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
VDD pins. After the device has been powered, signals may be
applied to the shutdown pins (see MICRO POWER SHUT-
DOWN) 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 out-
put, 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 dy-
namic 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 elim-
inates output transients ('clicks and pops'). This circuitry pre-
vents 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 2, the LM48820 has two internal opera-
tional amplifiers. The two amplifiers have internally configured
gain, the closed loop gain is set by selecting the ratio of Rf to
Ri. Consequently, the gain for each channel of the IC is
AV = -(Rf / Ri) = 1.5 (V/V) (1)
where RF = 30kΩ and Ri = 20kΩ.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a suc-
cessful design. Equation 1 states the maximum power dissi-
pation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
PDMAX = (VDD) 2 / (2π2RL) (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 tempera-
tures. 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:
PDMAX = (TJMAX - TA) / (θJA) (W) (3)
For this micro SMD package, θJA = 86°C/W and TJMAX = 150°
C. Depending on the ambient temperature, TA, of the system
surroundings, Equation 3 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 1 is greater than that of Equation 1, then
either the supply voltage must be decreased, the load
impedance increased or TA reduced. For the typical applica-
tion of a 3V power supply, with a 16Ω load, the maximum
ambient temperature possible without violating the maximum
junction temperature is approximately 127°C provided that
device operation is around the maximum power dissipation
point. Power dissipation is a function of output power and
thus, if typical operation is not around the maximum power
dissipation point, the ambient temperature may be increased
accordingly.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion. Applications that employ a 3V power supply typically use
a 4.7µF capacitor in parallel with a 0.1µF ceramic filter ca-
pacitor to stabilize the power supply output, reduce noise on
the supply line, and improve the supply's transient response.
Keep the length of leads and traces that connect capacitors
between the LM48820's power supply pin and ground as short
as possible.
MICRO POWER SHUTDOWN
The voltage applied to the SD_LC (shutdown left channel) pin
and the SD_RC (shutdown right channel) pin controls the
LM48820’s shutdown function. When active, the LM48820’s
micropower shutdown feature turns off the amplifiers’ bias
circuitry, reducing the supply current. The trigger point is
0.45V (max) for a logic-low level, and 1.2V (min) for logic-high
level. The low 0.05µA (typ) shutdown current is achieved by
applying a voltage that is as near as ground a possible to the
SD_LC/SD_RC pins. A voltage that is higher than ground may
increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a mi-
croprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
SD_LC/SD_RC pins and VDD. Connect the switch between
the SD_LC/SD_RC pins and ground. Select normal amplifier
operation by opening the switch. Closing the switch connects
the SD_LC/SD_RC pins to ground, activating micro-power
shutdown. The switch and resistor guarantee that the
SD_LC/SD_RC pins will not float. This prevents unwanted
state changes. In a system with a microprocessor or micro-
controller, use a digital output to apply the control voltage to
the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins
with active circuitry eliminates the pull-up resistor.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM48820's performance requires properly se-
lecting external components. Though the LM48820 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
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LM48820