AN47
DC voltage dimming
The ADJ pin can be overdriven by an external DC voltage (V
ADJ
), as shown, in order to override
the internal voltage reference and adjust the output current to a value above or below the nominal
value.
+
ADJ
ZXLD1350
GND
DC
GND
The nominal output current is then given by:
0.08
×
V
ADJ
I
OUT
≈
--------------------------------
R
S
[for 0.3< V
ADJ
<2.5V]
Note that 100% brightness setting corresponds to V
ADJ
= V
REF
.
If V
IN
is 2.5V max the R
SENSE
should be increased by 2X R
S
. This will slightly decrease the
efficiency by 1 to 2% .
The input impedance of the ADJ pin is 200k
relatively high output resistance.
±20%. This may be factor if the DC voltage with a
I
OUT
vs V
ADJ
(R
S
=300m )
350
300
250
200
mA
150
100
50
0
0
0.25
0.5
V
0.75
1
Series1
1.25
Figure 1
Typical output current versus ADJ voltage with R
S
= 300m
© Zetex Semiconductors plc 2006
ZETEX [ ZETEX SEMICONDUCTORS ]
