Datasheet

LM48820
www.ti.com SNAS370B –MAY 2007–REVISED MAY 2013
APPLICATION INFORMATION
SUPPLY VOLTAGE SEQUENCING
Before applying any signal to the inputs or shutdown pins of the LM48820, it is important to apply a supply
voltage to the V
DD
pins. After the device has been powered, signals may be applied to the shutdown pins (see
MICRO POWER SHUTDOWN) and input pins.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM48820 features a low noise inverting charge pump that generates an internal negative supply voltage.
This allows the outputs of the LM48820 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. In traditional headphone
amplifiers, 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
LM48820 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 LM48820 when compared to a traditional headphone amplifier operating from the
same supply voltage.
OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED
The LM48820 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.
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM48820 has two internal operational amplifiers. The two amplifiers have internally
configured gain, 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
) = 1.5 (V/V)
where
• R
F
= 30kΩ
• R
i
= 20kΩ (1)
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
) (W) (2)
Since the LM48820 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 large internal power dissipation, the
LM48820 does not require heat sinking over a large range of ambient temperatures. 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
) (W) (3)
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