LM4936MH National Semiconductor, LM4936MH Datasheet - Page 21

• Manufacturer Part Number: LM4936MH
• Description: IC,Audio Amplifier,DUAL,TSSOP,28PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4936MH

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
voltage applied to the HP SENSE pin at approximately
50mV. This 50mV puts the LM4936 into bridged mode op-
eration. The output coupling capacitor blocks the amplifier’s
half supply DC voltage, protecting the headphones.
The HP SENSE threshold is set so the output signal cannot
cause an output mode change. While the LM4936 operates
in bridge mode, the DC potential across the load is essen-
tially 0V. Connecting headphones to the headphone jack
disconnects the headphone jack contact pin from R2 and
allows R1 to pull the HP SENSE pin up to V
This enables the headphone function and mutes the bridged
speaker. The single-ended ’-’ outputs then drive the head-
phones, whose impedance is in parallel with resistors R2
and R3. These resistors have negligible effect on the
LM4936’s output drive capability since the typical impedance
of headphones is 32Ω.
Figure 3 also shows the suggested headphone jack electri-
cal connections. The jack is designed to mate with a three-
wire plug. The plug’s tip and ring should each carry one of
the two stereo output signals, whereas the sleeve should
carry the ground return. A headphone jack with one control
pin contact is sufficient to drive the HP-IN pin when connect-
ing headphones.
GAIN SELECT FUNCTION (Bass Boost)
The LM4936 features selectable gain, using either internal or
external feedback resistors. The GAIN SEL bit (B3) controls
which gain is selected. Loading a digital 0 into the GAIN SEL
bit sets the gain to internal resulting in a gain of 6dB for BTL
mode or unity for singled-ended mode. Loading a digital 1
into the GAIN SEL bit sets the gain to be determined by the
external resistors, R
In some cases a designer may want to improve the low
frequency 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. A resistor,
I
and R
F
.
(Continued)
DD
through R4.
21
R
with the feedback resistor of the bridged amplifier as seen in
Figure 2.
At low, frequencies C
frequencies, its nearly zero ohm impedance shorts R
result is increased bridge-amplifier gain at low frequencies.
The combination of R
quency at
The bridged-amplifier low frequency differential gain is:
Using the component values shown in Figure 2 (R
R
created at 120Hz. Assuming R
differential gain is 4V/V or 12dB. The input (C
(C
quency response that covers the range of frequencies af-
fected by the desired bass-boost operation.
VOLUME CONTROL
The LM4936 has an internal stereo volume control whose
setting is a function of the digital values in the V4 – V0 bits.
See Table 4.
The LM4936 volume control consists of 31 steps that are
individually selected. The range of the steps, are from 0dB -
78dB. The gain levels are 1dB/step from 0dB to -6dB, 2dB/
step from -6dB to -36dB, 3dB/step from -36dB to -47dB,
4dB/step from -47dB to -51dB, 5dB/step from -51dB to
-66dB, and 12dB to the last step at -78dB.
BS
BS
OUT
, and a capacitor, C
= 20kΩ, and C
) capacitor values must be selected for a low fre-
A
BS
VD
f
C
BS
BS
= 0.068µF), a first-order, -6dB pole is
BS
= 2(R
= 1/(2πR
is a virtual open circuit and at high
, in parallel, can be placed in series
and C
F
+ R
i
BS
BS
= 20kΩ, the low frequency
BS
C
form a -6dB corner fre-
BS
) / R
)
i
i
) and output
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F
= 20kΩ,
BS
. The
(10)
(9)