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'[EE] Constant current switcher?'
2007\03\10@134831 by Brooke Clarke

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Hi Robert:

You might look into LED driver constant current switcher ICs.  The
LM3404HV is rated to supply up to 1.2 Amps from 6 to 72 volts.
http://www.national.com/pf/LM/LM3404HV.html              
http://LED.national.com
LED drivers have a Vref of 200 mv instead of the more common 1.25 Volts
used in constant voltage type switchers to reduce the power wasted in
the current sense resistor.  I haven't checked this one, but the LM3404
(43 volt version) is only available in sample quantities now.  The
LM3402 (0.5 A, 42 volt) is available in quantity.

It's almost magical how these act as variable DC transformers.  In your
case 10 V & 0.8 amps is 8 watts.  From 70 volts that's about 150 ma
allowing a little for efficiency.  Maybe you could use a single series
diode and throw away half of the input to get the voltage down?

Note that more than one of the parts called out in the eval board app
note are state of the art and not available in small quantities.  Also
the SMT inductors are very state of the art.  My guess is that the
energy stored in an inductor depends on 1/2*L*I*I and so doubling the
current implies 4 times the current and probably volume.  Since in buck
switchers the inductor is in series with the load it sees the DC load
current.

It's truly amazing what these circuits do.  For more on the LM3402 and
LumiLED 1 & 3 Watt LEDs see:
http://www.pacificsites.com/~brooke/LED.shtml#HB

Have Fun,

Brooke Clarke

--
w/Java http://www.PRC68.com
w/o Java www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

What I need is the ability to have a regulated current output. In this case
about 800ma. The source voltage is 170DC (half bridge from 120VAC) and the
expected voltage across the load is between 2 and 5 volts (or preferably up
to 10 volts).

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