Datasheet

LM4810
SNAS125D –FEBRUARY 2001–REVISED APRIL 2013 www.ti.com
(5)
V
DD
≥ (2V
OPEAK
+ (V
ODTOP
+ V
ODBOT
)) (6)
Figure 29 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 LM4810 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 POWER DISSIPATION.
Remember that the maximum power dissipation point from Equation 1 must be multiplied by two since there are
two independent amplifiers inside the package. Once the power dissipation equations have been addressed, the
required gain can be determined from Equation 7.
(7)
Thus, a minimum gain of 1.497 allows the LM4810 to reach full output swing and maintain low noise and THD+N
performance. For this example, let A
V
=1.5.
The amplifiers overall gain is set using the input (R
i
) and feedback (R
f
) resistors. With the desired input
impedance set at 20kΩ, the feedback resistor is found using Equation 8.
A
V
= R
f
/R
i
(8)
The value of R
f
is 30kΩ.
The last step in this design is setting the amplifier's −3db frequency bandwidth. To achieve the desired ±0.25dB
pass 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 ±0.25dB desired limit. The results are an
f
L
= 100Hz/5 = 20Hz (9)
and a
f
H
= 20kHz
∗
5 = 100kHz (10)
As stated in External Components Description, both R
i
in conjunction with C
i
, and C
o
with R
L
, create first order
highpass filters. Thus to obtain the desired low frequency response of 100Hz within ±0.5dB, both poles must be
taken 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.
C
i
≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF. (11)
C
o
≥ 1 / (2π * 32Ω * 20Hz) = 249µF; use 330µF. (12)
The high frequency pole is determined by the product of the desired high frequency pole, f
H
, and the closed-loop
gain, A
V
. With a closed-loop gain of 1.5 and f
H
= 100kHz, the resulting GBWP = 150kHz which is much smaller
than the LM4810's GBWP of 900kHz. This figure displays that if a designer has a need to design an amplifier
with a higher gain, the LM4810 can still be used without running into bandwidth limitations.
16 Submit Documentation Feedback Copyright © 2001–2013, Texas Instruments Incorporated
Product Folder Links: LM4810