Datasheet

NCL30160
http://onsemi.com
7
Figure 6. Short-Circuit Protection
When V
IN
rises above the UVLO threshold voltage,
switching operation of the FET will begin. However, until
the V
IN
voltage reaches 8 V, the VCC regulator may not
provide the expected gate drive voltage to the FET. This
could result in the R
DS(on)
of the FET being higher than
expected or there not being enough gate drive capability to
operate at the maximum rated switching frequency. For
optimal performance, it is recommended to operate the part
at a V
IN
voltage of 8 V or greater.
Setting The Output Current
The average output current is determined as being the
middle of the peak and valley of the output current, set by the
CS comparator thresholds. The nominal average output
current will be the current value equivalent to 200 mV at the
CS pin. The proper R
SENSE
value for a desired average
output current can be calculated by:
R
SENSE
+
200 mV
I
LED
PWM Dimming
For a given R
SENSE
value, the average output current, and
therefore the brightness of the LED, can be set to a lower
value through the DIM/EN pin. When the DIM/EN pin is
brought low, the internal FET will turn off and switching
will remain off until the DIM/EN pin is brought back into its
high state.
Figure 7. Dimming Waveforms
By applying a pulsed signal to DIM/EN, the average
output current can be adjusted to the duty ratio of the pulsed
signal. It is recommended to keep the frequency of the
DIM/EN signal above 100 Hz to avoid any visible flickering
of the LED.
Figure 8. Dimming Performance
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:
+
L DI
VIN * V
LED
* I
OUT
ǒ
FET
R
DS
(on) ) DCR
L
) R
SENSE
Ǔ
t
ON