LM4949TLEVAL National Semiconductor, LM4949TLEVAL Datasheet - Page 32

• Manufacturer Part Number: LM4949TLEVAL
• Description: BOARD EVALUATION LM4949TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4949TLEVAL

• Amplifier Type: Class D
• Output Type: 2-Channel (Stereo) with Stereo Headphones
• Max Output Power X Channels @ Load: 2.5W x 2 @ 4 Ohm; 190mW x 2 @ 16 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4949
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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GENERAL AMPLIFIER FUNCTION
Class D Amplifier
The LM4949 features a high-efficiency, filterless, Class D
stereo amplifier. The LM4949 Class D amplifiers feature a fil-
terless modulation scheme, the differential outputs of each
channel switch at 300khz, from V
no input signal applied, the two outputs (_LS+ and _LS-)
switch with a 50% duty cycle, with both outputs in phase. Be-
cause the outputs of the LM4949 are differential, the two
signals cancel each other. This results in no net voltage
across the speaker, thus no load current during the idle state,
conserving power.
When an input signal is applied, the duty cycle (pulse width)
changes. For increasing output voltages, the duty cycle of
_LS+ increases, while the duty cycle of _LS- decreases. For
decreasing output voltages, the converse occurs, the duty
cycle of _LS- increases while the duty cycle of _LS+ decreas-
es. The difference between the two pulse widths yields the
differential output voltage.
Headphone Amplifier
The LM4949 headphone amplifier features three different op-
erating modes, output capacitorless (OCL), capacitor-cou-
pled (CC), and external amplifier mode.
The OCL architecture eliminates the bulky, expensive output
coupling capacitors required by traditional headphone ampli-
fiers. The LM4949 headphone section uses three amplifiers.
Two amplifiers drive the headphones while the third (VOC) is
set to the internally generated bias voltage (typically V
The third amplifier is connected to the return terminal of the
headphone jack. In this configuration, the signal side of the
headphones are biased to V
V
eliminating the need for an output coupling capacitor. Re-
External Headphone Amplifier
The LM4949 features the ability to drive and control a sepa-
rate headphone amplifier for applications that require a True
Ground headphone output (Figure 5). Configure the LM4949
into external headphone amplifier mode by setting bit D2
(OCL_LGC) in register 0.0 to 1 and bit D1 (OCL) to 0. In this
mode the VOC output becomes a logic output used to drive
the shutdown input of the external amplifier. The output level
of VOC is controlled by bits D1 (SDB_HPSEL) and D2
DD
/2, thus there is no net DC voltage across the headphone,
DD
/2, the return is biased to
DD
to GND. When there is
FIGURE 4. Capacitor Coupled Headphone Mode
DD
/2).
32
moving the output coupling capacitors from the headphone
signal path reduces component count, reducing system cost
and board space consumption, as well as improving low fre-
quency performance.
In OCL mode, the headphone return sleeve is biased to
V
sleeve is not an issue. However, if the headphone output is
used as a line out, the V
tential that a line-in would expect on the return sleeve. When
the return of the headphone jack is connected to GND, the
VOC amplifier of the LM4949 detects an output short circuit
condition and is disabled, preventing damage to the LM4949,
and allowing the headphone return to be biased at GND.
Capacitor Coupled Headphone Mode
In capacitor coupled (CC) mode, the VOC pin is disabled, and
the headphone outputs are coupled to the jack through series
capacitors, allowing the headphone return to be connected to
GND (Figure 4). In CC mode, the LM4949 requires output
coupling capacitors to block the DC component of the ampli-
fier output, preventing DC current from flowing to the load.
The output capacitor and speaker impedance form a high
pass filter with a -3dB roll-off determined by:
Where R
put coupling capacitor. Choose C
below the lowest frequency of interest. Setting f
results in poor low frequency performance. Select capacitor
dielectric types with low ESR to minimize signal loss due to
capacitor series resistance and maximize power transfer to
the load.
(SDB_MUXSEL) in register 0.1. SDB_MUXSEL determines
the source of the VOC control signal. With SDB_MUXSEL =
0, the VOC signal comes from the internal start-up circuitry of
the LM4949. This allows the external headphone amplifier to
be turned on and off simultaneously with the LM4949. When
SDB_MUXSEL = 1, the VOC signal comes from the I
bit D1. With SDB_HPSEL = 0, VOC is a logic low, with
SDB_HPSEL = 1, VOC is a logic high.
DD
/2. When driving headphones, the voltage on the return
L
is the headphone impedance, and C
f
-3dB
20200105
= 1 / 2πR
DD
/2 can conflict with the GND po-
L
OUT
C
OUT
such that f
OUT
-3dB
-3dB
is the out-
too high
2
C bus,
is well