LM4755_02 NSC [National Semiconductor], LM4755_02 Datasheet - Page 16

• Manufacturer Part Number: LM4755_02
• Description: Stereo 11W Audio Power Amplifier with Mute
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
PREVENTING OSCILLATIONS
With the integration of the feedback and bias resistors on-
chip, the LM4755 fits into a very compact package. However,
due to the close proximity of the non-inverting input pins to
the corresponding output pins, the inputs should be AC
terminated at all times. If the inputs are left floating, the
amplifier will have a positive feedback path through high
impedance coupling, resulting in a high frequency oscillation.
In most applications, this termination is typically provided by
the previous stage’s source impedance. If the application will
require an external signal, the inputs should be terminated to
ground with a resistance of 50 kΩ or less on the AC side of
the input coupling capacitors.
UNDERVOLTAGE SHUTDOWN
If the power supply voltage drops below the minimum oper-
ating supply voltage, the internal under-voltage detection
circuitry pulls down the half-supply bias line, shutting down
the preamp section of the LM4755. Due to the wide operat-
ing supply range of the LM4755, the threshold is set to just
under 9V. There may be certain applications where a higher
threshold voltage is desired. One example is a design requir-
ing a high operating supply voltage, with large supply and
bias capacitors, and there is little or no other circuitry con-
nected to the main power supply rail. In this circuit, when the
power is disconnected, the supply and bias capacitors will
discharge at a slower rate, possibly resulting in audible
output distortion as the decaying voltage begins to clip the
output signal. An external circuit may be used to sense for
the desired threshold, and pull the bias line (pin 6) to ground
to disable the input preamp. Figure 6 shows an example of
such a circuit. When the voltage across the zener diode
drops below its threshold, current flow into the base of Q1 is
interrupted. Q2 then turns on, discharging the bias capacitor.
This discharge rate is governed by several factors, including
the bias capacitor value, the bias voltage, and the resistor at
the emitter of Q2. An equation for approximating the value of
the emitter discharge resistor, R, is given below:
Note that this is only a linearized approximation based on a
discharge time of 0.1s. The circuit should be evaluated and
adjusted for each application.
As mentioned earlier in the Built-in Mute Circuit section,
when using an external circuit to pull down the bias line, the
rate of discharge will have an effect on the turn-off induced
distortions. Please refer to the Built-in Mute Circuit section
for more information.
R = (0.7v) / (Cb • (V
CC
/2) / 0.1s)
(Continued)
16
THERMAL CONSIDERATIONS
Heat Sinking
Proper heatsinking is necessary to ensure that the amplifier
will function correctly under all operating conditions. A heat-
sink that is too small will cause the die to heat excessively
and will result in a degraded output signal as the thermal
protection circuitry begins to operate.
The choice of a heatsink for a given application is dictated by
several factors: the maximum power the IC needs to dissi-
pate, the worst-case ambient temperature of the circuit, the
junction-to-case thermal resistance, and the maximum junc-
tion temperature of the IC. The heat flow approximation
equation used in determining the correct heatsink maximum
thermal resistance is given below:
T
where:
P
T
T
θ
θ
0.2 to 0.5 ˚C/W)
θ
When determining the proper heatsink, the above equation
should be re-written as:
θ
TO-263 HEATSINKING
Surface mount applications will be limited by the thermal
dissipation properties of printed circuit board area. The TO-
263 package is not recommended for surface mount appli-
cations with V
area. There are TO-263 package enhancements, such as
clip-on heatsinks and heatsinks with adhesives, that can be
used to improve performance.
Standard FR-4 single-sided copper clad will have an ap-
proximate Thermal resistance (θ
JC
CS
SA
SA
J
J
A
DMAX
–T
(˚C) = junction temperature of the IC
(˚C) = ambient temperature
(˚C/W) = junction-to-case thermal resistance of the IC
(˚C/W) = case-to-heatsink thermal resistance (typically
(˚C/W) = thermal resistance of heatsink
≤ [(T
A
FIGURE 6. External Undervoltage Pull-Down
= P
= maximum power dissipation of the IC
J
–T
DMAX
A
) / P
S
• (θ
>
DMAX
16V due to limited printed circuit board
JC
+ θ
] - θ
CS
JC
+ θ
–θ
CS
SA
SA
) ranging from:
)
10005932