LMZ22010 EN and SYNC

Hi Guenter,

1) When several of the modules are synced, and the EN is pulled low on one of them, that individual part will stop switching.  This will cause a transient response as the control loop adjusts for the change in curent gain of the loop.  While the EN is low that part will ignore the SYNC signal.

It will look like this screen shot below.

2) If the sync has been constantly high for a while and the part has stopped switching, when the clock resumes the part will also resume switching.  Again, if it is only one out of several synced parts you will see a transient response as the control loop adjusts.

3) Whether or not to use spread spectrum will depend on your application.  A spread spectrum clock can reduce the peak radiated EMI, but I would first recommend taking a look at out application notes on board layout and filtering.  For the buck converter power modules, most of your radiated noise is going to be produced on the input of the module as the large switching currents travel through the input leads to the part.  The leads act as antennas and can result in EMI without proper filtering.  So if the modules are stand-alone and not supplied from another source on your design, I would recommend adding an input filter to your design as your primary defense against unwanted EMI.

The spread spectrum will then be an additional measure of protection in your system, if you need it. However my experience is that it is not necessary with proper input filter design.

If you do use it, I don't think that you would need  +-50Khz of variation in your spread spectrum modulation +-10Khz should be enough.

However if your down stream components are sensitive to 10Khz noise, you would probably not want to use spreadspectrum because it will introduce 10Khz ripple to your output voltage. 

Regards,

Marc