LM48860TLBD National Semiconductor, LM48860TLBD Datasheet - Page 12

• Manufacturer Part Number: LM48860TLBD
• Manufacturer: National Semiconductor

Specifications of LM48860TLBD

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Application Information
SUPPLY VOLTAGE SEQUENCING
It is a good general practice to first apply the supply voltage
to a CMOS device before any other signal or supply on other
pins. This is also true for the LM48860 audio amplifier which
is a CMOS device.
Before applying any signal to the inputs or shutdown pins of
the LM48860, it is important to apply a supply voltage to the
V
applied to the shutdown pins (see MICRO POWER SHUT-
DOWN) and input pins.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM48860 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows the
outputs of the LM48860 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 LM48860 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 LM48860 when compared to a traditional
headphone amplifier operating from the same supply voltage.
OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED
The LM48860 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 LM48860 has two internal opera-
tional amplifiers. The two amplifiers have internally configured
gain.
Since this is an output ground-referenced amplifier, the
LM48860 does not require output coupling capacitors.
POWER DISSIPATION
From the graph (THD+N vs Output Power , V
16Ω, f = 1kHz, 22kH BW, two channels in phase, page 6)
assuming a 3V power supply and a 16Ω load, the maximum
power dissipation point and thus the maximum package dis-
sipation point is 281mW. The maximum power dissipation
point obtained must not be greater than the power dissipation
that results from Equation 1.
For the micro SMD package θ
for the LM48860. Depending on the ambient temperature,
T
find the maximum internal power dissipation supported by the
IC packaging. If the maximum power dissipation from the
A
DD
, of the system surroundings, Equation 1 can be used to
pins. After the device has been powered, signals may be
P
DMAX
= (T
JMAX
- T
JA
A
= 59.3°C/W. T
) / (θ
JA
)
DD
JMAX
= 3V, R
= 150°C
L
(1)
=
12
graph is greater than that of Equation 1, then either the supply
voltage must be decreased, the load impedance increased or
T
of a 5V power supply, with a 16Ω load, the maximum ambient
temperature possible without violating the maximum junction
temperature is approximately 110°C provided that device op-
eration is around the maximum power dissipation point. Pow-
er 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's 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 LM48860'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
LM48860’s shutdown function. When active, the LM48860’s
micropower shutdown feature turns off the amplifiers’ bias
circuitry, reducing the supply current. The trigger point is
0.45V for a logic-low level, and 1.2V for logic-high level. The
low 0.01µA (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 in-
crease the shutdown current. Do not let SD_LC/SD_RC float,
connect either to high or low.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM48860's performance requires properly se-
lecting external components. Though the LM48860 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
Charge Pump Capacitor Selection
Use low ESR (equivalent series resistance) (<100mΩ) ce-
ramic capacitors with an X7R dielectric for best performance.
Low ESR capacitors keep the charge pump output
impedance to a minimum, extending the headroom on the
negative supply. Higher ESR capacitors result in reduced
output power from the audio amplifiers.
Charge pump load regulation and output impedance are af-
fected by the value of the flying capacitor (C4). A larger valued
C4 (up to 3.3uF) improves load regulation and minimizes
charge pump output resistance. Beyond 3.3uF, the switch-on
resistance dominates the output impedance.
The output ripple is affected by the value and ESR of the out-
put capacitor (C3). Larger capacitors reduce output ripple on
the negative power supply. Lower ESR capacitors minimize
the output ripple and reduce the output impedance of the
charge pump.
The LM48860 charge pump design is optimized for 2.2uF, low
ESR, ceramic, flying and output capacitors.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C1 and C2 in Figure 1). A high val-
ue capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases, however, the
A
reduced (see power derating curves). For the application