- #1
ConeOfIce
- 13
- 0
Hi all,
At work we were working on a bus-hold issue that we fixed with a bit of an unconventional method. We had a circuit that looked somewhat like:
Buf>--R(33)------R2(2.7k)---->BH
.....|
......cap(220p)
.....|
......GND
Where Buf is a buffer, and BH is a bus-hold buffer. This is really just a switch circuit, with no AC component.
Now when more of the RC with BH is put in parallel the circuit starts to fail when the buffer is driving from high to low. The reason this seems to be is because the bus-hold requires -750uA to switch.
It would fail by dropping down to 1.5V(typical CMOS switching voltage) and getting stuck there.
The solution that happaned to fix this issue was placing a very large capacitor across the 33 ohm resistor and increasing the resistor to 260 ohms.
The larger the capacitor the better the fix. The fix also didn't work unless the resistor was increased.
I believe this worked for 2 reasons.
The first, was the cap essentially eliminated the resistor from the circuit in terms of creating a current, thus more current was available as the 33 ohms started to become more significant when more add-ons were put into parallel.
The second one, is that the capacitor would cause a very large temporary negative voltage on either side of it (sometimes about to about 1.5V more then seen without the cap). This temporary drop in voltage created a temporary inrush of current helping to make the bus-hold switch. The large cap also helps to decrease resonance upon returning to 0 volts.
I am failing to understand why the capacitor is causing this dip in voltage, and was wondering if anyone else could shine some light on this.
Thanks for the help!
At work we were working on a bus-hold issue that we fixed with a bit of an unconventional method. We had a circuit that looked somewhat like:
Buf>--R(33)------R2(2.7k)---->BH
.....|
......cap(220p)
.....|
......GND
Where Buf is a buffer, and BH is a bus-hold buffer. This is really just a switch circuit, with no AC component.
Now when more of the RC with BH is put in parallel the circuit starts to fail when the buffer is driving from high to low. The reason this seems to be is because the bus-hold requires -750uA to switch.
It would fail by dropping down to 1.5V(typical CMOS switching voltage) and getting stuck there.
The solution that happaned to fix this issue was placing a very large capacitor across the 33 ohm resistor and increasing the resistor to 260 ohms.
The larger the capacitor the better the fix. The fix also didn't work unless the resistor was increased.
I believe this worked for 2 reasons.
The first, was the cap essentially eliminated the resistor from the circuit in terms of creating a current, thus more current was available as the 33 ohms started to become more significant when more add-ons were put into parallel.
The second one, is that the capacitor would cause a very large temporary negative voltage on either side of it (sometimes about to about 1.5V more then seen without the cap). This temporary drop in voltage created a temporary inrush of current helping to make the bus-hold switch. The large cap also helps to decrease resonance upon returning to 0 volts.
I am failing to understand why the capacitor is causing this dip in voltage, and was wondering if anyone else could shine some light on this.
Thanks for the help!