LM4840LQ National Semiconductor, LM4840LQ Datasheet - Page 10

• Manufacturer Part Number: LM4840LQ
• Description: Stereo 2W Audio Power Amplifiers with Digital Volume Control and Input Mux
• Manufacturer: National Semiconductor

www.national.com
Application Information
DIGITAL VOLUME CONTROL
The LM4840 features a digital volume control which consists
of the CLOCK, UP, and DOWN pins. An external clock may
be fed to the CLOCK pin, or, by connecting a capacitor from
the CLOCK pin to ground, the internal clock may be used.
The internal clock frequency with respect to this capacitor
value is determined from the following formula:
When using an external clock, the clock is buffered and the
internal clock frequency is that of the external clock divided
by 2. Also, the maximum frequency should be kept below
100kHz.
Volume changes are then effected by toggling either the UP
or DOWN pins with a logic high. After a period of 4 clock
pulses with either the UP or DOWN pins held high, the
volume will change to the next specified step, either up or
down. Volume levels for each step vary and are specified in
Table 2. If either the UP or DOWN pin remains high after the
first volume transition the volume will change again, but this
time after 40 clock pulses. The next transition occurs at 20
clock pulses, then 12, then 8, and from then on 4 clock
pulses for each volume transtition. This cycle is shown in the
timing diagram shown in Figure 3. Releasing the held UP or
DOWN pin to ground at any time re-starts the cycle. This is
intended to provide the user with a volume control that
pauses briefly after initial application, then slowly increases
the rate of volume change as it is continuously applied.
If both the UP and DOWN pins are held high, no volume
change will occur. Trigger points for the UP and DOWN pins
are at 60% of V
maximum for a logic low. It is recommended, however, to
toggle UP and DOWN between V
performance. When using an external clock, clock pulses
should be a minimum 0f 3V for a high and maximum of 0.9V
for a low when using a 5V supply. Again, pulsing an external
clock from V
lowing these guidelines the volume may then be changed
with a microcontroller or manually using switches.
MEMORY FUNCTION
The LM4840 features a volume memory that saves the last
volume setting when power is turned off. This requires that
an auxiliary power source be connected to V
diode as shown in Figure 1. Connecting the circuit as shown
also provides that power to the V
V
at a voltage of 2.3V or greater to maintain volume memory
when V
cations as laptop computers, where V
and V
default volume setting for the LM4840 is -10dB in BTL mode,
and -16dB in single-ended mode. This default setting is only
achieved on power up when both V
been turned off, and the circuit had sufficient time to dis-
charge (
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that can switch be-
tween
single-ended (SE) headphones use a coupling capacitor on
each SE output. This capacitor blocks the half-supply volt-
age to which the output amplifiers are typically biased and
DD
f
CLK
when V
AUX
= (7.338 x 10
DD
<
driving
500ms depending on capacitor value at V
is absent. This feature is intended for such appli-
is connected to the real time clock battery. The
DD
DD
is on and is greater than V
DD
to GND ensures reliable performance. Fol-
bridge-tied-load
minimum for a logic high, and 20% of V
-7
) / C
AUX
(BTL)
DD
pin is being drawn from
DD
DD
is the system power
and V
and GND for best
AUX
speakers
. V
AUX
AUX
AUX
through a
had both
must be
AUX
).
and
DD
10
couples the audio signal to the headphones. The signal
return to circuit ground is through the headphone jack’s
sleeve.
The LM4840 eliminates these coupling capacitors. Amp2A is
internally configured to apply V
jack’s sleeve. This voltage matches the quiescent voltage
present on the Amp1A and Amp1B outputs that drive the
headphones. The headphones operate in a manner very
similar to a bridge-tied-load (BTL). The same DC voltage is
applied to both headphone speaker terminals. This results in
no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal’s output
amplitude increases on the speaker’s terminal.
When operating as a headphone amplifier, the headphone
jack sleeve is not connected to circuit ground. Using the
headphone output jack as a line-level output will place the
LM4840’s one-half supply voltage on a plug’s sleeve con-
nection.
audio-visual display equipment is possible. This presents no
difficulty when the external equipment uses capacitively
coupled inputs. For the very small minority of equipment that
is DC-coupled, the LM4840 monitors the current supplied by
the amplifier that drives the headphone jack’s sleeve. If this
current exceeds 500mA
tecting the LM4840 and the external equipment. For more
information, see the section titled ’Single-Ended Output
Power Performance and Measurement Considerations’.
EXPOSED-DAP MOUNTING CONSIDERATIONS
The LM4840’s exposed-DAP (die attach paddle) packages
(MH, LQ) provide a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. The result is a low voltage audio power amplifier that
produces 2W at
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4840’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MH and LQ packages must have their exposed DAPs
soldered to a grounded copper pad on the PCB. The DAP’s
PCB copper pad is connected to a large plane of continuous
unbroken copper. This plane forms a thermal mass and heat
sink and radiation area. Place the heat sink area on either
outside plane in the case of a two-sided PCB, or on an inner
layer of a board with more than two layers. Connect the DAP
copper pad to the inner layer or backside copper heat sink
area with 32(4x8) (MH ) or 6(3x2) (LQ) vias. The via diam-
eter should be 0.012in–0.013in with a 1.27mm pitch. Ensure
efficient thermal conductivity by plating-through and solder-
filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4
not placed on the same PCB layer as the should be 5in
(min) for the same supply voltage and load resistance. The
last two area recommendations apply for 25˚C ambient tem-
perature. Increase the area to compensate for ambient tem-
peratures above 25˚C. In systems using cooling fans, the
LM4840MH can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in
exposed copper or 5.0in
the LM4840MH can continuously drive a 3
power. The LM4840LQ achieves the same output power
Driving
1% THD with a 4
a
portable
2
PK
inner layer copper plane heatsink,
, the amplifier is shutdown, pro-
DD
/2 to a stereo headphone
notebook
load. This high power
load. Heatsink areas
2
computer
(min) area is
load to full
or
2
2