LM4652 datasheet(14/20 Pages) NSC

14 / 20 page

Application Information (Continued)

LM4652 is the R

DS(ON) of the FET times the RMS output

current when operating at maximum output power. The other

component of power dissipation in the LM4652 is the switch-

ing loss. If the output power is high enough and the DC

resistance of the filter coils is not minimized then significant

loss can occur in the output filter. This will not affect the

power dissipation in the LM4652 but should be checked to

be sure that the filter coils with not over heat.

The first step in determining the maximum power dissipation

is finding the maximum output power with a given voltage

and load. Refer to the graph Output Power verses Supply

Voltage to determine the output power for the given load and

supply voltage. From this power, the RMS output current can

be calculated as I

OUTRMS = SQRT(POUT/RL). The power

dissipation caused by the output current is P

DOUT

OUTRMS)

2 * (2*R

DS(ON)). The value for RDS(ON) can be

found from the Electrical Characteristics for the LM4652

table above. The percentage of loss due to the switching is

calculated by Equation (9):

%LOSS

SWITCH =(tr+tf +TOVERMOD) * fSW

(9)

r,tf and TOVERMOD can be found in the Electrical Charac-

teristic for the LM4651 and Electrical Characteristic for

the LM4652 sections above. The system designer deter-

mines the value for f

SW (switching frequency). Power dissi-

pation caused by switching loss is found by Equation (10).

OUTMAX is the 1% output power for the given supply voltage

and the load impedance being used in the application. P

OUT-

MAX

can be determined from the graph Output Power vs.

Supply Voltage in the Typical Performance Characteris-

tics section above.

DSWITCH = (%LOSSSWITCH * POUTMAX)/

(1−%LOSS

SWITCH)

(Watts)

(10)

DMAX for the LM4652 is found by adding the two compo-

nents (P

DSWITCH +PDOUT) of power dissipation together.

Determining the Correct Heat Sink

Once the LM4652’s power dissipation known, the maximum

thermal resistance (in ˚C/W) of a heat sink can be calculated.

This calculation is made using Equation (11) and is based on

the fact that thermal heat flow parameters are analogous to

electrical current flow properties.

DMAX =(TJMAX −TAMBIENTMAX)/ θJA

(Watts)

(11)

Where

JA = θJC + θCS + θSA

Since we know

JC, θCS, and TJMAX from the Absolute

Maximum Ratings and Operating Ratings sections above

(taking care to use the correct

JC for the LM4652 depending

on which package type is being used in the application) and

have calculated P

DMAX and TAMBIENTMAX, we only need θSA,

the heat sink’s thermal resistance. The following equation is

derived from Equation (11):

SA = [(TJMAX −TAMBIENTMAX)/PDMAX]− θJC − θCS

Again, it must be noted that the value of

SA is dependent

upon the system designer’s application and its correspond-

ing parameters as described previously. If the ambient tem-

perature surrounding the audio amplifier is higher than

AMBIENTMAX, then the thermal resistance for the heat sink,

given all other parameters are equal, will need to be lower.

Example Design of a Class D Amplifier

The following is an example of how to design a class D

amplifier system for a power subwoofer application utilizing

the LM4651 and LM4652 to meet the design requirements

listed below:

•

Output Power, 1% THD

125W

•

Load Impedance

Ω

•

Input Signal level

3V RMS (max)

•

Input Signal Bandwidth

10Hz − 150Hz

•

Ambient Temperature

50˚C (max)

Determine the Supply Voltage

From the graph Output Power verses Supply voltage at

1% THD the supply voltage needed for a 125 watt, 4

Ω

application is found to be ±20V.

Determine the Value for R

OSC(Modulation Frequency)

The oscillation frequency is chosen to obtain a satisfactory

efficiency level while also maintaining a reasonable THD

performance. The modulation frequency can be chosen us-

ing the Clipping Power Point and Efficiency verses

Switching Frequency graph. A modulation frequency of

125kHz is found to be a good middle ground for THD per-

formance and efficiency. The value of the resistor for R

OSC is

found from Equation (6) to be 3.9 k

Ω.

Determine the Value for R

SCKT (Circuit Limit)

The current limit is internally set as a failsafe to 10 amps.

The inductor ripple current and the peak output current must

be lower than 10 amps or current limit protection will turn on.

A typical 4

Ω load driven by a filter using 50µH inductors does

not require more than 10A. The current limit will have to be

increased when loads less than 4

Ω are used to acheive

higher output power. With R

SCKT equal to 100kΩ, the current

limit is 10A.

Determine the Value for R

DLY (Dead Time Control)

The delay time or dead time is set to the recommended

value so R

DLY equals 5kΩ. If a higher bandwidth of operation

is desired, R

DLY should be a lower value resistor. If a zero

value for R

DLY is desired, connect the LM4651’s pin 17 to

GND.

Determine the Value of L

1,CBYP,C1,Rfl1 Rfl2,Cfl1 Cfl2,Rf,

f (the Output and Feedback Filters)

All component values show in

Figure 1 Typical Audio Ap-

plication Circuit, are optimized for a subwoofer application.

Use the following guidelines when changing any component

values from those shown. The frequency response of the

output filter is controlled by L

1 and CBYP. Refer to the Ap-

plication Information section titled Output Stage Filtering

for a detailed explanation on calculating the correct values

for L

1 and CBYP.

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