ISL54405_14 Datasheet, PDF (11/20 Pages) Intersil Corporation – CD/MP3 Quality Stereo 2:1 Multiplexer with Click and Pop Elimination

ISL54405
AC Coupled or DC Coupled Operation
The Audio CODEC drivers can be directly coupled to the
ISL54405 when the audio signals from the drivers are ground
referenced or do not have a significant DC offset voltage,
<50mV. Otherwise, the signal should be AC coupled to the
ISL54405 part.
CLICK AND POP OPERATION
The ISL54405 has special circuitry to eliminate click and pops
in the speakers during power-up and power-down of the Audio
CODEC Drivers and during removal and insertion of
headphones.
A different click and pop scheme is required depending on
whether the audio CODEC drivers are AC coupled or DC
coupled to the inputs of the ISL54405 part.
AC COUPLED CLICK AND POP OPERATION
Single supply audio drivers have their signal biased at a DC
offset voltage, usually at 1/2 the DC supply voltage of the
driver. As this DC bias voltage comes up or goes down during
power up or down of the driver a transient can be coupled into
the speaker load through the DC blocking capacitor (see the
“Sound Card AC Coupled Application Block Diagrams” on
page 8).
When a driver is off and suddenly turned on, the rapidly
changing DC bias voltage at the output of the driver will cause
an equal voltage at the input side of the switch due to the fact
that the voltage across the blocking capacitor cannot change
instantly. If the switch is in audio mode or there is no low
impedance path to discharge the capacitor voltage at the input
of the switch, before turning on the switch, a transient
discharge will occur in the speaker, generating a click and pop
noise.
Proper elimination of a click/pop transient at the speaker load
while powering up or down of the audio driver requires that the
ISL54405 have its C/P shunts activated on the source side of
the switch and then placed in mute mode. This allows the
transient generated by the audio drivers to be discharged
through the click and pop shunt circuitry.
Once the driver DC bias has reached VDD/2 and the transient
on the switch side of the DC blocking capacitor has been
discharged to ground through the C/P shunt circuitry, the
switches can be turned on and connected through to the
speaker loads without generating an undesirable click/pop in
the speakers.
With a typical DC blocking capacitor of 220µF and the C/P
shunt circuitry designed to have a resistance of 40allowing
a 100ms wait time to discharge the transient before placing
the switch in the audio mode will prevent the transient from
getting through to the speaker load. See Figures 26 and 27 on
page 15.
CLICK AND POP ELIMINATION WHEN CONNECTED TO
HIGH IMPEDANCE SOURCE AND LOAD
By design, in order to flatten the RON resistance of the switch
across the signal range (±3V) a current gets added to the
signal path. When the ISL54405 part is connected to a high
impedance source (i.e. AC coupled to the input of the switch)
and a high impedance load, (such as the impedance of a 20k
to 100k preamplifier stage) a DC offset voltage will be
present on the signal line in the range of 35mV to 135mV.
When the switch is turned off, this offset voltage gets pulled to
ground. During switching, this change in the offset voltage can
cause a click and pop noise to be heard in the downstream
speaker.
Placing a 1kresistor from the output of the switch to ground
will lower the offset voltage to around 1.5mV, thereby
effectively eliminating the click and pop noise. The 1k
resistor is small enough to reduce the voltage offset
significantly while not increasing power dissipation
dramatically. Power consumption will need to be considered
when using a smaller impedance under this scenario.
When connected to a low impedance load such as
headphones (32), the current added to the signal line results
in a minimal DC offset voltage on the signal line and does not
cause click and pop noise when the switch is turned off.
DC COUPLED CLICK AND POP OPERATION
The ISL54405 can pass ground referenced audio signals which
allows it to be directly connected to audio drivers that output
ground referenced audio signals, eliminating the need for a DC
blocking capacitor.
Audio drivers that swing around ground, however, do generate
some DC offset, from a few millivolts to tens of millivolts.
When switching between audio channels or muting the audio
signal, these small DC offset levels of the drivers can generate
a transient that can cause unwanted clicks and pops in the
speaker loads.
In a DC coupled application the C/P shunt resistors placed at
the source side of the switch have no effect in eliminating the
transients at the speaker loads when transitioning in and out
of the mute state or switching between channels. In fact,
having these C/P shunts active on the source side
unnecessarily increases the power consumption. So, for DC
coupled connection, the C/P shunt circuitry should not be
applied at the source (driver) side of the switch.
For DC coupled applications the ISL54405 has a special
soft-start feature that slowly ramps the DC offset voltage from
the audio driver to the speaker load when turning on a switch
channel. The ramp rate at the load is determined by the
capacitor value connected at the CAP_SS pin.
Lab experimentation has shown that if you can slow the
voltage ramp rate at the speaker to <10V/s, you can eliminate
click/pop noise in a speaker. A soft-start capacitor value of
0.1µF provides for 4.5V/s ramp rate and is recommended. See
Figures 28 and 29 on page 16. See “MUTE to ON” on page 9
for more detail of how soft-start works.
Supply Sequencing and Overvoltage
Protection
With any CMOS device, proper power supply sequencing is
required to protect the device from excessive input currents
which might permanently damage the IC. All I/O pins contain
ESD protection diodes or diode stacks from the pin to V_DD and
to GND (see Figure 9). To prevent forward biasing these diodes,
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FN6699.2
May 6, 2014