Datasheet

LM4917
www.ti.com 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
minimize the RC combination of the feedback resistor R
F
and stray input capacitance at the amplifier inputs. A
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
configurable gain, and the closed loop gain is set by selecting the ratio of R
f
to R
i
. Consequently, the gain for
each channel of the IC is
A
V
= -(R
f
/ R
i
) (1)
Since this an output ground-referenced amplifier, by driving the headphone through R
OUT
(Pin 11) and L
OUT
(Pin
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.
P
DMAX
= (V
DD
)
2
/ (2π
2
R
L
) (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 16Ω 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:
P
DMAX
= (T
JMAX
- T
A
) / (θ
JA
) (3)
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