LM4816MTBD National Semiconductor, LM4816MTBD Datasheet - Page 10

• Manufacturer Part Number: LM4816MTBD
• Description: BOARD EVALUATION LM4816MT
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4816MTBD

• Amplifier Type: Class AB
• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 1.5W x 2 @ 8 Ohm
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4816
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4816’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation in the output signal. Keep the length of leads and
traces that connect capacitors between the LM4816’s power
supply pin and ground as short as possible. Connecting a
1µF capacitor, C
improves the internal bias voltage’s stability and improves
the amplifier’s PSRR. The PSRR improvements increase as
the bypass pin capacitor value increases. Too large, how-
ever, increases turn-on time and can compromise amplifier’s
click and pop performance. The selection of bypass capaci-
tor values, especially C
ments, click and pop performance (as explained in the sec-
tion, Proper Selection of External Components), system
cost, and size constraints.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4816’s shutdown function. Activate micro-power shut-
down by applying V
the LM4816’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
shutdown current is achieved by applying a voltage that is as
near as V
thrat is less than V
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 10kΩ pull-up resistor between the
SHUTDOWN pin and V
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
ing micro-power shutdown. The switch and resistor guaran-
tee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
TABLE 1. LOGIC LEVEL TRUTH TABLE FOR SHUT-
DOWN OPERATION
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4816’s performance requires properly se-
lecting external components. Though the LM4816 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4816 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 amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
SHUTDOWN
DD
as possible to the SHUTDOWN pin. A voltage
High
Low
B
, between the BYPASS pin and ground
DD
DD
DD
may increase the shutdown current.
to the SHUTDOWN pin. When active,
B
DD
, depends on desired PSRR require-
through the pull-up resistor, activat-
. Connect the switch between the
OPERATIONAL MODE
Full power, stereo BTL
Micro-power Shutdown
DD
/2. The low 0.6µA typical
amplifiers
(Continued)
10
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
the LM4816’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (R
capacitor, C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
ciency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
nected to the BYPASS pin. Since C
the LM4816 settles to quiescent operation, its value is critical
when minimizing turn−on pops. The slower the LM4816’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
1.0µF along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4816 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 is ramping to its final value, the LM4816’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 input and outputs track
DD
f
I
. Thus, pops can be minimized by selecting an input
shown in Figure 1 allows the LM4816 to drive high effi-
), the smaller the turn−on pop. Choosing C
I
produce a −3dB high pass filter cutoff frequency
I
, using Equation (4), is 0.063µF. The 1.0µF
RMS
i
in Figure 1). A high value capaci-
(2.83V
i
(in the range of 0.1µF to
P-P
B
i
B
no larger than neces-
determines how fast
). Please refer to the
, the capacitor con-
i
has an affect on
I
) and the input
B
equal to
DD
(7)
/2)