Datasheet

NCL30100
http://onsemi.com
12
Vcc
V
DRV
Ipk
I
mag
Imin
I
demag
I
LED
50 uA
I
CT
V
CT
V
Sense
I
CS
V
CS
38 mV
0
0
0
0
0
0
0
0
0
Figure 24. Voltage and Current Nodes in the Application Circuit
The current follows the red line in Figure 23 when Q1 is
turned on. The converter operates in continuous conduction
mode therefore the current through the inductor never goes
to zero. When the switch is on, the current creates a negative
voltage drop on the R
sense
resistor. This negative voltage can
not be measured directly by the IC so an R
shift
resistor is
connected to CS pin. Inside the IC there is a current source
connected to this pin. This current source creates constant
voltage drop on resistor R
sense
which shifts the negative
voltage drop presented on R
sense
positive. The magnetizing
current I
mag
increases linearly, the negative voltage on
R
sense
increased as well. Thus the voltage on CS pin
approaching zero. On the CS pin there is a comparator with
a reference level of 38 mV. Once the voltage on the CS pin
reaches this reference level, the DRV output is turned off and
current path I
mag
disappears. Energy stored in the inductor
as a magnetic field keeps current flowing in the same
direction. The current path is now closed via diode D1
(green line). Once the DRV is turned off, the internal current
source starts to charge the CT capacitor and the voltage on
this node increases. Once the CT capacitor voltage reaches
the V
CT
level, Q1 is turned on and an internal switch
discharges the CT capacitor to be ready for the next
switching cycle.