C

reach its quiescent DC voltage (nominally 1/2 V

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 LM4834 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 configured as unity gain buffers. An internal current

source ramps up the voltage of the bypass pin. Both the in-

puts and outputs ideally track the voltage at the bypass pin.

The device will remain in buffer mode until the bypass pin

has reached its half supply voltage, 1/2 V

bypass node is stable, the device will become fully opera-

tional.

Although the bypass pin current source cannot be modified,

the size of the bypass capacitor, C

the device turn-on time and the amount of “click and pop”. By

increasing C

However, the trade-off for using a larger bypass capacitor is

an increase in the turn-on time for the device. Reducing C

B

will decrease turn-on time and increase “click and pop”.

There is a linear relationship between the size of C

turn-on time. Here are some typical turn-on times for differ-

ent values of C

C

B

T

ON

0.01 µF

20 ms

0.1 µF

200 ms

0.22 µF

420 ms

0.47 µF

840 ms

1.0 µF

2 sec

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.

In systems where the line out and headphone jack are the

same, the output coupling cap, C

C

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 32

Ω and 64Ω. Whereas, the

input impedance of powered speakers can vary from 1k

Ω

top 100k

Ω. As the RC time constant of the load and the out-

put coupling capacitor increases, the turn off transients are

increased.

To improve click and pop performance in this situation, exter-

nal resistors R6 and R7 should be added. The recom-

mended value for R6 is between 150

Ω to 1kΩ. The recom-

mended value for R7 is between 100

Ω to 500Ω. To achieve

virtually clickless and popless performance R6 = 150

Ω,R7=

100

Ω,C

values of R6 will result in better click and pop performance.

However, it should be understood that lower resistance val-

ues of R6 will increase quiescent current.

LOW FREQUENCY ENHANCEMENT

In some cases a designer may want to improve the low fre-

quency response of the bridged amplifier. This low frequency

boost can be useful in systems where speakers are housed

in small enclosures. A resistor, R

in parallel, can be placed in series with the feedback resistor

of the bridged amplifier as seen in

Figure 5.

At low frequencies the capacitor will be virtually an open cir-

cuit. At high frequencies the capacitor will be virtually a short

circuit. As a result of this, the gain of the bridge amplifier is

increased at low frequencies. A first order pole is formed with

a corner frequency at:

f

The resulting low frequency differential gain of this bridged

amplifier becomes:

2(R

With R

order pole is formed with a corner frequency of 120 Hz. At

low frequencies the differential gain will be 4, assuming R

20k. The low frequency boost formulas assume that C

f

DS100015-33

FIGURE 4. Resistors for Varying Output Loads

DS100015-32

FIGURE 5. Low Frequency Enhancement

11

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