Diodes ZXLD1356/ ZXLD1356Q - Manual (Page 19)

ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
19 of 29
www.diodes.com
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
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
> 0.36Ω]
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)
0.36 555
0.56 357
1.33 150
The above values assume that the ADJ pin is floating and at a nominal voltage of V
REF
(= 1.25V). Note that R
S
= 0.36V 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
.
GND
ZXLD1356 ADJ
GND
+
DC
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 550mA maximum.
The input impedance of the ADJ pin is 50k
Ω ±25% for voltages below V
REF
and 14.2k Ω ±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 D
PWM
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
:
PWM
GND
0V
V
ADJ
GND
ZXLD1356 ADJ