LM4666SD National Semiconductor, LM4666SD Datasheet - Page 13

• Manufacturer Part Number: LM4666SD
• Description: IC AMP AUDIO 1.2W STER D 14LLP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class Dr

Specifications of LM4666SD

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 1.2W x 2 @ 8 Ohm
• Voltage - Supply: 2.8 V ~ 5.5 V
• Features: Depop, Differential Inputs, Short-Circuit Protection, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 14-LLP
• For Use With: LM4666SDBD - BOARD EVALUATION LM4666SD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4666SD/NOPB; LM4666SDTR

Application Information
REFERENCE DESIGN BOARD SCHEMATIC
In addition to the minimal parts required for the application
circuit, a measurement filter is provided on the evaluation
circuit 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 com-
ponent on the outputs. The grounded capacitive filter ele-
ments 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 differ-
ential 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.
The commonly used Audio Precision analyzer is differential
but its ability to accurately reject fast pulses of 160ns width is
questionable necessitating the on board measurement filter.
When the signal needs to be single-ended, use an audio
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FIGURE 4.
13
signal transformer to convert the differential output 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 -0.35dB (4%). 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 out-
puts. 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.
The AUX-0025 Switching Amplifier Measurement Filter from
Audio Precision may be used instead of the on board mea-
surement filter. The AUX-0025 filter should be connected to
the high current direct outputs on the evaluation board and in
series with the measurement equipment. Attaching oscillo-
scope probes on the outputs of the AUX-0025 filter will
display the audio waveforms. The AUX-0025 filter may also
be connected to the on board filter without any adverse
effects.
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