LM4850MM datasheet(14/20 Pages) NSC

14 / 20 page

Application Information (Continued)

extend to at least one−fifth the lower bandwidth limit and the

high frequency response must extend o at least five times

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 = 20kHz x 5 = 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 LM4850 GBWP of

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

the LM4850 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.

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