LM4917

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SNAS238G – AUGUST 2004 – REVISED MAY 2013

APPLICATION INFORMATION

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM4917 features a low noise inverting charge pump that generates an internal negative supply voltage. This

allows the outputs of the LM4917 to be biased about GND instead of a nominal DC voltage, like traditional

headphone amplifiers. Because there is no DC component, the large DC blocking capacitors (typically 220µF)

are not necessary. The coupling capacitors are replaced by two, small ceramic charge pump capacitors, saving

board space and cost.

Eliminating the output coupling capacitors also improves low frequency response. The headphone impedance

and the output capacitor form a high pass filter that not only blocks the DC component of the output, but also

attenuates low frequencies, impacting the bass response. Because the LM4917 does not require the output

coupling capacitors, the low frequency response of the device is not degraded by external components.

In addition to eliminating the output coupling capacitors, the ground referenced output nearly doubles the

available dynamic range of the LM4917 when compared to a traditional headphone amplifier operating from the

same supply voltage.

OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED

The LM4917 contains advanced circuitry that virtually eliminates output transients ('clicks and pops'). This

circuitry prevents all traces of transients when the supply voltage is first applied or when the part resumes

operation after coming out of shutdown mode.

To ensure optimal click and pop performance under low gain configurations (less than 0dB), it is critical to

more reliable way to lower gain or reduce power delivered to the load is to place a current limiting resistor in

series with the load as explained in the Minimizing Output Noise / Reducing Output Power section.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4917 has two operational amplifiers internally. The two amplifiers have externally

each channel of the IC is

(1)

8), the LM4917 does not require output coupling capacitors. The typical single-ended amplifier configuration

where one side of the load is connected to ground requires large, expensive output capacitors.

POWER DISSIPATION

Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to

ensure a successful design. Equation 2 states the maximum power dissipation point for a single-ended amplifier

operating at a given supply voltage and driving a specified output load.

(2)

Since the LM4917 has two operational amplifiers in one package, the maximum internal power dissipation point

is twice that of the number which results from Equation 2. Even with the large internal power dissipation, the

LM4917 does not require heat sinking over a large range of ambient temperature. From Equation 2, assuming a

3V power supply and a 16

Ω load, the maximum power dissipation point is 28mW per amplifier. Thus the

maximum package dissipation point is 56mW. The maximum power dissipation point obtained must not be

greater than the power dissipation that results from Equation 3:

(3)

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