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LM4961LQBD Folha de dados(PDF) 11 Page - Texas Instruments

Nome de Peças LM4961LQBD
Descrição Electrónicos  Ceramic Speaker Driver
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SNAS242J – AUGUST 2004 – REVISED MARCH 2007
SELECTING INPUT CAPACITOR FOR AUDIO AMPLIFIER
One of the major considerations is the closed loop bandwidth of the amplifier. To a large extent, the bandwidth is
dictated by the choice of external components shown in Figure 2. The input coupling capacitor, Ci, forms a first
order high pass filter which limits low frequency response. This value should be chosen based on needed
frequency response for a few distinct reasons.
High value input capacitors are both expensive and space hungry in portable designs. Clearly, a certain value
capacitor is needed to couple in low frequencies without severe attenuation. But ceramic speakers used in
portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to 150Hz.
Thus, using a high value input capacitor may not increase actual system performance.
In addition to system cost and size, click and pop performance is affected by the value of the input coupling
capacitor, Ci. A high value input coupling capacitor requires more charge to reach its quiescent DC voltage
(nominally 1/2 VDD). This charge comes from the output via the feedback and is apt to create pops upon device
enable. Thus, by minimizing the capacitor value based on desired low frequency response, turn-on pops can be
minimized.
SELECTING BYPASS CAPACITOR FOR AUDIO AMPLIFIER
Besides minimizing the input capacitor value, careful consideration should be paid to the bypass capacitor value.
Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the
amplifier turns on. The slower the amplifier’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the
smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci (in the range of 0.039µF to
0.39µF), should produce a virtually clickless and popless shutdown function. Although the device will function
properly, (no oscillations or motorboating), with CB equal to 0.1µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of CB equal to 1.0µF is recommended in all but the most cost sensitive
designs.
SELECTING FEEDBACK CAPACITOR FOR AUDIO AMPLIFIER
The LM4961 is unity-gain stable which gives the designer maximum system flexibility. However, to drive ceramic
speakers, a typical application requires a closed-loop differential gain of 10. In this case a feedback capacitor
(Cf2) will be needed as shown in Figure 1 to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that eliminates possible high frequency noise. Care should be
taken when calculating the -3dB frequency because an incorrect combination of Rf and Cf2 will cause rolloff
before the desired frequency
SELECTING VALUE FOR RC
The audio power amplifier integrated in the LM4961 is designed for very fast turn on time. The Cchg pin allows
the input capacitors (CinA and CinB) to charge quickly to improve click/pop performance. Rchg1 and Rchg2
protect the Cchg pins from any over/under voltage conditions caused by excessive input signal or an active input
signal when the device is in shutdown. The recommended value for Rchg1 and Rchg2 is 1k
Ω. If the input signal
is less than VDD+0.3V and greater than -0.3V, and if the input signal is disabled when in shutdown mode, Rchg1
and Rchg2 may be shorted out.
SELECTING OUTPUT CAPACITOR (CO) FOR BOOST CONVERTER
A single 4.7µF to 10µF ceramic capacitor will provide sufficient output capacitance for most applications. If larger
amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can
be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually
prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500
kHz because of significant ringing and temperature rise due to self-heating from ripple current. An output
capacitor with excessive ESR can also reduce phase margin and cause instability.
In general, if electrolytics are used, we recommended that they be paralleled with ceramic capacitors to reduce
ringing, switching losses, and output voltage ripple.
Copyright © 2004–2007, Texas Instruments Incorporated
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