Datasheet

R
I
R
F
C
LFE
R
LFE
Right Gain 1
or
Left Gain 1
Right Gain 2
or
Left Gain 2
Right Out -
or
Left Out -
(CBS)
(RBS)
LM4938
www.ti.com SNAS245B –FEBRUARY 2005–REVISED MAY 2013
The HP-IN threshold is set at 4V. While the LM4938 operates in bridged mode, the DC potential across the load
is essentially 0V. Therefore, even in an ideal situation, the output swing cannot cause a false single-ended
trigger. 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
DD
through R4. This enables the headphone function, turns off both of
the "+" output amplifiers, and mutes the bridged speaker. The remaining single-ended amplifiers then drive the
headphones, whose impedance is in parallel with resistors R2 and R3. These resistors have negligible effect on
the LM4938's output drive capability since the typical impedance of headphones is 32Ω.
Figure 45 also shows the suggested headphone jack electrical 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 connecting headphones.
A microprocessor or a switch can replace the headphone jack contact pin. When a microprocessor or switch
applies a voltage greater than 4V to the HP-IN pin, a bridge-connected speaker is muted and the single ended
output amplifiers 1A and 2A will drive a pair of headphones.
GAIN SELECT FUNCTION (Bass Boost)
The LM4938 features selectable gain, using either internal or external feedback resistors. Either set of feedback
resistors set the gain of the output amplifiers. The voltage applied to the GAIN SELECT pin controls which gain is
selected. Applying V
DD
to the GAIN SELECT pin selects the external gain mode. Applying 0V to the GAIN
SELECT pin selects the internally set unity gain.
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, R
LFE
, and a capacitor, C
LFE
, in parallel, can be placed in series with the feedback resistor
of the bridged amplifier as seen in Figure 46.
Figure 46. Low Frequency Enhancement
At low, frequencies C
LFE
is a virtual open circuit and at high frequencies, its nearly zero ohm impedance shorts
R
LFE
. The result is increased bridge-amplifier gain at low frequencies. The combination of R
LFE
and C
LFE
form a -
6dB corner frequency at
f
C
= 1/(2πR
LFE
C
LFE
) (9)
The bridged-amplifier low frequency differential gain is:
A
VD
= 2(R
F
+ R
LFE
) / R
i
(10)
Using the component values shown in Figure 1 (R
F
= 20kΩ, R
LFE
= 20kΩ, and C
LFE
= 0.068µF), a first-order, -
6dB pole is created at 120Hz. Assuming R
i
= 20kΩ, the low frequency differential gain is 4. The input (C
i
) and
output (C
O
) capacitor values must be selected for a low frequency response that covers the range of frequencies
affected by the desired bass-boost operation.
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