LM4995TMBD National Semiconductor, LM4995TMBD Datasheet - Page 11

• Manufacturer Part Number: LM4995TMBD
• Description: BOARD EVALUATION LM4995TM
• Manufacturer: National Semiconductor
• Series: Boomer®, PowerWise®r

Specifications of LM4995TMBD

• Amplifier Type: Class AB
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1.3W x 1 @ 8 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4995
• Lead Free Status / RoHS Status: Not applicable / Not applicable

width is dictated by the choice of external components shown
in
high pass filter which limits low frequency response. This val-
ue should be chosen based on needed frequency response
for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenua-
tion. But in many cases the speakers used in portable sys-
tems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a large
input capacitor may not increase actual system performance.
In addition to system cost and size, click and pop performance
is effected by the size of the input coupling capacitor, C
larger input coupling capacitor requires more charge to reach
its quiescent DC voltage (nominally 1/2 V
comes from the output via the feedback and is apt to create
pops upon device enable. Thus, by minimizing the capacitor
size based on necessary low frequency response, turn-on
pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
on pops since it determines how fast the LM4995 turns on.
The slower the LM4995's outputs ramp to their quiescent DC
voltage (nominally 1/2 V
Choosing C
the range of 0.1µF to 0.39µF), should produce a virtually
clickless and popless shutdown function. While the device will
function properly, (no oscillations or motorboating), with C
equal to 0.1µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C
recommended in all but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found.
Given:
Figure
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
1. The input coupling capacitor, C
B
B
, is the most critical component to minimize turn-
equal to 1.0µF along with a small value of C
DD
), the smaller the turn-on pop.
100 Hz–20 kHz ± 0.25 dB
i
, forms a first order
B
DD
equal to 1.0µF is
). This charge
1 Wrms
1 Vrms
20 kΩ
8Ω
i
i.
(in
A
B
11
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4995 to reproduce peaks in excess of 1W with-
out producing audible distortion. At this time, the designer
must make sure that the power supply choice along with the
output impedance does not violate the conditions explained
in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 2.
From Equation 2, the minimum A
Since the desired input impedance was 20 kΩ, and with a
A
allocation of R
is to address the bandwidth requirements which must be stat-
ed as a pair of −3 dB frequency points. Five times away from
a −3 dB point is 0.17 dB down from passband response which
is better than the required ±0.25 dB specified.
As stated in the External Components section, R
junction with C
The high frequency pole is determined by the product of the
desired frequency pole, f
a A
which is much smaller than the LM4995 GBWP of 2.5MHz.
This figure displays that if a designer has a need to design an
amplifier with a higher differential gain, the LM4995 can still
be used without running into bandwidth limitations.
The LM4995 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor (C4) may be needed
as shown in Figure 2 to bandwidth limit the amplifier. This
feedback capacitor creates a low pass filter that eliminates
possible high frequency oscillations. Care should be taken
when calculating the -3dB frequency in that an incorrect com-
bination of R
combination of feedback resistor and capacitor that will not
produce audio band high frequency rolloff is R
C
proximately 320kHz.
VD
4
= 25pf. These components result in a -3dB point of ap-
VD
impedance of 2, a ratio of 1.5:1 of R
f
f
C
L
H
i
= 100Hz/5 = 20Hz
= 3 and f
= 20kHz * 5 = 100kHz
≥
1/(2
3
π
and C
i
i
*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
= 20 kΩ and R
create a highpass filter.
H
= 100kHz, the resulting GBWP = 300kHz
4
will cause rolloff before 20kHz. A typical
R
H
f
, and the differential gain, A
/R
i
= A
f
= 30 kΩ. The final design step
VD
VD
/2
is 2.83; use A
f
to R
i
3
results in an
= 20kΩ and
www.national.com
VD
i
VD
= 3.
in con-
. With
(2)