L4952TS NSC [National Semiconductor], L4952TS Datasheet - Page 14

• Manufacturer Part Number: L4952TS
• Description: 3.1W Stereo-SE Audio Power Amplifier with DC Volume Control
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
soft. Since the gain is not critical, there may be a volume
variation from part-to-part even with the same applied DC
volume control voltage. The gain of a given LM4952 can be
set with fixed external voltage, but another LM4952 may
require a different control voltage to achieve the same gain.
Figure 5 is a curve showing the volume variation of five
typical LM4952s as the voltage applied to the DC VOL input
pin is varied. For gains between –20dB and +16dB, the
typical part-to-part variation is typically
control voltage.
VOLUME CONTROL VOLTAGE GENERATION
Figure 6 shows a simple circuit that can be used to create an
adjustable DC control voltage that is applied to the DC Vol
input. The 91kΩ series resistor and the 50kΩ potentiometer
create a voltage divider between the supply voltage, V
and GND. The series resistor’s value assumes a 12V power
supply voltage. The voltage present at the node between the
series resistor and the top of the potentiometer need only be
a nominal value of 3.5V and must not exceed 9.5V, as stated
in the LM4952’s Absolute Maximum Ratings.
FIGURE 5. Typical part-to-part gain variation as a
function of DC Vol control voltage
±
(Continued)
1dB for a given
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DD
,
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UNREGULATED POWER SUPPLIES AND THE DC VOL
CONTROL
As an amplifier’s output power increases, the current that
flows from the power supply also increases. If an unregu-
lated power supply is used, its output voltage can decrease
(“droop” or “sag”) as this current increases. It is not uncom-
mon for an unloaded unregulated 15V power supply con-
nected to the LM4952 to sag by as much as 2V when the
amplifier is drawing 1A to 2A while driving 4Ω stereo loads to
full power dissipation. Figure 7 is an oscilloscope photo
showing an unregulated power supply’s voltage sag while
powering an LM4952 that is driving 4Ω stereo loads. The
amplifier’s input is a typical music signal supplied by a CD
player. As shown, the sag can be quite significant.
This sagging supply voltage presents a potential problem
when the voltage that drives the DC Vol pin is derived from
the voltage supplied by an unregulated power supply. This is
the case for the typical volume control circuit (a 50kΩ poten-
tiometer in series with a 91kΩ resistor) shown in Figure 6.
The potentiometer’s wiper is connected to the DC Vol pin.
With this circuit, power supply voltage fluctuations will be
control. Capacitor connected to DC VOL pin minimizes
that could cause changes in perceived volume setting
FIGURE 6. Typical circuit used for DC voltage volume
V
voltage fluctuation when using unregulated supplies
FIGURE 7. LM4952 operating on an unregulated 12V
(nominal) power supply. Wave forms shown include
the DC voltage applied to the DC VOL pin (Trace D)
DD
(Trace A), V
OUT A
(Trace B), V
OUT B
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(Trace C), and
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