Re: [SI-LIST] : Diodes, EMC, ESD Protection networks

Magnus Homann (
Tue, 12 May 1998 08:22:44 +0200

> 3) A Multiple VCC Supplies//Partial Power Down question. Think of it
> as a Bus Analyser sitting on a CPU bus.
> I have a CPU circuit (50MHz clocks...) and a monitoring Bus Analyser
> (not a product as such but a circuit I am developing). If power is
> applied to both devices, all is fine. However, if power to the Bus
> Analyser fails (or is turned on after the CPU circuit is turned on), it
> then loads the data lines on the CPU circuit corrupting operation. The
> loading is primarily done by the Bus Analyser's Input IC's ESD
> protection network, clamping the input to 0.7 volts above VCC, which in
> this case is 0 volts. The Bus Analyser's power supply has a very low DC
> resistance when the power is switched off due to other things.
> The only solutions I have seen (so far) is from Texas Instruments
> Application Information.

I see two options:

1) Use an IC that doesn't have diodes from input to VCC, e.g. 74ABT.

2) Use a buffer like QS3245 between your IC and the bus.