HWD2119 Datasheet, PDF (10/21 Pages) List of Unclassifed Manufacturers – 350mWAudio Power Amplifier with Shutdown Mode

Application Information (Continued)
There are a few ways to activate micro-power shutdown.
These included using a single-pole, single-throw switch, a
microcontroller, or a microprocessor. When using a switch,
connect an external 10kΩ to 100kΩ pull-up resistor between
the SHUTDOWN pin and V_DD. Connect the switch between
the SHUTDOWN pin and ground. Select normal amplifier
operation by closing the switch. Opening the switch con-
nects the shutdown pin to V_DD through the pull-up resistor,
activating micro-power shutdown. The switch and resistor
guarantee that the SHUTDOWN pin will not float. This pre-
vents unwanted state changes. In a system with a micropro-
cessor or a microcontroller, use a digital output to apply the
control voltage to the SHUTDOWN pin. Driving the SHUT-
DOWN pin with active circuitry eliminates the pull-up resistor
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the HWD2119’s performance requires properly se-
lecting external components. Though the HWD2119 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The HWD2119 is unity gain stable, giving the designer maxi-
mum 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
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V_RMS (2.83V_P-P). Please refer to the
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Another important consideration is the amplifier’s close-loop
bandwidth. To a large extent, the bandwidth is dictated by
the choice of external components shown in Figure 1. The
input coupling capacitor, C_i, forms a first order high pass filter
that limits low frequency response. This value should be
chosen based on needed frequency response for a few
distinct reasons discussed below
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (C_i in Figure 1). A high value
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with limited frequency response reap little
improvement by using a large input capacitor.
Besides affecting system cost and size, C_i has an effect on
the HWD2119’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 value. Higher value capacitors need
more time to reach a quiescent DC voltage (usually 1/2 V_DD)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R_F. Thus, selecting an input capacitor value that is no higher
than necessary to meet the desired -3dB frequency can
minimize pops.
As shown in Figure 1, the input resistor (R_i) and the input
capacitor, C_i produce a -3dB high pass filter cutoff frequency
that is found using Equation (5).
f_-3dB = 1/(2 πR_iC_i) (Hz)
(5)
As an example when using a speaker with a low frequency
limit of 150Hz, C_i, using Equation (5) is 0.063µF. The 0.39µF
C_i shown in Figure 1 allows the HWD2119 to drive a high
efficiency, full range speaker whose response extends down
to 20Hz.
Besides optimizing the input capacitor value, the bypass
capacitor value, C_B requires careful consideration. The by-
pass capacitor’s value is the most critical to minimizing
turn-on pops because it determines how fast the HWD2119
turns on. The slower the HWD2119’s outputs ramp to their
quiescent DC voltage (nominally 1/2V_DD), the smaller the
turn-on pop. While the device will function properly (no os-
cillations or motorboating), with C_B less than 1.0µF, the
device will be much more susceptible to turn-on clicks and
pops. Thus, a value of C_B equal to or greater than 1.0µF is
recommended in all but the most cost sensitive designs.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C_B, the capacitor
connected to the BYPASS pin. Since C_B determines how
fast the HWD2119 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
HWD2119’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2V_DD), the smaller the turn-on pop. Choosing C_B
equal to 1.0µF along with a small value of C_i (in the range of
0.1µF to 0.39µF) produces a click-less and pop-less shut-
down function. As discussed above, choosing C_i no larger
than necessary for the desired bandwidth helps minimize
clicks and pops.
Optimizing Click and Pop Reduction Performance
The HWD2119 contains circuitry that minimizes turn-on and
shutdown transients or ’clicks and pops’. For this discussion,
turn on refers to either applying the power or supply voltage
or when the shutdown mode is deactivated. While the power
supply is ramping to it’s final value, the HWD2119’s internal
amplifiers are configured as unity gain buffers. An internal
current source charges the voltage of the bypass capacitor,
C_B, connected to the BYPASS pin in a controlled, linear
manner. Ideally, the input and outputs track the voltage
charging on the bypass capacitor. The gain of the internal
amplifiers remains unity until the bypass capacitor is fully
charged to 1/2V_DD. As soon as the voltage on the bypass
capacitor is stable, the device becomes fully operational.
Although the BYPASS pin current cannot be modified,
changing the size of the bypass capacitor, C_B, alters the
device’s turn-on time and magnitude of ’clicks and pops’.
Increasing the value of C_B reduces the magnitude of turn-on
pops. However, this presents a tradeoff: as the size of C_B
increases, the turn-on time (Ton) increases. There is a linear
relationship between the size of C_B and the turn on time.
Below are some typical turn-on times for various values of
C_B:
C_B
0.01µF
0.1µF
0.22µF
0.47µF
1.0µF
T_ON
20ms
200ms
440ms
940ms
2S
10