LM4853LD datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

the upper bandwidth limit. The variation for both response

limits is 0.17dB, well within the ± 0.25dB desired limit. This

results in:

L = 100Hz/5= 20Hz

H = 20kHzx5= 100kHz

As stated in the External Components section, R

I in con-

junction with C

I create a highpass filter. Find the coupling

capacitor’s value using Equation 9.

≥ 1/(2πR

IfL)

(9)

≥ 1/(2π x 20kΩ x 20Hz) = 0.397µF

Use a 0.39µF capacitor, the closest standard value.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the differential gain,

VD. With AVD = 3 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4853 GBWP of

10MHz. This difference indicates that a designer can still use

the LM4853 at higher differential gains without bandwidth

limitations.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 3

Ω AND 4Ω LOADS

Power dissipated by a load is a function of the voltage swing

across the load and the load’s impedance. As load imped-

ance decreases, load dissipation becomes increasingly de-

pendant on the interconnect (PCB trace and wire) resistance

between the amplifier output pins and the load’s connec-

tions. Residual trace resistance causes a voltage drop,

which results in power dissipated in the trace and not in the

load as desired. For example, 0.1

Ω trace resistance reduces

the output power dissipated by a 4

Ω load from 2.0W to

1.95W. This problem of decreased load dissipation is exac-

erbated as load impedance decreases. Therefore, to main-

tain the highest load dissipation and widest output voltage

swing, PCB traces that connect the output pins to a load

must be as wide as possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply’s output voltage

decreases with increasing load current. Reduced supply

voltage causes decreased headroom, output signal clipping,

and reduced output power. Even with tightly regulated sup-

plies, trace resistance creates the same effects as poor

supply regulation. Therefore, making the power supply

traces as wide as possible helps maintain full output voltage

swing.

Demonstration Board Layout

200334A1

FIGURE 3. Recommended MM PC Board Layout:

Component-Side SilkScreen

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