LMZ12001 Duty Cycle Limit

TI E2E,

I'm designing a step down regulator and came across the LMZ12001 module. It seems perfect for my application. It only requires a few external caps and resistors, it's pcb footprint is small, and most importantly it's hand solderable. I'd like to use it, but I'm concerned about a discrepancy between WEBENCH, the data sheet, and the eval board test reults.

My basic requirements are Vin=4.5V to 15V, Vo=3.3V, Io=0.35A.

WEBBENCH will not show the LMZ12001 as an option unless my input voltage is above about 6V. It rejects it because the duty cycle is above 0.66. But the LMZ12001's should work down to 4.5V.

The data sheet only mentions duty cycle three times and refers the reader to the graphs. But the graphs do not show duty cycle. And the tabular part of the data sheet does not show a limit for duty cycle. However, on-time and off-time limits are mentioned, and I meet those limits in the test data below.

I have the eval board with an LMZ12001EXT up and running with my applicaiton load. It works fine down to below 4.5V. Probing pin-2 I can see the switch times. Here is some test data showing input voltage, on time, off time, switching frequency, and duty cycle.

- vin=4.5v, ton=1310ns, toff=410ns, fsw=580kHz, on duty cycle=0.76
- vin=5.0v, ton=1120ns, toff=520ns, fsw=610kHz, on duty cycle=0.68
- vin=5.5v, ton=960ns, toff=600ns, fsw=640kHz, on duty cycle=0.61
- vin=7.2v, ton=690ns, toff=800ns, fsw=675kHz, on duty cycle=0.46
- vin=15v, ton=324ns, toff=1120ns, fsw=690kHz, on duty cycle=0.22

The only parts I changed on the eval board were the output voltage adjust resistors. I'm using Rfbt=3.65k||56k and Rfbb=1.1k for Vo=3.3V.

Am I OK using the LMZ12001 down to Vin=4.5V? Is the duty cycle limit of 0.66 imposed by WEBENCH a hard limit? Is it associated with meeting other specs, like temperature (I need it to work up to 60degC)?

Thanks in advance for any advice you can offer.

Best regards, Charles.