NCP1028
www.onsemi.com
13
Fault Condition – Output Short−Circuit
As soon as V
CC
reaches VCC
ON
, drive pulses are
internally enabled. If everything is correct, the auxiliary
winding increases the voltage on the V
CC
pin as the output
voltage rises. During the start−sequence, the controller
smoothly ramps up the peak current to Imax setting, e.g.
Ipeak_HI, which is reached after a typical period of 1.0 ms.
As soon as the peak current setpoint reaches its maximum
(during the startup period but also anytime an overload
occurs), an internal error flag is asserted, Ipflag, indicating
that the system has reached its maximum current limit set
point (Ip = Ip max). The assertion of this flag triggers a
55 ms counter. If at counter completion Ipflag remains
asserted, all driving pulses are stopped and the part stays off
during eight periods of 55 ms (440 ms). A new attempt to
restart occurs and will last 55 ms providing the fault is still
present. If the fault still affects the output, a safe burst mode
is entered, affected by a low duty−cycle operation (11%).
When the fault disappears, the power supply quickly
resumes operation. Figure 26 depicts this particular mode.
Figure 26. In case of short−circuit or overload, the NCP1028 protects itself and the power supply via a low
frequency burst mode. The V
CC
is maintained by the current source and self−supplies the controller.
