NCP1203
www.onsemi.com
11
Figure 19. Another Way of Shutting Down the IC without a Definitive Latch−Off State
ON/OFF
Q1
8
7
6
5
1
2
3
4
Full Latching Shutdown
Other applications require a full latching shutdown, e.g.
when an abnormal situation is detected (overtemperature or
overvoltage). This feature can easily be implemented
through two external transistors wired as a discrete SCR.
When the V
CC
level exceeds the zener breakdown voltage,
the NPN biases the PNP and fires the equivalent SCR,
permanently bringing down the FB pin. The switching
pulses are disabled until the user unplugs the power supply.
Figure 20. Two Bipolars Ensure a Total Latch−Off of the SMPS in Presence of an OVP
LAux
NCP1203
CV
CC
Rhold
12 k
0.1 mF
10 k
10 k
8
7
6
5
1
2
3
4
OVP
Rhold ensures that the SCR stays on when fired. The bias
current flowing through Rhold should be small enough to let
the V
CC
ramp up (12.8 V) and down (4.9 V) when the SCR
is fired. The NPN base can also receive a signal from a
temperature sensor. Typical bipolars can be MMBT2222
and MMBT2907 for the discrete latch. The MMBT3946
features two bipolars NPN+PNP in the same package and
could also be used.
Protecting the Controller Against Negative Spikes
As with any controller built upon a CMOS technology, it
is the designer’s duty to avoid the presence of negative
spikes on sensitive pins. Negative signals have the bad habit
to forward bias the controller substrate and induce erratic
behaviors. Sometimes, the injection can be so strong that
internal parasitic SCRs are triggered, engendering
irremediable damages to the IC if they are a low impedance
path is offered between V
CC
and GND. If the current sense
pin is often the seat of such spurious signals, the
high−voltage pin can also be the source of problems in
certain circumstances. During the turn−off sequence, e.g.
when the user un−plugs the power supply, the controller is
still fed by its V
CC
capacitor and keeps activating the
MOSFET ON and OFF with a peak current limited by
Rsense. Unfortunately, if the quality coefficient Q of the
resonating network formed by Lp and Cbulk is low (e.g. the
MOSFET Rdson + Rsense are small), conditions are met to
make the circuit resonate and thus negatively bias the
controller. Since we are talking about ms pulses, the amount
of injected charge (Q = I x t) immediately latches the
controller which brutally discharges its V
CC
capacitor. If this
V
CC
capacitor is of sufficient value, its stored energy
damages the controller. Figure 21 depicts a typical negative
shot occurring on the HV pin where the brutal V
CC
discharge
testifies for latchup.
