Re: 4420 driver ICs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 1 Apr 2004, at 19:12, Tesla list wrote:

 > Original poster: Jan Wagner <jwagner-at-cc.hut.fi>
 >
 >
 > On Thu, 1 Apr 2004, Tesla list wrote:
 >  > Umm im not sure what you mean...The input to the 4420?Or the
 >  MOSFET?The > input to the 4420 is connected to the antenna...
 >
 > Relying heavily on the chip's ESD protection to handle it, right? ;-)
 >
 > I mean it seems a bit odd, with potentially 100V... x kV antenna
 > signals going directly to the 4420 input pin. Now OTOH the 4420
 > datasheet shows internal protection diodes, so maybe your setup
 > actually _does_ have some chances of working properly (?) :)

The point I was trying to hint at was that if all that is connected
to the input is the antenna and the input is simply a gate, it will
have no way of discharging its input capacitance and can be biased on
forever by a tiny voltage on the gate. I would stick a high value
resistance between the input and ground so at least it had a fighting
chance of working. Protection diodes will not do the job. The
datasheets will no doubt tell the story.

Malcolm


 > To be on the safe side I'd probably add a 18V zener diode accross the
 > chips Vdd and ground, and a very small 6.8V zener between the input
 > pin and ground, so that the circuit is better protected from the
 > kilovolts. Oh and a 100ohm..1kOhm resistor in series with the antenna,
 > on the tc4420 side.
 >
 > good luck,
 >   - Jan
 >
 > --
 > ****************************************************
 >   Helsinki University of Technology
 >   Dept. of Electrical and Communications Engineering
 >   http://www.hut.fi/~jwagner/ - jwagner-at-cc.hut.fi
 >
 >
 >