LM4961LQBD/NOPB National Semiconductor, LM4961LQBD/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM4961LQBD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4961LQBD/NOPB

• Lead Free Status / Rohs Status: Compliant

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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 V1 and V
to the device as possible.
SELECTING INPUT CAPACITOR FOR AUDIO AMPLIFIER
One of the major considerations is the closedloop bandwidth
of the amplifier. To a large extent, the bandwidth is dictated
by the choice of external components shown in Figure 1. The
input coupling capacitor, C
which limits low frequency response. This value should be
chosen based on needed frequency response for a few dis-
tinct 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 atten-
uation. 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, C
high value input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca-
pacitor 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 consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
on pops since it determines how fast the amplifer turns on.
The slower the amplifier’s outputs ramp to their quiescent DC
voltage (nominally 1/2 V
Choosing C
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
C
to turn-on clicks and pops. Thus, a value of C
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 speak-
ers, a typical application requires a closed-loop differential
gain of 10. In this case a feedback capacitor (C
needed as shown in Figure 1 to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that elimi-
nates possible high frequency noise. Care should be taken
when calculating the -3dB frequency because an incorrect
combination of R
frequency
SELECTING VALUE FOR R
The audio power amplifier integrated in the LM4961 is de-
signed 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
B
equal to 0.1µF, the device will be much more susceptible
B
B
, is the most critical component to minimize turn-
equal to 1.0µF along with a small value of C
f
and C
f
2 will cause rolloff before the desired
DD
i
, forms a first order high pass filter
), the smaller the turn-on pop.
C
DD
pins should be as close
B
equal to 1.0µF
f
2) will be
DD
). This
i
i
. A
(in
10
1kΩ. If the input signal is less than V
-0.3V, and if the input signal is disabled when in shutdown
mode, Rchg1 and Rchg2 may be shorted out.
SELECTING OUTPUT CAPACITOR (C
CONVERTER
A single 4.7µF to 10µF ceramic capacitor will provide suffi-
cient 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 Os-
con 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.
SELECTING INPUT CAPACITOR (Cs1) FOR BOOST
CONVERTER
An input capacitor is required to serve as an energy reservoir
for the current which must flow into the coil each time the
switch turns ON. This capacitor must have extremely low
ESR, so ceramic is the best choice. We recommend a nomi-
nal value of 4.7µF, but larger values can be used. Since this
capacitor reduces the amount of voltage ripple seen at the
input pin, it also reduces the amount of EMI passed back
along that line to other circuitry.
SETTING THE OUTPUT VOLTAGE (V
CONVERTER
The output voltage is set using the external resistors R
R
mended for R
92µA. R
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM4961's internal Boost converter is internally
compensated, the external feed-forward capacitor C
quired for stability (see Figure 1). Adding this capacitor puts
a zero in the loop response of the converter. The recom-
mended frequency for the zero fz should be approximately
6kHz. C
SELECTING DIODES
The external diode used in Figure 1 should be a Schottky
diode. A 20V diode such as the MBR0520 from Fairchild
Semiconductor is recommended.
The MBR05XX series of diodes are designed to handle a
maximum average current of 0.5A. For applications exceed-
ing 0.5A average but less than 1A, a Microsemi UPS5817 can
be used.
3
(see Figure 1). A value of approximately 13.3kΩ is recom-
f
2
1 can be calculated using the formula:
is calculated using the formula:
V
3
to establish a divider current of approximately
1
C
= V
f
1 = 1 / (2π x R
FB
[1 + R
2
(R
3
1
+ 170)]
x fz)
DD
+0.3V and greater than
1
O
) OF BOOST
) FOR BOOST
f
is re-
2
and
(5)
(6)