Datasheet

LM4940
SNAS219C –OCTOBER 2003–REVISED MAY 2013 www.ti.com
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires
amplification and desired output transient suppression.
As shown in Figure 26, the input resistor (R
IN
) and the input capacitor (C
IN
) produce a high pass filter cutoff
frequency that is found using Equation 5.
f
C
= 1/2πR
i
C
i
(5)
As an example when using a speaker with a low frequency limit of 50Hz, C
i
, using Equation 5 is 0.159µF. The
0.39µF C
INA
shown in Figure 26 allows the LM4940 to drive high efficiency, full range speaker whose response
extends below 30Hz.
Output Coupling Capacitor Value Selection
The capacitors C
OUTA
and C
OUTB
that block the V
DD
/2 output DC bias voltage and couple the output AC signal to
the amplifier loads also determine low frequency response. These capacitors, combined with their respective
loads create a highpass filter cutoff frequency. The frequency is also given by Equation 5.
Using the same conditions as above, with a 4Ω speaker, C
OUT
is 820µF (nearest common valve).
Bypass Capacitor Value
Besides minimizing the input capacitor size, careful consideration should be paid to value of C
BYPASS
, the
capacitor connected to the BYPASS pin. Since C
BYPASS
determines how fast the LM4940 settles to quiescent
operation, its value is critical when minimizing turn-on pops. The slower the LM4940's outputs ramp to their
quiescent DC voltage (nominally V
DD
/2), the smaller the turn-on pop. Choosing C
BYPASS
equal to 10µF along with
a small value of C
IN
(in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As
discussed above, choosing C
IN
no larger than necessary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4940 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this
discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode
is deactivated.
As the V
DD
/2 voltage present at the BYPASS pin ramps to its final value, the LM4940's internal amplifiers are
configured as unity gain buffers and are disconnected from the AMP
A
and AMP
B
pins. An internal current source
charges the capacitor connected between the BYPASS pin and GND in a controlled manner. Ideally, the input
and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until
the voltage applied to the BYPASS pin.
The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches V
DD
/2. As soon as
the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are
reconnected to their respective output pins. Although the BYPASS pin current cannot be modified, changing the
size of C
BYPASS
alters the device's turn-on time. Here are some typical turn-on times for various values of
C
BYPASS
:
C
B
(µF) T
ON
(ms)
1.0 120
2.2 120
4.7 200
10 440
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