CSD87355Q5D: Operating frequency limit

Hi Sam,

Thanks for the inquiry. 1.5MHz is the recommended maximum switching frequency for the CSD87355Q5D. It can be operated at higher switching frequency but we have only characterized power loss up to 1.5MHz. If you plan to operate it at greater than 1.5MHz, please keep in mind that the switching loss in the FETs and the gate drive loss in the gate driver will increase linearly with switching frequency. Also, for low duty cycle operation (e.g. 12Vin to 1.x Vout) in a synchronous buck converter, the on time of the upper FET will be very short. You need to make sure that the controller and gate driver can support the minimum on time for stable operation.

Hi Sam,

There is not one easy answer to your question regarding maximum recommended frequency of a MOSFET. We provide recommendations for our power blocks because they are optimized for switch mode operation in a power supply or motor drive. Discrete FETs can be used in many different applications. The first thing to look at is the Applications section on the first page of the datasheet. If it includes synchronous buck converter (control FET or sync FET) then the FET can be operated at frequencies up to and greater than 500kHz. If it says Motor Drive or Secondary Synchronous rectifier Then the switching frequency will usually be 10's of kHz to a few hundred kHz. If it says load switch or hot swap, then the FET is not designed for and will not perform well in switch mode applications. You also need to look at the Rds(on), Qg, Qgd and Qgs specs. Lower Rds(on) indicates a larger die size and higher charge. We have some tools to estimate power loss for a sync buck converter: . .

I hope this helps. If you're considering a particular FET for an application, please feel free to email me directly using my TI email address.

Hi John,

Many thanks for your explanation. I understand now.

Regards,

Hao