Datasheet

LM48511
SNAS416G –JULY 2007–REVISED MAY 2013 www.ti.com
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers, and switching DC-DC
converters, is critical for optimizing device and system performance. Consideration to component values must be
used to maximize overall system quality. The best capacitors for use with the switching converter portion of the
LM48511 are multi-layer ceramic capacitors. They have the lowest ESR (equivalent series resistance) and
highest resonance frequency, which makes them optimum for high frequency switching converters. When
selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as Z5U and
Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may
provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor
manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from
Taiyo-Yuden and Murata.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both PV1, V1 and V
DD
pins should be as close to the device as possible.
AUDIO AMPLIFIER GAIN SETTING RESISTOR SELECTION
The amplifier gain of the LM48511 is set by four external resistors, the input resistors, R5 and R7, and the feed
back resistors R6 and R8.. The amplifier gain is given by:
Where R
IN
is the input resistor and R
F
is the feedback resistor.
A
VD
= 2 X R
F
/R
IN
(3)
Careful matching of the resistor pairs, R6 and R8, and R5 and R7, is required for optimum performance. Any
mismatch between the resistors results in a differential gain error that leads to an increase in THD+N, decrease
in PSRR and CMRR, as well as an increase in output offset voltage. Resistors with a tolerance of 1% or better
are recommended.
The gain setting resistors should be placed as close to the device as possible. Keeping the input traces close
together and of the same length increases noise rejection in noisy environments. Noise coupled onto the input
traces which are physically close to each other will be common mode and easily rejected.
AUDIO AMPLIFIER INPUT CAPACITOR SELECTION
Input capacitors may be required for some applications, or when the audio source is single-ended. Input
capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of
the audio source and the bias voltage of the LM48511. The input capacitors create a high-pass filter with the
input resistors R
IN
. The -3dB point of the high pass filter is found by:
f = 1 / 2πR
IN
C
IN
(4)
In single-ended configurations, the input capacitor value affects click-and-pop performance. The LM48511
features a 50mg turn-on delaly. Choose the input capacitor / input resistor values such that the capacitor is
charged before the 50ms turn-on delay expires. A capacitor value of 0.18μF and a 20kΩ input resistor are
recommended. In differential applications, the charging of the input capacitor does not affect click-and-pop
significantly.
The input capacitors can also be used to remove low frequency content from the audio signal. High pass filtering
the audio signal helps protect speakers that can not reproduce or may be damaged by low frequencies. When
the LM48511 is using a single-ended source, power supply noise on the ground is seen as an input signal.
Setting the high-pass filter point above the power supply noise frequencies, 217Hz in a GSM phone, for example,
filters out the noise such that it is not amplified and heard on the output. Capacitors with a tolerance of 10% or
better are recommended for impedance matching and improved CMRR and PSRR.
SELECTING REGULATOR OUTPUT CAPACITOR
A single 100µF low ESR tantalum capacitor provides sufficient output capacitance for most applications. Higher
capacitor values improve line regulation and transient response. Typical electrolytic capacitors are not suitable
for switching converters that operate above 500kHz because of significant ringing and temperature rise due to
self-heating from ripple current. An output capacitor with excessive ESR reduces phase margin and causes
instability.
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