LM4836MT National Semiconductor, LM4836MT Datasheet - Page 13

• Manufacturer Part Number: LM4836MT
• Description: Stereo 2W Audio Power Amplifiers with DC Volume Control/ Bass Boost/ and Input Mux
• Manufacturer: National Semiconductor

Application Information
BASS BOOST FUNCTION
The Bass Boost Function can be toggled by changing the
logic at pin 19. A logic low will switch the power amplifiers to
bass boost mode. In bass boost mode the low frequency
gain of the amplifier is set by the external capacitor.
Whereas a logic high sets the amplifiers to unity gain.
In some cases a designer may want to improve the low fre-
quency response of the bridged amplifier or incorporate a
bass boost feature. This bass boost can be useful in systems
where speakers are housed in small enclosures. If the de-
signer wishes to disable the bass boost feature, pin 19 can
be tied to V
When bass boost is enabled, the output amplifiers will be in-
ternally set at a gain of 2 at low frequencies (gain of 4 in
bridged mode). As shown in Figure 2, C
frequency for the bass boost. At low frequencies the capaci-
tor will be virtually an open circuit. At high frequencies the ca-
pacitor will be virtually a short circuit. As a result of this, the
gain of the bridge amplifier is increased at low frequencies. A
first order pole is formed with a corner frequency at:
DD
.
f
c
= 1/(2 10k C
BASS
)
BASS
(Continued)
sets the cutoff
13
The resulting low frequency differential gain of this bridged
amplifier becomes:
With C
frequency of 160 Hz. The low frequency boost formulas as-
sume that C
quency response as explained in the Proper Selection of
External Components section. See the Typical Perfor-
mance Characteristics section for a graph that includes
bass boost performance with various values of C
DC VOLUME CONTROL
The DC voltage at the DC Volume Control pin (pin 5) deter-
mines the attenuation of output of the amplifiers. If the DC
potential of pin 5 is above 4V (typical 80% V
amplifiers are set at unity gain. The attenuator range is from
0 dB (pin 5 = 80% V
age greater than 4V (or 80% V
unity. Refer to the Typical Performance Characteristics for
detailed information of the attenuation characteristics of the
DC Volume Control pin.
BASS
= 0.1 µF, a first order pole is formed with a corner
O
, C
2(10k
i
, f
DD
IC
, f
) to -81 dB (pin 5 = 0V). Any DC volt-
+10k ) /10k
OC
allow the appropriate low fre-
DD
) will result in a gain of
= 4
DD
) the internal
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BASS
.