Datasheet

NCL30081
http://onsemi.com
18
Internal Soft−Start
At startup or after recovering from a fault, there is a small
internal soft−start of 40 ms.
In addition, during startup, as the output voltage is zero
volts, the demagnetization time is long and the constant
current control block will slowly increase the peak current
towards its nominal value as the output voltage grows.
Figure 44 shows a soft−start simulation example for a 9 W
LED power supply.
Figure 44. Startup Simulation Showing the Natural Soft−start
0
4.00
8.00
12.0
16.0
1
0
200m
400m
600m
800m
2
604u 1.47m 2.34m 3.21m
time in seconds
4.07m
0
200m
400m
600m
800m
3
4
I
out
V
CS
V
out
V
Control
(A)(V) (V)
Cycle−by−Cycle Current Limit
When the current sense voltage exceeds the internal
threshold V
ILIM
, the MOSFET is turned off for the rest of the
switching cycle (Figure 45).
Winding and Output Diode Short−Circuit Protection
In parallel with the cycle−by−cycle sensing of the CS pin,
another comparator with a reduced LEB (t
BCS
) and a higher
threshold (1.5 V typical) is able to sense winding
short−circuit and immediately stops the DRV pulses. The
controller goes into auto−recovery mode in version B.
In version A, the controller is latched. In latch mode, the
DRV pulses stop and VCC ramps up and down. The circuit
un−latches when VCC pin voltage drops below V
CC(reset)
threshold.