Dear Dennis... analogZONE's Dennis Feucht answers your design queries in his new Circuit Design Clinic! February 2006

analogZONE presents a new, interactive analog design service to readers! Send us your design questions (with relevant data; schematics in JPEG or GIF, please) for some free engineering advice from analogZONE's circuit consultant, Dennis Feucht, on how you might solve a design problem or improve circuit performance. Submissions may be edited for clarity or brevity, and submitters and their email addresses will remain anonymous (unless otherwise indicated). Please send your questions to Dennis here.

Low Output-Voltage CATV Flyback Converter

Analog circuit design covers plenty of ground, from narrow-band, frequency-domain RF to power converters. The following interaction is with an RF designer working on a CATV application. He is designing a 5-W converter using an LM5021 controller IC from National Semiconductor Corp. As an RF designer, he's feeling his way along on power-converter design.

Current Sensing Turn-On Spike

"Dear Dennis:

There is one thing you mention regarding MOSFET source current monitoring that I believe has been taken care of by the designers at National Semi, and that is keeping the turn-on transient, which can be rather sharp, from tripping the current-sense circuit. From Page 8 of the LM5021 data sheet, "Current sense/Current Limit: The discharge switch remains on for an additional 90 ns leading edge blanking interval to attenuate the current sense transient that occurs when the external power FET is turned on." I believe this obviates the need for clever current-sense resistors and such. Please let me know if I am incorrect on this point."

For Dennis' response and more of this month's circuit design queries, download the current PDF file here.