UC1846-SP: 2 questions on use of the SYNC Pin

Hey,

1) A 0-5 V input should be sufficient see the electrical characteristics:

2) when set up as a "slave" there will not be a frequency it defaults to as RT/CT will not be used.

Thanks,
Daniel

Daniel, 

I've used various TI PWM's and on two occasions the amplitude and duty cycle of the sync wave form was critical.   As an example the UC1526, requires a pulse with a period of about 1us in order to sync properly.

1) Is a 50% duty cycle square wave between 3.9V to 5V the correct signal to use?  

2) I want to know what happens when there is no external sync during turn-on... are you saying that the PWM will not switch at all? 

Hey,

1) It would have to be under 2.5 V as thats the input low level, but yes.
2) RT and CT are used to set the frequency in normal operation. To set the device up to use the SYNC pin, there is no resistor or capacitor on the pins, thus not having a reference of what frequency to run.

Thanks,
Daniel

2) so it's free running or not switching at all?

Hey,

It shouldnt switch as what frequency would it run at?
Are you seeing something different?

Thanks,
Daniel

I'm in the design phase and have not seen anything.  But if I understand correctly the PWM will stop switching if the external sync is removed?