Q: A customer wants to run MAX20078 at 2.1MHz to stay above the AM band. However, the MAX20078 datasheet reads that the part's on-time can be programmed for switching frequencies ranging from 100kHz up to 1MHz. Is it possible to run at 2.1MHz?
A: Yes, if the input and output voltages are within range. The datasheet EC table shows MAX20078 has a minimum off-time of 200ns.
The maximum duty cycle Dmax = Ton_max*fsw = (1/fsw-Toff_min)*fsw = 1-fsw*Toff_min
For a Buck, duty cycle D=Vo/Vin. This is an approximation because it does not take efficiency into account.
Using these two equations to rewrite Dmax>D below:
1-fsw*200ns > Vo/Vin
The customer's input voltage range is 6-17V. The output specs are 3-4V at 6A.
2.1MHz switching frequency is not possible at the lower input voltages, especially when considering efficiency at 6A output. At lower input voltages the frequency will dip, but the design will still regulate. The Vo and Vin range for which 2.1MHz switching frequency is possible will depend on efficiency, which will vary with the customer’s choice of external components and thermal considerations.