LM4675TLBD National Semiconductor, LM4675TLBD Datasheet - Page 17

• Manufacturer Part Number: LM4675TLBD
• Description: BOARD EVALUATION LM4675TL
• Manufacturer: National Semiconductor
• Series: Boomer®, PowerWise®r

Specifications of LM4675TLBD

• Amplifier Type: Class D
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 2.7W x 1 @ 4 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4675
• Lead Free Status / RoHS Status: Not applicable / Not applicable

REFERENCE DESIGN BOARD SCHEMATIC
In addition to the minimal parts required for the application
circuit, a measurement filter is provided on the evaluation cir-
cuit board so that conventional audio measurements can be
conveniently made without additional equipment. This is a
balanced input, grounded differential output low pass filter
with a 3dB frequency of approximately 35kHz and an on board
termination resistor of 300Ω (see schematic). Note that the
capacitive load elements are returned to ground. This is not
optimal for common mode rejection purposes, but due to the
independent pulse format at each output there is a significant
amount of high frequency common mode component on the
outputs. The grounded capacitive filter elements attenuate
this component at the board to reduce the high frequency
CMRR requirement placed on the analysis instruments.
Even with the grounded filter the audio signal is still differen-
tial, necessitating a differential input on any analysis instru-
ment connected to it. Most lab instruments that feature BNC
connectors on their inputs are NOT differential responding
because the ring of the BNC is usually grounded.
FIGURE 8.
17
The commonly used Audio Precision analyzer is differential,
but its ability to accurately reject high frequency signals is
questionable necessitating the on board measurement filter.
When in doubt or when the signal needs to be single-ended,
use an audio signal transformer to convert the differential out-
put to a single ended output. Depending on the audio
transformer's characteristics, there may be some attenuation
of the audio signal which needs to be taken into account for
correct measurement of performance.
Measurements made at the output of the measurement filter
suffer attenuation relative to the primary, unfiltered outputs
even at audio frequencies. This is due to the resistance of the
inductors interacting with the termination resistor (300Ω) and
is typically about -0.25dB (3%). In other words, the voltage
levels (and corresponding power levels) indicated through the
measurement filter are slightly lower than those that actually
occur at the load placed on the unfiltered outputs. This small
loss in the filter for measurement gives a lower output power
reading than what is really occurring on the unfiltered outputs
and its load.
20182728
www.national.com