Diodes ZXLD1366 - Manual (Page 17)

ZXLD1366
Document number: DS31992 Rev. 8 - 2
17 of 30
www.diodes.com
October 2013
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A Product Line of
Diodes Incorporated
ZXLD1366
Application Information
Setting Nominal Average Output Current with External Resistor R
S
The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (R
S
) connected between
V
IN
and I
SENSE
and is given by:
I
OUTnom
= 0.2/R
S
for R
S
≥ 0.2 Ω
The table below gives values of nominal average output current for several preferred values of current setting resistor (R
S
) in the typical
application circuit shown on page 1:
R
S
( Ω)
Nominal Average Output
Current (mA)
0.20 1000
0.27 740
0.56 357
The above values assume that the ADJ pin is floating and at a nominal voltage of V
REF
(= 1.25V). Note that R
S
= 0.2 Ω is the minimum
allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value.
It is possible to use different values of R
S
if the ADJ pin is driven from an external voltage. (See next section).
Output Current Adjustment by External DC Control Voltage
The ADJ pin can be driven by an external dc voltage (V
ADJ
), as shown, to adjust the output current to a value above or below the nominal
average value defined by R
S
.
The nominal average output current in this case is given by:
I
OUTdc
= (V
ADJ
/1.25) x (0.2/R
S
) for 0.3< V
ADJ
<2.5V
Note that 100% brightness setting corresponds to V
ADJ
= V
REF
. When driving the ADJ pin above 1.25V, R
S
must be increased in proportion
to prevent I
OUTdc
exceeding 1A maximum.
The input impedance of the ADJ pin is 50kΩ ±25% for voltages below V
REF
and 14.2kΩ ±25% for voltages above V
REF
+100mV.
Output Current Adjustment by PWM Control
Directly Driving ADJ Input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as shown below, to adjust the output current to
a value above or below the nominal average value set by resistor R
S
:
GND
ZXLD1366 ADJ
GND
+
DC
PWM
GND
0V
V
ADJ
GND
ZXLD1366 ADJ