AN47
Resistor dimming
By connecting a variable resistor between ADJ and GND, simple dimming can be achieved.
Capacitor C is optional for better AC mains interference and HF noise rejection. Recommend
ADJ
value of C
is 0.22F.
ADJ
R
S
ADJ
ZXLD1350
R
ADJ
GND
*C
ADJ
GND
The current output can be determined using the equation:
(0.08 ⁄ R ) × R
S
ADJ
------------------------------------------------
=
I
OUT
(R
+ 200k)
ADJ
Note that continuous dimming is not possible with a resistor. At some point the shutdown
threshold will be reached and the output current reduced to zero. This can occur below 300mV.
Note that a 1M⍀ resistor will load the V
on the ADJ pin. The V
will now be divided down by
REF
REF
the nominal 200k V
resistance and the 1M R
. The nominal voltage will now be
REF
ADJ
approximately 1V. R will need to be adjusted to set the maximum current.
S
The +/20% tolerance of the input resistance should also be understood. See table below:
Table 1
R
k⍀
Rint nom.
Rint min.
Rint max.
V
% error
from
nominal
Due to
% error
from
nominal
Due to
ADJ
ADJ
k⍀
k⍀
k⍀
nominal
Rint min.
Rint max.
1000
500
200
100
200
200
200
200
160
160
160
160
240
240
240
240
1.041
0.892
0.625
0.416
3.4%
6.1%
-3.3%
-5.7%
11.1%
15.4%
-10.0%
-13.3%
Issue 1 - August 2006
© Zetex Semiconductors plc 2006
5
www.zetex.com
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