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From: markp on 20 Jan 2010 14:13
Hi All,

Anyone use the Zetex ZXSC400 LED driver? I've got a very strange problem -
below about 2.6V or so input voltage the efficiency goes out the window.
I've got a circuit similar to the datasheet with 6 LEDs.

It looks like the base drive voltage of the transistor isn't doing what it
should (in fact when it seems to be running 'normally' at higher input
voltage the base voltage drives down to -0.4V and stays there between burst
cycles. According to the simulation it should only ever dip to -0.1V in the
middle of bursts and should be 0V inbetween bursts. Also the -0.4V actually
violates the chip spec for the drive pin.

The layout looks reasonable, there's a solid ground plane and devices are
close, loop areas are small. I'm using the exact components specified in the
app note. I'm at a loss, if anyone has seen anything similar I'd be
interested in how you fixed it.

Mark


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