LM4920TLBD National Semiconductor, LM4920TLBD Datasheet - Page 13

• Manufacturer Part Number: LM4920TLBD
• Description: BOARD EVALUATION LM4920TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4920TLBD

• Amplifier Type: Class AB
• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 80mW x 2 @ 16 Ohm
• Voltage - Supply: 1.6 V ~ 4.2 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4920
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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 LM4920 audio amplifier which is
a CMOS device.
Before applying any signal to the inputs or shutdown pins of
the LM4920, it is important to apply a supply voltage to the
V
be applied to the shutdown pins (see MICRO POWER
SHUTDOWN) and input pins.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM4920 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows
the outputs of the LM4920 to be biased about GND instead
of a nominal DC voltage, like traditional headphone amplifi-
ers. 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 LM4920 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 LM4920 when compared to a traditional
headphone amplifier operating from the same supply volt-
age.
OUTPUT TRANSIENT (’CLICK AND POPS’)
ELIMINATED
The LM4920 contains advanced circuitry that virtually elimi-
nates 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 2, the LM4920 has two internal opera-
tional amplifiers. The two amplifiers have internally config-
ured gain, the closed loop gain is set by selecting the ratio of
R
where R
Since this is an output ground-referenced amplifier, by driv-
ing the headphone through R
D2), the LM4920 does not require output coupling capaci-
tors. The typical single-ended amplifier configuration re-
quires large, expensive output capacitors.
DD
f
to R
pins. After the device has been powered, signals may
i
. Consequently, the gain for each channel of the IC is
F
= 30kΩ and R
A
V
= -(R
i
= 20kΩ.
f
/ R
i
OUT
) = 1.5 V/V
(Pin C2) and L
OUT
(Pin
13
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
Since the LM4920 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation 1. Even
with large internal power dissipation, the LM4920 does not
require heat sinking over a large range of ambient tempera-
tures. From Equation 1, 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 2:
For the micro SMD package, θ
for the LM4920. Depending on the ambient temperature, T
of the system surroundings, Equation 2 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 2, then either the supply voltage must be de-
creased, the load impedance increased or T
the typical application of a 3V power supply, with a 16Ω load,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 144˚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
critical for low noise performance and high power supply
rejection. Applications that employ a 3V power supply typi-
cally use a 4.7µF capacitor in parallel with a 0.1µF ceramic
filter capacitor 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 LM4920’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
LM4920’s shutdown function. When active, the LM4920’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
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
microprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
P
P
DMAX
DMAX
= (T
= (V
JMAX
DD
JA
)
2
- T
= 105˚C/W. T
/ (2π
A
) / (θ
2
R
L
JA
)
)
A
JMAX
reduced. For
www.national.com
= 150˚C
(1)
(2)
A
,