Application Information (Continued)
CB
4.7 µF
TON
1sec
put signals to obtain a given output power. Input signals
equal to or greater than 1Vrms are available from sources
such as audio codecs.
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de-
vice or shutdown function may cause the “click and pop” cir-
cuitry to not operate fully, resulting in increased “click and
pop” noise.
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 3. Both the input coupling capacitor, CI, and
the output coupling capacitor form first order high pass filters
which limit low frequency response given in Equations 4 and
5.
In systems where the line out and headphone jack are the
same, the output coupling cap, CO, is of particular concern.
CO is chosen for a desired cutoff frequency with a head-
phone load. This desired cutoff frequency will change when
the headphone load is replaced by a high impedance line out
load(powered speakers). The input impedance of head-
phones are typically between 32Ω and 64Ω. Whereas, the
input impedance of powered speakers can vary from 1kΩ to
100kΩ. As the RC time constant of the load and the output
coupling capacitor increases, the turn off transients are in-
creased.
fIC = 1/(2πRiCi) (4)
fOC = 1/(2πRLCO) (5)
These values should be chosen based on required fre-
quency response.
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without se-
vere attenuation. In many cases the speakers used in por-
table systems, whether internal or external, have little ability
to reproduce signals below 100 Hz–150 Hz. In this case, us-
inga large input or output capacitor may not increase system
performance.
To improve click and pop performance in this situation, exter-
nal resistor R7 should be added as shown in Figure 4. The
recommended value for R7 is between 150Ω to 1kΩ. To
achieve virtually clickless and popless performance R7 =
150Ω, CO = 220µF, and CB = 1.0µF should be used. Lower
values of R6 will result in better click and pop performance.
However, it should be understood that lower resistance val-
ues of R7 will increase current consumption.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
Ci. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 VDD.) This
charge comes from the output through the feedback and is
apt to create pops once the device is enabled. By minimizing
the capacitor size based on necessary low frequency re-
sponse, turn-on pops can be minimized.
CLICK AND POP CIRCUITRY
The LM4835 contains circuitry to minimize turn-on transients
or “click and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the device is turning on, the amplifiers are inter-
nally muted. An internal current source ramps up the voltage
of the bypass pin. Both the inputs and outputs ideally track
the voltage at the bypass pin. The device will remain in mute
mode until the bypass pin has reached its half supply volt-
age, 1/2 VDD. As soon as the bypass node is stable, the de-
vice will become fully operational.
DS100139-5
FIGURE 3. Resistor for Varying Output Loads
DOCKING STATION
In an application such as a notebook computer, docking sta-
tion or line level outputs may be required. Pin 9 and Pin 13
can drive loads greater than 1kΩ rail to rail. These pins are
tied to the output of the input op-amp to drive powered
speakers and other high impedance loads. Output coupling
capacitors need to be placed in series with the load. The rec-
ommended values of the capacitors are between 0.33µF to
1.0µF with the positive side of the capacitors toward the IC.
The outputs of the docking station pins cannot be attenuated
with the DC volume control. However the gain of the outputs
can be configured by adjusting the feedback and input resis-
tors for the input op-amp. The input op-amp is in an inverting
configuration where the gain is:
Although the bypass pin current source cannot be modified,
the size of the bypass capacitor, CB, can be changed to alter
the device turn-on time and the amount of “click and pop”. By
increasing CB, the amount of turn-on pop can be reduced.
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing CB
will decrease turn-on time and increase “click and pop”.
There is a linear relationship between the size of CB and the
turn-on time. Here are some typical turn-on times for differ-
ent values of CB:
CB
TON
0.01 µF
0.1 µF
0.22 µF
0.47 µF
1.0 µF
2 ms
RF / Ri = - Av
20 ms
42 ms
84 ms
200 ms
Note that by adjusting the gain of the input op-amp the over-
all gain of the output amplifiers are also affected. Although
the single ended outputs of the output amplifiers can be used
to drive line level outputs, it is recommended to use Pins 9
and 13 to achieve better performance.
11
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