LM4674ATL datasheet(14/18 Pages) NSC

14 / 18 page

Application Information (Continued)

PCB LAYOUT GUIDELINES

As output power increases, interconnect resistance (PCB

traces and wires) between the amplifier, load and power

supply create a voltage drop. The voltage loss due to the

traces between the LM4674A and the load results in lower

output power and decreased efficiency. Higher trace resis-

tance between the supply and the LM4674A has the same

effect as a poorly regulated supply, increasing ripple on the

supply line, and reducing peak output power. The effects of

residual trace resistance increases as output current in-

creases due to higher output power, decreased load imped-

ance or both. To maintain the highest output voltage swing

and corresponding peak output power, the PCB traces that

connect the output pins to the load and the supply pins to the

power supply should be as wide as possible to minimize

trace resistance.

The use of power and ground planes will give the best

THD+N performance. In addition to reducing trace resis-

tance, the use of power planes creates parasitic capacitors

that help to filter the power supply line.

The inductive nature of the transducer load can also result in

overshoot on one of both edges, clamped by the parasitic

diodes to GND and VDD in each case. From an EMI stand-

point, this is an aggressive waveform that can radiate or

conduct to other components in the system and cause inter-

ference. In is essential to keep the power and output traces

short and well shielded if possible. Use of ground planes

beads and micros-strip layout techniques are all useful in

preventing unwanted interference.

As the distance from the LM4674A and the speaker in-

creases, the amount of EMI radiation increases due to the

output wires or traces acting as antennas become more

efficient with length. Ferrite chip inductors places close to the

LM4674A outputs may be needed to reduce EMI radiation.

LM4674ATL Datasheet(PDF) 14 Page - National Semiconductor (TI)