
• Single Supply Voltage (min): 3V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 20
• Package Type: TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4884MHX

Application Information
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4884’s performance requires properly se-
lecting external components. Though the LM4884 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues. The LM4884 is unity-gain stable, giving a designer
maximum design flexibility. The gain should be set to no
more than a given application requires. This allows the am-
plifier to achieve minimum THD+N and maximum signal-to-
noise ratio. These parameters are compromised as the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maxi-
mum output power. Fortunately, many signal sources such
as audio CODECs have outputs of 1V
Please refer to the Audio Power Amplifier Design section for
more information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
high value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals with
frequencies below 150Hz. Applications using speakers with
this limited frequency response reap little improvement by
using large input capacitor.
Besides effecting system cost and size, CI - C4 can also
affect on the LM4884’s turn-on and turn-off transient ("click
and pop") performance. When the supply voltage is first
applied, a transient may be created as the charge on the
input capacitor changes from zero to a quiescent state. The
magnitude of the transient is proportional to the value of, and
more importantly, the mismatch between, the capacitors
connected to a given pair of inverting and non-inverting
inputs. The better the match, the less the transient magni-
tude.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
rent. This fixed current is supplied through amplifiers input
pins. Thus, selecting an input capacitor value that is no
higher than necessary to meet the desired -3dB frequency
will reduce turn-on time and help ensure that transients are
minimized.
The LM4884’s nominal input resistance (R
minimum) and the input capacitor, C
with a -3dB low frequency limit defined by equation (5).
Table 2. Logic Level Truth Table for Shutdown
SHUTDOWN
High
Low
f
-3dB
DD
Operation
/2) when charged with a fixed cur-
= 1/2π(25kΩ)C
I
, C
2
Full Power, stereo
OPERATIONAL
and C
BTL amplifiers
Micro-power
Shutdown
i
, form high pass filter
MODE
3
i
, C
(Continued)
i
) is 25kΩ (20kΩ,
RMS
4
) in Figure 1. A
(2.83V
P-P
(5)
).
17
As an example when using a speaker with a low frequency
limit of 150Hz, C
1 allows the LM4884 to drive high efficiency, full range
speaker whose response extends below 30Hz.
Bypass Capacitor Value Selection
Besides optimizing the input capacitor value, careful consid-
eration should be paid to value of C
nected between the BYPASS pin and ground. Since C
determines how fast the LM4884 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4884’s outputs ramp to their
quiescent DC voltage (nominally
turn-on transient. Choosing C
small value of C
duces a transient-free turn-on and shutdown function. As
discussed above, choosing C
the desired bandwidth helps minimize turn-on transients.
OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK
AND POP PERFORMANCE)
The LM4884 contains circuitry to minimize turn-on and shut-
down transients or ’clicks and pop’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply voltage is ramping to its final value, the LM4884’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the BYPASS
pin in a controlled, linear manner. Ideally, the amplifier inputs
and outputs track the voltage applied to the BYPASS pin.
The gain of the internal amplifiers remains unity until the
voltage on the bypass pin reaches 1/2 V
voltage on the BYPASS pin is stable, the device becomes
fully operational. Although the bypass pin current can not be
modified, changing the size of C
time and the magnitude of output transients. Increasing the
value of C
However, this presents a tradeoff: as the size of C
creases, the turn-on time increases. There is a linear rela-
tionships between the size of C
time. The table shows some typical turn-on times for various
values of C
In order to eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
’clicks and pops’.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Power Output:
Load Impedance:
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
B
B
B
:
reduces the magnitude of turn-on transients.
I
, is 0.047µF. The 0.47µF C
i
(in the range of 0.047µF to 0.47µF), pro-
C
i
120ms
140ms
170ms
240ms
= 0.47µF
110ms
B
i
no larger than necessary for
equal to 0.47µF along with a
B
B
alters the device’s turn-on
1
⁄
+ 2(C
2
Ton
V
B
DD
, the capacitor con-
DD
I
), the smaller the
) and the turn-on
I
C
. As soon as the
shown in Figure
i
100ms
140ms
210ms
= 0.33µF
80ms
90ms
DD
www.national.com
1 W
may not
B
RMS
8Ω
in-
B