LM4808LDBD National Semiconductor, LM4808LDBD Datasheet - Page 14

• Manufacturer Part Number: LM4808LDBD
• Description: BOARD EVALUATION LM4808LD
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4808LDBD

• Amplifier Type: Class AB
• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 150mW x 2 @ 16 Ohm
• Voltage - Supply: 2 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4808
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
Bypass Capacitor Value
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of the bypass capacitor,
C
quiescent operation, its value is critical when minimizing
turn-on pops. The slower the LM4808’s outputs ramp to their
quiescent DC voltage (nominally 1/2 V
turn-on pop. Choosing C
minimize turn-on pops. As discussed above, choosing C
larger than necessary for the desired bandwith helps mini-
mize clicks and pops.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-
acteristics section. Another way, using Equation (5), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (5). For a
single-ended application, the result is Equation (6).
The Output Power vs Supply Voltage graph for a 32
indicates a minimum supply voltage of 4.8V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4808 to produce peak output power in excess of 70mW
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section. Remember that the maximum
power dissipation point from Equation (1) must be multiplied
by two since there are two independent amplifiers inside the
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
B
. Since C
V
DD
B
determines how fast the LM4808 settles to
(2V
OPEAK
B
+ (V
Audio Amplifier
equal to 1.0µF or larger, will
OD TOP
100Hz–20kHz
+ V
DD
OD BOT
), the smaller the
(Continued)
1Vrms (max)
))
±
0.50dB
70mW
20k
32
load
i
(5)
(6)
no
14
package. Once the power dissipation equations have been
addressed, the required gain can be determined from Equa-
tion (7).
Thus, a minimum gain of 1.497 allows the LM4808 to reach
full output swing and maintain low noise and THD+N perfro-
mance. For this example, let A
The amplifiers overall gain is set using the input (R
feedback (R
set at 20k , the feedback resistor is found using Equation
(8).
The value of R
The last step in this design is setting the amplifier’s −3db
frequency bandwidth. To achieve the desired
band magnitude variation limit, the low frequency response
must extend to at lease one−fifth the lower bandwidth limit
and the high frequency response must extend to at least five
times the upper bandwidth limit. The gain variation for both
response limits is 0.17dB, well within the
limit. The results are an
and a
As stated in the External Components section, both R
conjunction with C
pass filters. Thus to obtain the desired low frequency re-
sponse of 100Hz within
into consideration. The combination of two single order filters
at the same frequency forms a second order response. This
results in a signal which is down 0.34dB at five times away
from the single order filter −3dB point. Thus, a frequency of
20Hz is used in the following equations to ensure that the
response is better than 0.5dB down at 100Hz.
The high frequency pole is determined by the product of the
desired high frequency pole, f
A
resulting GBWP = 150kHz which is much smaller than the
LM4808’s GBWP of 900kHz. This figure displays that if a
designer has a need to design an amplifier with a higher
gain, the LM4808 can still be used without running into
bandwidth limitations.
V
. With a closed-loop gain of 1.5 and f
C
i
C
o
1 / (2 * 20 k
1 / (2 * 32
f
) resistors. With the desired input impedance
f
is 30k .
f
i
H
, and C
f
L
= 20kHz
= 100Hz/5 = 20Hz
* 20 Hz) = 0.397µF; use 0.39µF.
* 20 Hz) = 249µF; use 330µF.
A
±
V
0.5dB, both poles must be taken
o
= R
with R
*
5 = 100kHz
H
V
f
, and the closed-loop gain,
=1.5.
/R
i
L
, create first order high-
H
±
0.25dB desired
= 100kHz, the
±
0.25dB pass
i
) and
(10)
i
(7)
(8)
(9)
in