LM4860

SNAS096C – AUGUST 1994 – 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 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 10

μF and a 0.1 μF bypass capacitors which aid in supply stability, but do not eliminate the need

for bypassing the supply nodes of the LM4860. 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 LM4860 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. There is a

built-in threshold which produces a drop in quiescent current to 500

μA typically. For a 5V power supply, this

threshold occurs when 2V–3V is applied to the shutdown pin. A typical quiescent current of 0.6

μA 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 47 k

Ω will disable the LM4860. There are no soft

pull-down resistors inside the LM4860, 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.

HEADPHONE CONTROL INPUTS

The LM4860 possesses two headphone control inputs that disable the amplifier and reduce IDD to less than 1 mA

when either one or both of these inputs have a logic-high voltage placed on their pins.

Unlike the shutdown function, the headphone control function does not provide the level of current conservation

that is required for battery powered systems. Since the quiescent current resulting from the headphone control

function is 1000 times more than the shutdown function, the residual currents in the device may create a pop at

the output when coming out of the headphone control mode. The pop effect may be eliminated by connecting the

headphone sensing output to the shutdown pin input as shown in Figure 21. This solution will not only eliminate

the output pop, but will also utilize the full current conservation of the shutdown function by reducing IDD to

0.6

μA. The amplifier will then be fully shutdown. This configuration also allows the designer to use the control

inputs as either two headphone control pins or a headphone control pin and a shutdown pin where the lowest

level of current consumption is obtained from either function.

Figure 22 shows the implementation of the LM4860's headphone control function using a single-supply

headphone amplifier. The voltage divider of R1 and R2 sets the voltage at the HP-IN1 pin to be approximately

50 mV when there are no headphones plugged into the system. This logic-low voltage at the HP-IN1 pin enables

the LM4860 to amplify AC signals. Resistor R3 limits the amount of current flowing out of the HP-IN1 pin when

the voltage at that pin goes below ground resulting from the music coming from the headphone amplifier. The

output coupling cap protects the headphones by blocking the amplifier's half-supply DC voltage. The capacitor

also protects the headphone amplifier from the low voltage set up by resistors R1 and R2 when there aren't any

headphones plugged into the system. The tricky point to this setup is that the AC output voltage of the

headphone amplifier cannot exceed the 2.0V HP-IN1 voltage threshold when there aren't any headphones

plugged into the system, assuming that R1 and R2 are 100k and 1k, respectively. The LM4860 may not be fully

shutdown when this level is exceeded momentarily, due to the discharging time constant of the bias-pin voltage.

This time constant is established by the two 50k resistors (in parallel) with the series bypass capacitor value.

When a set of headphones are plugged into the system, the contact pin of the headphone jack is disconnected

from the signal pin, interrupting the voltage divider set up by resistors R1 and R2. Resistor R1 then pulls up the

HP-IN1 pin, enabling the headphone function and disabling the LM4860 amplifier. The headphone amplifier then

drives the headphones, whose impedance is in parallel with resistor R2. Since the typical impedance of

headphones are 32

Ω, resistor R2 has negligible effect on the output drive capability. Also shown in Figure 22 are

the electrical connections for the headphone jack and plug. A 3-wire plug consists of a Tip, Ring, and Sleave,

where the Tip and Ring are signal carrying conductors and the Sleave is the common ground return. One control

pin contact for each headphone jack is sufficient to indicate to control inputs that the user has inserted a plug into

a jack and that another mode of operation is desired.

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