NCP1239
www.onsemi.com
20
The controller bias current is reduced during power up by
disabling most of the circuit blocks including I
FAULT(OTP)
.
This current source is enabled once V
CC
reaches V
CC(min)
.
A bypass capacitor is usually connected between the Fault
and GND pins. It will take some time for V
FAULT
to reach
its steady state value once I
OTP
is enabled. Therefore, the
lower fault comparator (i.e. over-temperature detection) is
ignored during soft-start. In addition, in order to speed up
this fault pin capacitor, OTP current is doubled during the
soft-start period.
Figure 38. Fault Detection Schematic
Fault
NTC
Vaux
Latch
S
R
Q
Q
Up counter
4
Power on
reset
OVP/OTP gone
RST
Vfault(clamp)
VFault(OTP)
VFault(OVP)
IOTP
Vdd
DRV
Rfault(clamp)
600 ns
Time constant
1
Blanking Time
sm
Falling edge
As a matter of fact, the controller operates normally while
the fault pin voltage is maintained within the upper and
lower fault thresholds. Upper and lower fault detectors have
blanking delays to prevent noise from triggering them. Both
OVP and OTP comparator output are validated only if its
high-state duration lasts a minimum of 600 ns. Below this
value, the event is ignored. Then, a counter ensures that
OVP/OTP events occurred for 4 successive drive clock
pulses before actually latching the part.
When the part is latched-off, the drive is immediately
turned off and V
CC
goes in endless hiccup mode. The power
supply needs to be un-plugged to reset the part (V
CC(reset)
or
BO event). Please note that this protection on the Fault pin
is autorecovery for the E version.
AUTO-RECOVERY SHORT-CIRCUIT PROTECTION
In case of output short-circuit or if the power supply
experiences a severe overloading situation, an internal error
flag is raised and starts a countdown timer. If the flag is
asserted longer than the timer’s programmed value, the
driving pulses are stopped and a 1-s auto-recovery timer
starts. If V
CC
voltage is below V
CC(min)
, HV current source
is activated to build up the voltage to V
CC(on)
. On the
contrary, if V
CC
voltage is above V
CC(min)
, HV current
source is not activated, V
CC
falls down as the auxiliary
pulses are missing and the controller waits that V
CC(min)
is
crossed to enable the stat-up current source. During the timer
count down, the controller purposely ignores the re-start
when V
CC
crosses V
CC(on)
and waits for another V
CC
cycle.
By lowering the duty cycle in fault condition, it naturally
reduces the average input power and the rms current in the
output cable. Illustration of such principle appears in
Figure 39. Please note that soft-start is activated upon
re-start attempt.
