LM4835MTX National Semiconductor, LM4835MTX Datasheet - Page 19

• Manufacturer Part Number: LM4835MTX
• Manufacturer: National Semiconductor

Specifications of LM4835MTX

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 1.5W
• Audio Amplifier Function: Headphone/Speaker
• Total Harmonic Distortion: 0.3@8Ohm@1W%
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 74dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 28
• Package Type: TSSOP
• Lead Free Status / Rohs Status: Not Compliant

Application Information
HP-IN FUNCTION
Applying a voltage between 4V and V
HP-IN headphone control pin turns off Amp2A and Amp2B,
muting a bridged-connected load. Quiescent current con-
sumption is reduced when the IC is in this single-ended
mode.
Figure 4 shows the implementation of the LM4835’s head-
phone control function. With no headphones connected to
the headphone jack, the R1-R2 voltage divider sets the
voltage applied to the HP-IN pin (pin 16) at approximately
50mV. This 50mV enables Amp1B and Amp2B, placing the
LM4835 in bridged mode operation. The output coupling
capacitor blocks the amplifier’s half supply DC voltage, pro-
tecting the headphones.
The HP-IN threshold is set at 4V. While the LM4835 operates
in bridged mode, the DC potential across the load is essen-
tially 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 head-
phone jack contact pin from −OUTA and allows R1 to pull the
HP Sense pin up to V
tion, turns off Amp2A and Amp2B, and mutes the bridged
speaker. The amplifier then drives the headphones, whose
impedance is in parallel with resistor R2 and R3. These
resistors have negligible effect on the LM4835’s output drive
capability since the typical impedance of headphones is
32Ω.
Figure 4 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.
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 Amp1A and Amp2A drive a pair of
headphones.
GAIN SELECT FUNCTION (Bass Boost)
The LM4835 features selectable gain, using either internal
and external feedback resistors. Either set of feedback re-
sistors set the gain of the output amplifiers. The voltage
applied to pin 3 (GAIN SELECT pin) controls which gain is
selected. Applying V
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
series with the feedback resistor of the bridged amplifier as
seen in Figure 5.
LFE
, and a capacitor, C
DD
DD
to the GAIN SELECT pin selects the
. This enables the headphone func-
LFE
, in parallel, can be placed in
DD
(Continued)
to the LM4835’s
19
At low, frequencies C
frequencies, its nearly zero ohm impedance shorts R
The result is increased bridge-amplifier gain at low frequen-
cies. The combination of R
corner frequency at
The bridged-amplifier low frequency differential gain is:
Using the component values shown in Figure 1 (R
R
created at 120Hz. Assuming R
differential gain is 4. The input (C
values must be selected for a low frequency response that
covers the range of frequencies affected by the desired
bass-boost operation.
DC VOLUME CONTROL
The LM4835 has an internal stereo volume control whose
setting is a function of the DC voltage applied to the DC VOL
CONTROL pin. The volume control’s voltage input range is
0V to V
voltage = 80% V
volume remains at 0dB for DC control voltages greater than
80% V
ates at unity gain, bypassing the volume control. A graph
showing a typical volume response versus DC control volt-
age is shown in the Typical Performance Characteristics
section.
Like all volume controls, the LM4835’s internal volume con-
trol is set while listening to an amplified signal that is applied
to an external speaker. The actual voltage applied to the DC
VOL CONTROL pin is a result of the volume a listener
desires. As such, the volume control is designed for use in a
feedback system that includes human ears and preferences.
This feedback system operates quite well without the need
for accurate gain. The user simply sets the volume to the
desired level as determined by their ear, without regard to
the actual DC voltage that produces the volume. Therefore,
the accuracy of the volume control is not critical, as long as
the volume changes monotonically, matches well between
LFE
FIGURE 5. Figure 5. Low Frequency Enhancement
= 20kΩ, and C
DD
DD
. When the MODE input is 0V, the LM4835 oper-
. The volume range is from 0dB (DC control
DD
A
f
) to -80dB (DC control voltage = 0V). The
C
VD
LFE
LFE
= 1 / (2πR
= 2(R
= 0.068µF), a first-order, -3dB pole is
is a virtual open circuit and at high
LFE
F
+ R
LFE
i
and C
i
= 20kΩ, the low frequency
) and output (C
LFE
C
LFE
) / R
10013911
LFE
)
i
form with a -3dB
O
www.national.com
) capacitor
F
= 20kΩ,
(10)
LFE
(9)
.