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'[EE]:: Flexible LED drivers'
2008\01\21@180002 by Apptech

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There are many LED driver ICs available.
The MC34063 is one of the older general purpose SMPS ICs
available and with a little external glue and some head
scratching can be used to implement high efficiency LED
driver solutions. A major advantage is that it is amenable
to implementing high or low side drives or both at once with
a little extra stuff, making it suitable for any topology.
The two most major limitations of the IC are its low maximum
operating frequency (about 200 kHz max actual and it's
stupid non saturating darlington output switch*.).

More recently ON Semi has reworked the MC34063 to produce
the NCP3065 and NCV3065. These have similar basic nature but
a much lower 0.235V sense voltage (which was previously
stupidly set at the band gap reference level), notionally
double maximum oscillation frequency (still low by modern
standards at nominally 200 kHz (more in practice according
to their graphs), perhaps slightly lower high side cycle by
cycle current sense, hard turn off of cycle on over current
(rather than progressive on time reduction with increasing
current), and a bit more. The criminally stupid darlington
output stage is retained - not a major problem if an
external FET is to be driven. They also raise the Vdd_min
from 2.5V to 3V. Vdd_max is still a commendable 40V.

Overall these ICs provide an extremely flexible range of
solutions for essentially any desired topology.

A negative is the price - using Digikey as a comparative
guide pricing is 3 to 5 times that of the MC34063 which
asymptotes to about $US0.20 in volume at Digikey pricing.

I am sourcing MC34063 clones produced by an ON Semi partner
in China at $US0.08 in 10,000 quantity, so I won't be
switching to the new ICs any time soon, but they may be
useful to know about for some people.

Magic almost no cost solution to improving MC34063
efficiency: Drive external FET switch with two emitter
follower connected NPN/PNP 500 mA jellybean bipolars, 1K IC
output to ground, IC output to both bases joined, FET gate
drive from both emitters joined, each collector to
appropriate rail. Utterly transforms the performance. If
desired, add a FET gate drive resistor of 10 ohms or so to
improve EMC and reduce gate losses. Operates to below 3V Vdd
with suitable FET.



       Russell


* The non saturating darlington is used to allow old
technology processes to achieve marginally acceptable
switching speeds.


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