LM4782 Datasheet, PDF (21/33 Pages) National Semiconductor (TI) – 3 Channel 25W Audio Power Amplifier with Mute and Standby

LM4782, LM4782TABD
www.ti.com
SNAS231B – FEBRUARY 2004 – REVISED MARCH 2013
CLICKS AND POPS
In the typical application of the LM4782 as a split-supply audio power amplifier, the IC exhibits excellent “click”
and “pop” performance when utilizing the mute and standby modes. In addition, the device employs Under-
Voltage Protection, which eliminates unwanted power-up and power-down transients. The basis for these
functions are a stable and constant half-supply potential. In a split-supply application, ground is the stable half-
supply potential. But in a single-supply application, the half-supply needs to charge up at the same rate as the
supply rail, V_CC. This makes the task of attaining a clickless and popless turn-on more challenging. Any uneven
charging of the amplifier inputs will result in output clicks and pops due to the differential input topology of the
LM4782.
To achieve a transient free power-up and power-down, the voltage seen at the input terminals should be ideally
the same. Such a signal will be common-mode in nature, and will be rejected by the LM4782. In Figure 5, the
resistor R_INP serves to keep the inputs at the same potential by limiting the voltage difference possible between
the two nodes. This should significantly reduce any type of turn-on pop, due to an uneven charging of the
amplifier inputs. This charging is based on a specific application loading and thus, the system designer may need
to adjust these values for optimal performance.
As shown in Figure 5, the resistors labeled R_BI help bias up the LM4782 off the half-supply node at the emitter of
the 2N3904. But due to the input and output coupling capacitors in the circuit, along with the negative feedback,
there are two different values of R_BI, namely 10kΩ and 200kΩ. These resistors bring up the inputs at the same
rate resulting in a popless turn-on. Adjusting these resistors values slightly may reduce pops resulting from
power supplies that ramp extremely quick or exhibit overshoot during system turn-on.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet the design targets of an application. The choice of
external component values that will affect gain and low frequency response are discussed below.
The gain of each amplifier is set by resistors R_f and R_i for the non-inverting configuration shown in Figure 1. The
gain is found by Equation 5 below:
A_V = 1 + R_f / R_i (V/V)
(5)
For best noise performance, lower values of resistors are used. A value of 1kΩ is commonly used for R_i and then
setting the value of R_f for the desired gain. For the LM4782 the gain should be set no lower than 10V/V and no
higher than 50V/V. Gain settings below 10V/V may experience instability and using the LM4782 for gains higher
than 50V/V will see an increase in noise and THD.
The combination of R_i with C_i (see Figure 1) creates a high pass filter. The low frequency response is determined
by these two components. The -3dB point can be found from Equation 6 shown below:
f_i = 1 / (2πR_iC_i) (Hz)
(6)
If an input coupling capacitor is used to block DC from the inputs as shown in Figure 6, there will be another high
pass filter created with the combination of C_IN and R_IN. When using a input coupling capacitor R_IN is needed to
set the DC bias point on the amplifier's input terminal. The resulting -3dB frequency response due to the
combination of C_IN and R_IN can be found from Equation 7 shown below:
f_IN = 1 / (2πR_INC_IN) (Hz)
(7)
With large values of R_IN oscillations may be observed on the outputs when the inputs are left floating. Decreasing
the value of R_IN or not letting the inputs float will remove the oscillations. If the value of R_IN is decreased then the
value of C_IN will need to increase in order to maintain the same -3dB frequency response.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such as C_IN and C_i (see Figure 1, Figure 3, Figure 4,
Figure 5, and Figure 6) will result in very good performance. However, electrolytic capacitors are less linear than
other premium capacitors. Higher THD+N performance may be obtained by using high quality polypropylene
capacitors in the signal path. A more cost effective solution may be the use of smaller value premium capacitors
in parallel with the larger electrolytic capacitors. This will maintain signal quality in the upper audio band where
any degradation is most noticeable while also coupling in the signals in the lower audio band for good bass
response.
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