NCP1217, NCP1217A
www.onsemi.com
13
Figure 24. When Vadj is Pulled Above 3.1 V, NCP1217 Permanently Latches−Off the Output Pulses
Default
adj level
Fault brings adj above latching level
Latched−off
Reset level
The startup current source keeps the
device latched until reset occurs.
V
CC
Time
Time
Time
Driver
Pulses
Drv
Adj
VCC
ON
= 12.8 V
VCC
min
= 7.6 V
VCC
latch
= 5.6 V
In normal operation, the Adj pin level is kept at a fixed
level, the default one or lower. As soon as some external
signal pulls this Adj pin level above 3.1 V typical, the output
pulses are permanently disabled. Care must be taken to limit
the injected current into pin 1 to less than 2.0 mA, e.g.
through a series resistor of 5.6 k with a 10 V V
CC
. The
startup switch is activated every time V
CC
reaches 5.6 V and
maintains a V
CC
voltage ramping up and down between
5.6 V and 12.8 V. Reset occurs when V
CC
falls below 5.6 V,
e.g. when the user cycle the SMPS down. Figure 25
illustrates the operation. Adding a zener diode from Q1 base
to ground makes a cheap OVP, protecting the supply from
any lethal open−loop operation. If a thermistor (NTC) is
added in parallel with the Zener−diode, overtemperature
protection is also ensured.
Figure 25. A Thermistor and a Zener Diode Offer
Both OVP and Overtemperature Latched−Off
Protection
Laux
8
7
6
5
1
2
3
4
Vaux
CV
CC
t16 V
OVP
T
Nonlatching Shutdown
In some cases, it might be desirable to shut off the part
temporarily and authorize its restart once the default has
disappeared. This option can easily be accomplished
through a single NPN bipolar transistor wired between FB
and ground. By pulling FB below the Adj Pin 1 level, the
output pulses are disabled as long as FB is pulled below
Pin 1. As soon as FB is relaxed, the IC resumes its operation.
Figure 26 depicts the application example.
