NCP1288
http://onsemi.com
28
OVERCURRENT PROTECTION WITH FAULT TIMER
Classical Overcurrent Protection
When an overcurrent occurs on the output of the power
supply, the feedback loop asks for more power than the
controller can deliver, and the CS setpoint reaches V
ILIM
.
When this event occurs, an internal t
fault
timer is started:
once the timer times out, DRV pulses are stopped and the
controller is either latched off (latched protection,
Version A), or it enters an autorecovery mode (Version B).
The timer is reset when the CS setpoint goes back below
V
ILIM
before the timer elapses. The fault timer is also started
if the driver signal is reset by the max duty ratio.
Figure 56. Timer−Based Overcurrent Protection
CS
FB
−
+
tLEB
blanking
/ 5
−
+
+
VILIM
Protection
Mode
Brown−out
tfault
timer
release
t autorec
timer
Reset
Autorecovery
protection
mode only
R
S
Q
PWM
Reset DRV
Fault Flag
DC MAX
DRV
In autorecovery mode, the controller tries to restart after
t
autorec
. If the fault has gone, the supply resumes operation;
if not, the system starts a new burst cycle (see Figure 57).
