ncp1001 ON Semiconductor, ncp1001 Datasheet - Page 10

• Manufacturer Part Number: ncp1001
• Description: Integrated Off-line Switching Regulator
• Manufacturer: ON Semiconductor

improves reliability in overload conditions. Figure 20 shows
the timing waveforms in auto restart mode.
voltage start−up circuit. Once the undervoltage lockout trip
point is exceeded, the high voltage start−up circuit turns off,
and the V
winding of the power transformer. Once the converter is
enabled, the V
limiter. Since the voltage limiter will regulate the V
voltage at 8.6 volts, it must shunt all excess current based on
the input impedance to this pin. A resistor is required
between the auxiliary winding filter capacitor and the V
pin to limit the current.
Optocoupler Fail−safe Circuit
and protect the load in the event of a failure. This circuit
operates by sensing the current in the V
detecting a high current which is an indication of an open
optocoupler.
is created by the voltage drop between the auxiliary winding
and the V
will clamp the V
current that is available at this pin, that is not needed for
either the chip bias current, or the opto current is shunted
through this limiter.
under an open opto condition, the opto current will all be
shunted through the V
therefore the auxiliary winding voltage) will increase. The
increase in auxiliary winding voltage will cause an
amplified increase in the current into the V
an open opto condition, the current in the limiter is measured
and if it exceeds 10 milliamps, the chip will shut down and
go into burst mode operation. After a shutdown signal, the
optocoupler
divide−by−eight counter and attempt to restart the unit after
every eight V
be chosen prudently. There is a range of values for R
will determine the operation of this circuit. On one extreme,
a large value of R
have the effect of maximizing efficiency, while reducing the
response to an open optocoupler. The other extreme is the
minimum value of R
into the chip and minimize the voltage overshoot in the event
of an open optocoupler
The V
The NCP100x has the ability to sense an open optocoupler
The V
The opto fail−safe circuit operates on the premise that
For this circuit to operate properly, the shunt resistor must
CC
CC
CC
CC
pin receives the output of a current source which
pin receives its start−up power from the high
limiter across the shunt resistor. The Vcc limiter
CC
pin receives its power from the auxiliary
CC
fail−safe
CC
cycles.
S
voltage will be clamped by the 8.6 volt
will minimize the bias current, which will
voltage to approximately 8.6 volts. Any
S,
CC
which will maximize the bias current
.
limiter, and the output voltage (and
circuit
will
NCP1000, NCP1001, NCP1002
CC
CC
pin. To detect
enable
limiter, and
http://onsemi.com
S
that
the
CC
CC
10
For minimum bias current:
where:
from it’s nominal value.
used. This value can found in the tables on the data sheet.
For the best optocoupler fail−safe response:
where:
fail−safe. This information can be found in the tables under
Current Limit and Thermal Protection, as well as in
Figure 12.
the two extreme values calculated. Setting it closer to R
will optimize the optocoupler fail−safe feature, while setting
it closer to the R
Thermal Shutdown and Package
in the event that the maximum junction temperature is
exceeded. When activated, typically at 140 C, one input of
the Driver is held low to disable the Power Switch Circuit.
Thermal shutdown activation is non−latching and the Power
Switch Circuit is allowed resume operation when the
junction temperature falls below 110 C. The thermal
shutdown feature is provided to prevent catastrophic device
failures from accidental overheating. It is not intended to be
used as a substitute for proper heatsinking.
a special heat tab copper alloy lead frame. The tab consists
of pins 3, 6, 7, 8 is specifically designed to improve the
thermal conduction from the die to the printed circuit board.
This permits the use of standard layout and mounting
practices while having the ability to halve the junction to air
thermal resistance.
V
auxiliary winding.
Typically, this voltage will vary between "5% to "10%
I
V
auxiliary winding.
I
The value of R
The internal Thermal Shutdown block protects the device
The die in the 8−pin dual−in−line package is mounted on
CC1max
trip
AUXmin
AUXmax
Rs max +
Rs
is the minimum trip current for the optocoupler
min
is the maximum rated bias current for the device
is the minimum expected DC voltage from the
is the maximum expected DC voltage from the
+
(V
Smax
S
(V
AUX max
that is used in the circuit must be between
AUX min
1.0mA ) I
value will minimize the bias current
I
CC1max
* 8.8 volts)
* 7.2 volts)
trip
Smin