NCL30161 Datasheet, PDF (7/8 Pages) ON Semiconductor – Constant-Current Buck Regulator for Driving High Power LEDs

NCL30161
Inductor Selection
The inductor that is used directly affects the switching
frequency the driver operates at. The value of the inductor
sets the slope at which the output current rises and falls
during the switching operation. The slope of the current, in
turn, determines how long it takes the current to go from the
valley point of the current ripple to the peak when the FET
is on and the current and rising, and how long it takes the
current to go from the peak point of the current to the valley
when the FET is off and the current is falling. These times
can be approximated from the following equations:
t_ON
+
VIN * V_LED * I_OUT
L DI
ǒ Ǔ FET_RDS(on) ) DCR_L ) R_SENSE
t_OFF
+
V_LED
)
L
V_diode
DI
) 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
+
t_ON
1
)
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 the 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
NCL30161 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. Selecting an appropriate R_OT
value allows the system to quickly achieve the intended
current regulation. The R_OT value can be calculated using
the following equation:
R_OT + t_OFF 10¹¹ W
Where t_OFF is the expected off time during normal switching
operation, calculated in the Inductor Selection section
above.
Every time the DIM/EN pin is brought from a low state to
a high state, the initial off−time program is reset. The first
off−time of the MOSFET after the DIM/EN is brought high
will be set by the ROT value. The off−time will then be
adjusted in subsequent switching cycles.
Input Capacitor
A decoupling capacitor from VIN to ground should be
used to provide the current needed when the power
MOSFET turns on. A 10 mF ceramic capacitor is
recommended.
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