NCL30160
http://onsemi.com
6
Figure 4. Typical Operation Waveforms
(V
CC
= 12 V, V
LED
= 6.5 V, R
SENSE
= 0.68 W, L = 100 mH)
THEORY OF OPERATION
This switching power supply is comprised of an inverted
buck regulator controlled by a current mode, hysteretic
control circuit. The buck regulator operates exactly like a
conventional buck regulator except the power device
placement has been inverted to allow for a low side power
FET. Referring to Figure 1, when the FET is conducting,
current flows from the input,through the inductor, the LED
and the FET to ground.
When the FET shuts off, current continues to flow through
the inductor and LED, but is diverted through the diode
(D1). This operation keeps the current in the LED
continuous with a continuous current ramp.
The control circuit controls the current hysteretically.
Figure 2 illustrates the operation of this circuit. The CS
comparator thresholds are set to provide a 10% current
ripple. The peak current comparator threshold of 220 mV
sets I
peak
at 10% above the average current while the valley
current comparator threshold of 180 mV sets I
valley
at 10%
below the average current.
When the FET is conducting, the current in the inductor
ramps up. This current is sensed by an external sense resistor
that is connected from CS to ground. When the CS pin
reaches 220 mV, the peak current comparator turns off the
power FET. A conventional hysteretic controller would
monitor the load current and turn the switch back on when
the CS pin reaches 180 mV. But in this topology, the current
information is not available to the control circuit when the
FET is off. To set the proper FET off time, the CS voltage is
sensed when the FET is turned back on and a correction
signal is sent to the off time circuit to adjust the off time as
necessary.
Figure 5. Typical Current Waveforms
The current waveshape is triangular, and the peak and
valley currents are controlled. The average value for a
triangular waveshape is halfway between the peak and
valley, so even with changes in duty cycle due to input
voltage variations or load changes, the average current will
remain constant.
In the event there is a short−circuit across the LEDs, a
large amount of current could potentially flow through the
circuit during startup. To protect against this, the NCL30160
comes with a short circuit protection feature. If the voltage
on the CS pin is detected to be greater than 500 mV
(equating to 2.5 times the intended average output current),
the NCL31060 will turn off the FET, and prevent the FET
from turning on again until power is recycled to NCL30160.
