LM4651 datasheet(12/20 Pages) NSC

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Application Information (Continued)

Introduction

National Semiconductor (NSC) is committed to providing

application information that assists our customers in obtain-

ing the best performance possible from our products. The

following information is provided in order to support this

commitment. The reader should be aware that the optimiza-

tion of performance was done using a reference PCB de-

signed by NSC and shown in Figure 7 through 11. Variations

in performance can occur because of physical changes in

the printed circuit board and the application. Therefore, the

designer should know that component value changes may

be required in order to optimize performance in a given

application. The values shown in this data sheet can be used

as a starting point for evaluation purposes. When working

with high frequency circuits, good layout practices are also

critical to achieving maximum performance.

Input Pre-Amplifier with Subwoofer Filter

The LM4651 and LM4652 Class D solution is designed for

low frequency audio applications where low gain is required.

This necessitates a pre−amplifier stage with gain and a low

pass audio filter. An inexpensive input stage can be de-

signed using National’s LM833 audio operational amplifier

and a minimum number of external components. A gain of 10

(20dB) is recommended for the pre−amplifier stage. For a

subwoofer application, the pole of the low pass filter is

normally set within the range of 60Hz − 180Hz. For a clean

sounding subwoofer the filter should be at least a

second-order filter to sharply roll off the high frequency audio

signals. A higher order filter is recommended for stand-alone

self-powered subwoofer applications. Figure 6 shows a

simple input stage with a gain of 10 and a second-order low

pass filter.

Supply Bypassing

Correct supply bypassing has two important goals. The first

is to ensure that noise on the supply lines does not enter the

circuit and become audible in the output. The second is to

help stabilize an unregulated power supply and provide cur-

rent under heavy current conditions. Because of the two

different goals multiple capacitors of various types and val-

ues are recommended for supply bypassing. For noise

de-coupling, generally small ceramic capacitors (.001µF to

.1µF) along with slightly larger tantalum or electrolytic ca-

pacitors (1µF to 10µF) in parallel will do an adequate job of

removing most noise from the supply rails. These capacitors

should be placed as close as possible to each IC’s supply

pin(s) using leads as short as possible. For supply stabiliz-

ing, large electrolytic capacitors (3,300µF to 15,000µF) are

needed. The value used is design and cost dependent.

High Frequency PCB Design

A double-sided PCB is recommended when designing a

class D amplifier system. One side should contain a ground

plane with the power traces on the other side directly over

the ground plane. The advantage is the parasitic capaci-

tance created between the ground plane and the power

planes. This parasitic capacitance is very small (pF) but is

the value needed for coupling high frequency noise to

ground. At high frequencies, capacitors begin to act more

like inductors because of lead and parasitic inductance in the

capacitor. For this reason, bypassing capacitors should be

surface mount because of their low parasitic inductance.

Equation (8) shows how to determine the amount of power to

ground plane capacitance.

C= eoerA/d

(Farads)

(8)

where eo = 0.22479pF/in and er = 4.1

A is the common PCB area and d is the distance between

the planes. The designer should target a value of 100pF or

greater for both the positive supply to ground capacitance

and negative supply to ground capacitance. Signal traces

that cross over each other should be laid out at 90˚ to

minimized any coupling.

Output Offset Voltage Minimization

The amount of DC offset voltage seen at the output with no

input signal present is already quite good with the LM4651/

52. With no input signal present the system should be at

50% duty cycle. Any deviation from 50% duty cycle creates a

DC offset voltage seen by the load. To completely eliminate

the DC offset, a DC voltage divider can be used at the input

to set the DC offset to near zero. This is accomplished by a

simple resistor divider that applies a small DC voltage to the

input. This forces the duty cycle to 50% when there is no

input signal. The result is a LM4651 and LM4652 system

with near zero DC offset. The divider should be a 1.8M

Ω

from the +6V output (pin 6) to the input (other side of 25k,

1). R1 acts like the second resistor in the divider. Also use

a 1µF input capacitor before R

1 to block the DC voltage from

the source. R

1 and the 1µF capacitor create a high pass filter

with a 3dB point at 6.35Hz. The value of R

OFFSET is set

according to the application. Variations in switching fre-

quency and supply voltage will change the amount of offset

voltage requiring a different value than stated above. The

value above (1.8M

Ω)isfor ±20V and a switching frequency

of 125kHz.

Output Stage Filtering

As common with Class D amplifier design, there are many

trade-offs associated with different circuit values. The output

stage is not an exception. National has found good results

with a 50µF inductor and a 5µF Mylar capacitor (see Figure

1, Typical Audio Application Circuit) used as the output

LC filter. The two-pole filter contains three components; L

and C

BYP because the LM4651 and LM4652 have a bridged

output. The design formula for a bridge output filter is f

C =

1/[2

π(L

12CBYP)

⁄2

A common mistake is to connect a large capacitor between

ground and each output. This applies only to single-ended

DS101277-77

FIGURE 6. Pre−amplifier Stage with Low Pass Filter

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