Datasheet

NCL30160
http://onsemi.com
8
t
OFF
+
L DI
V
LED
) V
diode
) I
OUT
DCR
L
Where DCR
L
is the dc resistance of the inductor, V
LED
is
the forward voltages of the LEDs, FET
RDS(ON)
is the
on-resistance of the power MOSFET, and V
diode
is the
forward voltage of the catch diode.
The switching frequency can then be approximated from
the following:
f
SW
+
1
t
ON
) t
OFF
Higher values of inductance lead to slower rates of rise
and fall of the output current. This allows for smaller
discrepancies between the expected and actual output
current ripple due to propagation delays between sensing at
the CS pin and the turning on and off of the power MOSFET.
However, the inductor value should be chosen such that the
peak output current value does not exceed the rated
saturation current of the inductor.
Catch Diode Selection
The catch diode needs to be selected such that average
current through the diode does not exceed the rated average
forward current of the diode. The average current through
the diode can be calculated as:
I
avg_diode
+ I
OUT
t
OFF
t
ON
) t
OFF
It is also important to select a diode that is capable of
withstanding the peak reverse voltage it will see in the
application. It is recommended to select a diode with a rated
reverse voltage greater than VIN. It is also recommended to
use a low-capacitance Schottky diode for better efficiency
performance.
Selecting The Off-Time Setting Resistor
The off-time setting resistor (R
OT
) programs the
NCL30160 with the initial time duration that the MOSFET
is turned off when the switching operation begins. During
subsequent switching cycles, the voltage at the CS pin is
sensed every time the MOSFET is turned on, and the
off-time will be adjusted depending on how much of a
discrepancy exists between the sensed value and the CS
lower limit threshold value. The R
OT
value can be calculated
using the following equation:
R
OT
+ t
OFF
10
11
W
Where t
OFF
is the expected off time during normal
switching operation, calculated in the Inductor Selection
section above.
Input Capacitor
A decoupling capacitor from VIN to ground should be
used to provide the current needed when the power
MOSFET turns on. A 4.7 mF ceramic capacitor is
recommended.