LM4861
SNAS095C –MAY 1997–REVISED MAY 2013
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POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as
possible. As displayed in the Typical Performance Characteristics, the effect of a larger half supply bypass
capacitor is improved low frequency THD+N due to increased half-supply stability. Typical applications employ a
5V regulator with 10μF and a 0.1μF bypass capacitors which aid in supply stability, but do not eliminate the need
for bypassing the supply nodes of the LM4861. The selection of bypass capacitors, especially CB, is thus
dependant upon desired low frequency THD+N, system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4861 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. The shutdown feature turns the amplifier off when a logic high is placed on the
shutdown pin. Upon going into shutdown, the output is immediately disconnected from the speaker. A typical
quiescent current of 0.6μA results when the supply voltage is applied to the shutdown pin. In many applications,
a microcontroller or microprocessor output is used to control the shutdown circuitry which provides a quick,
smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch that when closed, is
connected to ground and enables the amplifier. If the switch is open, then a soft pull-up resistor of 47kΩ will
disable the LM4861. There are no soft pull-down resistors inside the LM4861, so a definite shutdown pin voltage
must be applied externally, or the internal logic gate will be left floating which could disable the amplifier
unexpectedly.
HIGHER GAIN AUDIO AMPLIFIER
The LM4861 is unity-gain stable and requires no external components besides gain-setting resistors, an input
coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential
gain of greater than 10 is required, a feedback capacitor may be needed, as shown in Figure 3, to bandwidth
limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency
oscillations. Care should be taken when calculating the −3dB frequency in that an incorrect combination of Rf and
Cf will cause rolloff before 20kHz. A typical combination of feedback resistor and capacitor that will not produce
audio band high frequency rolloff is Rf = 100kΩ and Cf = 5pF. These components result in a −3dB point of
approximately 320kHz. Once the differential gain of the amplifier has been calculated, a choice of Rf will result,
and Cf can then be calculated from the formula stated in External Components Description .
VOICE-BAND AUDIO AMPLIFIER
Many applications, such as telephony, only require a voice-band frequency response. Such an application
usually requires a flat frequency response from 300Hz to 3.5kHz. By adjusting the component values of Figure 3,
this common application requirement can be implemented. The combination of Ri and Ci form a highpass filter
while Rf and Cf form a lowpass filter. Using the typical voice-band frequency range, with a passband differential
gain of approximately 100, the following values of Ri, Ci, Rf, and Cf follow from the equations stated in External
Components Description .
Ri = 10kΩ, Rf = 510k ,Ci = 0.22μF, and Cf = 15pF
(4)
Five times away from a −3dB point is 0.17dB down from the flatband response. With this selection of
components, the resulting −3dB points, fL and fH, are 72Hz and 20kHz, respectively, resulting in a flatband
frequency response of better than ±0.25dB with a rolloff of 6dB/octave outside of the passband. If a steeper
rolloff is required, other common bandpass filtering techniques can be used to achieve higher order filters.
SINGLE-ENDED AUDIO AMPLIFIER
Although the typical application for the LM4861 is a bridged monoaural amp, it can also be used to drive a load
single-endedly in applications, such as PC cards, which require that one side of the load is tied to ground.
Figure 4 shows a common single-ended application, where VO1 is used to drive the speaker. This output is
coupled through a 470μF capacitor, which blocks the half-supply DC bias that exists in all single-supply amplifier
configurations. This capacitor, designated CO in Figure 4, in conjunction with RL, forms a highpass filter. The
−3dB point of this high pass filter is 1/(2πRLCO), so care should be taken to make sure that the product of RL and
CO is large enough to pass low frequencies to the load. When driving an 8Ω load, and if a full audio spectrum
reproduction is required, CO should be at least 470μF. VO2, the output that is not used, is connected through a
0.1 μF capacitor to a 2kΩ load to prevent instability. While such an instability will not affect the waveform of VO1
,
it is good design practice to load the second output.
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