LM4752 datasheet(16/22 Pages) NSC

16 / 22 page

Application Information (Continued)

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 (pin5) to ground

to disable the input preamp. Figure 7 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:

R = (0.7V) / (C

B • (V S / 2) / 0.1s)

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 Application Circuit with Mute

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 Application Circuit

with Mute section for more information.

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 internal

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:

J–TA =P DMAX • (

JC +

CS +

SA)

where:

DMAX = maximum power dissipation of the IC

J(˚C) = junction temperature of the IC

A(˚C) = ambient temperature

JC(˚C/W) = junction-to-case thermal resistance of the IC

CS(˚C/W) = case-to-heatsink thermal resistance (typically

0.2 to 0.5 ˚C/W)

SA(˚C/W) = thermal resistance of heatsink

When determining the proper heatsink, the above equation

should be re-written as:

≤ [(T

J −TA)/PDMAX]−

JC −

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

S > 16V due to limited printed circuit board

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 (

SA) ranging from:

1.5 x 1.5 in. sq.

20–27˚C/W

A=28˚C, Sine wave

testing, 1 oz. Copper)

2 x 2 in. sq.

16–23˚C/W

The above values for

SA vary widely due to dimensional

proportions (i.e. variations in width and length will vary

SA).

For audio applications, where peak power levels are short in

duration, this part will perform satisfactory with less

heatsinking/copper clad area. As with any high power design

proper bench testing should be undertaken to assure the

design can dissipate the required power. Proper bench test-

ing requires attention to worst case ambient temperature

and air flow. At high power dissipation levels the part will

show a tendency to increase saturation voltages, thus limit-

ing the undistorted power levels.

Determining Maximum Power Dissipation

For a single-ended class AB power amplifier, the theoretical

maximum power dissipation point is a function of the supply

voltage, V

S, and the load resistance, RL and is given by the

following equation:

(single channel)

DMAX (W)=[VS

2 /(2 •

π2 • R

L)]

The above equation is for a single channel class-AB power

amplifier. For dual amplifiers such as the LM4752, the equa-

tion for calculating the total maximum power dissipated is:

(dual channel)

DMAX (W)=2 • [V S

2 /(2 •

π2 • R

L)]

2 /(

π 2 • R

(Bridged Outputs)

DMAX (W) = 4[VS

2 /(2

π2 • R

L)]

Heatsink Design Example

Determine the system parameters:

S = 24V

Operating Supply Voltage

L =4

Ω

Minimum load impedance

A = 55˚C

Worst case ambient temperature

10003932

FIGURE 7. External Undervoltage Pull-Down

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