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ISL54405 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL54405 Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  11 FN6699.1 June 5, 2008 COM side (L,R) are activated and inoperable on the signal side (L1, R1, L2, R2) of the switch. When DIR_SEL = Logic “0” the C/P shunts are activated on the signal side (L1, R1, L2, R2) and inoperable on the COM side (L, R). Logic Levels: AC/DC, DIR_SEL = Logic “0” (Low) when ≤ 0.5V AC/DC, DIR_SEL = Logic “1” (High) when ≥1.4V or Floating. The AC/DC and DIR_SEL have internal pull-up resistors to the internal 3.3V supply rail and can be driven high or tri-stated (floated by the µprocessor). They should be driven to ground for a logic “0” (Low). Note: For 5V applications, the AC/DC and DIR_SEL pins should never be driven to the external 5V rail. They need to be driven with 1.8V logic or 3V logic circuit. 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 “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 Ω, 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 25 and 26 in the “Typical Performance Curves” beginning on page 13. 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 un-wanted 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 only increase un-neccesary 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 27 and 28 in the “Typical Performance Curves” beginning on page 13. See “MUTE to ON” section on page 10 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 VDD and to GND (see Figure 8). To prevent forward biasing these ISL54405 |
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