LM4755TS National Semiconductor, LM4755TS Datasheet - Page 16

• Manufacturer Part Number: LM4755TS
• Description: IC,Audio Amplifier,DUAL,BIPOLAR,SIP,9PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4755TS

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 11x2@4OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.08@8Ohm@2W%
• Single Supply Voltage (typ): 12/15/18/24/28V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Power Dissipation: 62.5W
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 50dB
• Single Supply Voltage (min): 9V
• Single Supply Voltage (max): 32V
• 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: 9 +Tab
• Package Type: TO-263
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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 ter-
minated 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 ap-
plications, this termination is typically provided by the previ-
ous 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 cir-
cuitry pulls down the half-supply bias line, shutting down the
preamp section of the LM4755. Due to the wide operating
supply range of the LM4755, the threshold is set to just under
9V. There may be certain applications where a higher thresh-
old voltage is desired. One example is a design requiring a
high operating supply voltage, with large supply and bias ca-
pacitors, and there is little or no other circuitry connected 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 thresh-
old, 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 gov-
erned 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)
16
THERMAL CONSIDERATIONS
Heat Sinking
Proper heatsinking is necessary to ensure that the amplifier
will function correctly under all operating conditions. A
heatsink that is too small will cause the die to heat excessively
and will result in a degraded output signal as the thermal pro-
tection 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 equa-
tion 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 dis-
sipation properties of printed circuit board area. The TO-263
package is not recommended for surface mount applications
with V
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 approx-
imate Thermal resistance (θ
1.5 × 1.5 in. sq.
2 × 2 in. sq.
JC
CS
SA
SA
J
DMAX
J
A
–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
≤
A
S
[(T
FIGURE 6. External Undervoltage Pull-Down
= P
= maximum power dissipation of the IC
> 16V due to limited printed circuit board area. There
J
–T
DMAX
A
) / P
• (θ
DMAX
JC
20–27°C/W
16–23°C/W
+ θ
] - θ
CS
JC
+ θ
SA
–θ
) ranging from:
SA
CS
)
(T
testing, 1 oz. Copper)
A
=28°C, Sine wave
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