LM4936 datasheet(21/28 Pages) NSC

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Application Information (Continued)

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

DD through R4.

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 32

Ω.

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

I and RF.

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,

BS, and a capacitor, CBS, in parallel, can be placed in series

with the feedback resistor of the bridged amplifier as seen in

Figure 2.

At low, frequencies C

BS is a virtual open circuit and at high

frequencies, its nearly zero ohm impedance shorts R

BS. The

result is increased bridge-amplifier gain at low frequencies.

The combination of R

BS and CBS form a -6dB corner fre-

quency at

C = 1/(2

πR

BSCBS)

(9)

The bridged-amplifier low frequency differential gain is:

VD = 2(RF +RBS)/Ri

(10)

Using the component values shown in Figure 2 (R

F = 20k

Ω,

BS = 20k

Ω, and C

BS = 0.068µF), a first-order, -6dB pole is

created at 120Hz. Assuming R

i = 20k

Ω, the low frequency

differential gain is 4V/V or 12dB. The input (C

i) and output

OUT) capacitor values must be selected for a low fre-

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.

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