On 25/10/2010 01:02 PM, scott wrote:
>> Well, the iPhone runs on battery power, so yeah, the power supply is
>> probably a little unstable.
>
> Not just that, but from a single voltage (the battery) several very
> stable voltages must be generated. This involves lots of analogue
> components (eg how do you efficiently make accurate 3.3V, 5V and 8V
> lines from a LiPo battery that can be between 3 and 4.5V?).

1. Why do you need several different voltages?

2. I'm not aware of any way to change the voltage of a DC circuit. You 
can use a resistor to limit current, but AFAIK there's no way to 
actually change voltages.

>> I'm thinking more about desktop PC motherboards. Why do *they* need so
>> many million capacitors?
>
> Noise reduction, basically they allow everything to run correctly at
> much higher frequencies than would be possible without them. It works
> two ways, the capacitors around each IC help to keep a smooth voltage
> level for that IC, ie shielding it from any noise on the power supply
> lines. Also the capacitor then prevents a lot of noise being put back
> onto the supply line from the ICs.
>
> There are also a lot of resistors on the signal lines, these in
> conjunction with the tiny parasitic capacitances cause a low-pass filter
> effect on the signals. This also helps to reduce noise emission from the
> lines.

Mmm, interesting. (And ridiculously complicated.) I wonder how much of 
this stuff is to do with electromagnetic radiation? (You don't want EM 
interference stopping your motherboard working, and you don't want the 
board to radiate EM either...)

Post a reply to this message