Re: sensing the signal from a IC Pin

Hi PeteS,
Thanks for your reply.
Yes I added the circuit with wire coz i do not have any other choice. I
tried to keep the wire length as short as possible. CT is connected
close to the pin. The driving resistor of 18k (range is 10k-75K)is
connected between the ref and Ct pin. I am not using the SYN pin. It is
connected firmly to the ground. I doubt its the wire connection
creating this problem. But i do not have choice, the main circuit is
built with all SMD stuck in position. I need to add this in and see how
i can use it.
krish

PeteS wrote:
> <<
> Its actually a PFC IC FA5502P from Fuji . I read the dataheet, it
> mentions capacitor can range from 330pF to 1000pF and resistance can
> vary from 10-75Kohm. The buffer i am connecting to this pin has 1Megh
> input resistance and 3pF capacitance. The capacitor
> now connected to this pin is 500pF. Adding this circuit there keeps the
> net cap within the range. I am not able to understand random behaviour
> >>
>
> Some questions for you to answer / look at:
>
> Are you 'adding' this circuit with wires?
>
> Are the timing components close to the Ct pin?
>
> Are you driving the resistor from the ref pin?
>
> What value of resistor are you using? (The app note shows a 22k
> device). This is particularly important - in my experience RC
> oscillators can get very unstable in very low current situations
> (depends on the design of the oscillator, obviously, but we're dealing
> with a precanned device here)
>
> Are you using the SYNC pin to lock to an external signal? If not, do
> you have the pin tied hard to ground (well away from any high switching
> currents)? - Not doing this will **definitely** give you the problem
> you are seeing.
>
> The timing diagrams shown have a nice beefy signal at the oscillator
> (Ct) pin, so I can only assume you haven't followed some (perhaps
> hidden) piece of application advice. The above might get you started.
>
> Cheers
>
> PeteS

.

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