LM4869 NSC [National Semiconductor], LM4869 Datasheet - Page 16

• Manufacturer Part Number: LM4869
• Description: 1.9W Differential Input, BTL Output Stereo Audio Amplifier with Selectable Gain and Shutdown
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
pin that is as near to GND as possible. A voltage that is
greater than GND may increase the shutdown current.
There are a few methods to control the micro-power shut-
down. These include using a single-pole, single-throw switch
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩ pull-down resistor between the
SHUTDOWN pin and GND and the SPST switch between
the SHUTDOWN pin and V
eration by closing the switch. Opening the switch applies
GND to the SHUTDOWN pin, activating micro-power shut-
down. The switch and resistor guarantee that the SHUT-
DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the active-state voltage to
the SHUTDOWN pin. Driving the SHUTDOWN pin with ac-
tive circuitry eliminates the pull-down resistor.
Table 1. LOGIC LEVEL TRUTH TABLE FOR
SHUTDOWN OPERATION
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4869’s performance requires properly se-
lecting external components. Though the LM4869 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues. The LM4869 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 LM4869’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
SHUTDOWN
High
Low
DD
I
, C
. Select normal amplifier op-
2
Full Power, stereo
OPERATIONAL
and C
BTL amplifiers
Micro-power
Shutdown
MODE
3
, C
(Continued)
RMS
4
) in Figure 1. A
(2.83V
P-P
).
16
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 LM4869’s nominal input resistance (R
minimum) and the input capacitor, C
with a -3dB low frequency limit defined by equation (5).
As an example when using a speaker with a low frequency
limit of 150Hz, C
1 allows the LM4869 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 LM4869 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4869’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 LM4869 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 LM4869’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
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-
f
-3dB
DD
/2) when charged with a fixed cur-
= 1/2π(25kΩ)C
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
i
, form high pass filter
V
B
DD
, the capacitor con-
i
DD
I
), the smaller the
) and the turn-on
i
) is 25kΩ (20kΩ,
I
. As soon as the
shown in Figure
B
(5)
in-
B