Hello Guys
I build a circuit with a LMC555 Timer, i used this circuit very often and had no problems with it.
The circuit is a PFM Generator for a flyback, the frequency can be selected by applying a voltage on REG. The turn on time is determined with R19 and C14. To limit the current in the transformer a shunt resistor is used which will reset the timer at a certain current level.
But in a recent project i have problems with the reset entry of the Timer. The following figure shows the normal behavior of the circuit. In yellow we can see the voltage over the shunt resistor, which represents the current. After a certain limit, an over current is detected and the reset signal drops. The reset function is working well.
But when i apply a load on my flyback circuit under certain conditions, some serious problems with the reset entry appear. As you can see in the following figure, the timer ignores the reset signal and my transformer passes in to saturation.
I know the threshold of the reset entry is quiet low, min. 0.4V. But the reset signal applied is lower than this threshold.
Does someone have an idea, why this problem appears and whats the solution?
Thank you
I build a circuit with a LMC555 Timer, i used this circuit very often and had no problems with it.
The circuit is a PFM Generator for a flyback, the frequency can be selected by applying a voltage on REG. The turn on time is determined with R19 and C14. To limit the current in the transformer a shunt resistor is used which will reset the timer at a certain current level.
But in a recent project i have problems with the reset entry of the Timer. The following figure shows the normal behavior of the circuit. In yellow we can see the voltage over the shunt resistor, which represents the current. After a certain limit, an over current is detected and the reset signal drops. The reset function is working well.
But when i apply a load on my flyback circuit under certain conditions, some serious problems with the reset entry appear. As you can see in the following figure, the timer ignores the reset signal and my transformer passes in to saturation.
I know the threshold of the reset entry is quiet low, min. 0.4V. But the reset signal applied is lower than this threshold.
Does someone have an idea, why this problem appears and whats the solution?
Thank you