Hello, my name is Chris Ludeman I am senior marketing manager with Intersil.

I am going to talk today about some recent developments in the area of analog switches particularly to do with handheld products. Many of you already own MP3 players ***** very large older ones or may be even some of the smaller new ones and you will be familiar with the trend towards miniaturization in consumer products.

One of the things that allows these parts to be miniaturize is the combination of functions with different connectors for example this small MP3 here has a separate USB and headphone connector but this even smaller one has a combined USB and headphone connector. And what I would like to talk about here is that technology that makes that happen or allows it to happen. On the board here is a simplified block diagram of the main components that we are gonna talk about in an MP3 player. You have your MP3 CODEC with its USB port and its audio port and the analog switch that allows you to combine these USB and audio sequence through a single connector, as you can see USB and audio share the same pins on this connector.

Now if you were to try and use a conventional analog switch for this function you are going to a number of problems. First of all the USB path needs a DC path to their laptop that we download the songs you ***** songs from, whereas the CODEC needs an AC couple path of a headphone because this CODEC runs of a single supply and its output sits at VDD of the two, so you have to block that DC, so the first problem you run up again is that the signal on the input of a switch swings above and below ground.

Now for traditional analog switch that’s a problem because on the input of switch you have ESD diodes and they are going to want to clip that signal, in fact down below you can see the signal coming out of a CODEC is sitting on VDD of the two, but on the input of the switch its swinging above and below ground and as soon as you ***** ESD dioden you are gonna have clipping or at lowest signal levels you are going to have distortion.

The other problem here is that while audio has a fairly narrow bandwidth of about 20kHz USB has to run at 40 megabits and so there is some conflicting requirements here. For audio a switch has to be linear, which usually infers a very low resistance switch but unfortunately that type of switch has very high capacitance and that’s gonna kill the USB bandwidth. So the needs of this application for a combined **** DC blocking, negative signal swing, low capacitance, and flat resistance and these are pretty much conflicting requirements.

So if we go across the other side of the board here again look at the couple of representative analogs switches and compare the differences between them. Analog switches come in all sorts of resistances and I picked a couple here, a low capacitance switch of 5Ω and high capacitance switch of 1Ω. The 5Ω switch is good, because it has low capacitance and will tend not to impact the USB signal. The low resistance switch has high capacitance and will have fairly low distortion for the audio signal, but unfortunately low capacitance comes with high insertion loss and also comes in relatively poor flatness of resistance.

Now 5Ω switch has a resistance that varies by about 1 to 2Ω over the input signal range, but the low resistance switch only has much less well much better flatness about ½Ω to 1Ω and little loss the 50pF picofarads is pretty much killer for high speed USB applications. So what sort of switch do you really need for passing high quality MP3? MP3 source material has a typical THD of around – (70 – 80) dB, so if you put a switch in the same path as MP3 the switch between it and the USB signal is going to be at least 10dB better than that, say (80 – 90) dB.

Now typical listening levels for ear bud speakers of 32Ω **** are about 1mW or 0.5 Vpp and clipping levels for those ear buds are about 4mW or about 1 Vpp so what sort of flatness do you really need in a switch at those signal levels to meet these (80 – 90) dB THD levels. Well from our work on switches we estimate that you need about 30mΩ of flatness, which you can get by the way out of an ISL54206 path now this path enables you to swing above and below ground by about 1.5V it has a relatively low resistance of 2.5Ω which is represents insertion loss of .65dB more important is completely flat over that signal range having a flatness of only 30 mΩ. So this is this is the type of switch you would need to use in MP3 player if you want to merge with the USB and audio connectors.