LMR62421 Isense blanking during switch transitions

Hi Jeff,

Let me check with our design engineering team on the blanking scheme of this architecture.

Best Regards,

-Juan

Hi Jeff,

Yes, this architecture has a switch sense blanking scheme to avoid any false triggering.  This parameter is not production tested and is not disclosed in our datasheets.  Please evaluate your application and let us know if you have any additional questions.

Best Regards,

-Juan

Hi Jeff,

THe LMR62421 has either a blanking time, or a filter on the current sense I can't remember which right now.  The important thing to remember if using it with a transformer is that the magnetizing inductance of the transformer be in the same recommended range as an inductor used for a boost converter.  This will help to keep the peak to peak ripple current in the device in the approprate range.

Regards,

Marc

Hi Marc,

We have performed testing of the LMR62421 using the evaluation board and transformer (SRF0703-8R2M) that we are using to provide a flyback supply from the LMR62421's switch current. We are not able to generate a stable flyback supply with either the circuit in the attached schematic or adding a capacitor across pins 1 and 2 fo the transformer (as you had suggested and simulation and reason seemed to support; we don't require galvanic isolation). The attached document also provides a couple of scope screenshots that detail the primary and secondary supplies (VDD and V_Above_Rail), as well as the inductor current and switch pin. It appears that when the switch turns on the inductor current spike that results (as simulation suggests) is causing the LMR62421 to enter over current shutdown. I thought you said that the LMR62421's current sense circuitry incorporates blanking or filtering during SW transitions? Any thoughts about what is going on here? For our rev. A design it seemed that the LM27313 worked well, but because of the higher duty cycle requirements of the rev. B design we were forced to switch to the another boost converter. We are losing faith in the LMR62421 for our application. Your or a colleague's help in making this topology work with the LMR62421, or pointing us to another component that will work (preferably in same package, since we are currently in layout and have a very compressed schedule), is appreciated.

Thanks,

Jeff6574.LMR62421_transformer_2.docx