1207A ON Semiconductor, 1207A Datasheet - Page 13

• Manufacturer Part Number: 1207A
• Description: Search -----> NCP1207A
• Manufacturer: ON Semiconductor

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Calculating the Vcc Capacitor
depends upon the V
V
on the startup sequence of your system, i.e. when you first
apply the power to the IC. The corresponding transient fault
duration due to the output capacitor charging must be less
than the time needed to discharge from 12 V to 10 V,
otherwise the supply will not properly start. The test consists
in either simulating or measuring in the lab how much time
the system takes to reach the regulation at full load. Let’s
suppose that this time corresponds to 6.0 ms. Therefore a
V
to not trigger the overload detection circuitry. If the
corresponding IC consumption, including the MOSFET
drive, establishes at 1.8 mA (e.g. with an 11 nC MOSFET),
we can calculate the required capacitor using the following
formula:
Dt of 10 ms, C equals 9.0 mF or 22 mF for a standard value.
When an overload condition occurs, the IC blocks its
internal circuitry and its consumption drops to 330 mA
typical. This happens at V
until V
using the calculated 22 mF and 330 mA current consumption,
this latchoff phase lasts: 313 ms.
HV Pin Recommended Protection
controller such as the NCP1207A, two phenomena can
appear:
high−voltage diode like a 1N4007 between the bulk
capacitor and the V
naturally shuts the controller down and eradicates the two
above problems.
CC
CC
As the above section describes, the fall down sequence
When the user unplugs a power supply built with a QR
For the above reasons, we strongly recommend to add a
1. A negative ringing can take place on pin8 due to a
2. When the bulk capacitor discharges, the internal
line to go from 12 V to 10 V? The required time depends
fall time of 10 ms could be well appropriated in order
CC
resonance between the primary inductance and
the bulk capacitor. As any CMOS device, the
NCP1207A is sensitive to negative voltages that
could appear on it’s pins and could create an
internal latch−up condition.
latch is reset by the voltage developed over the
sense resistor and the ON time expands as less
voltage is available. When the high−voltage rail
becomes too low, the gate drives permanently
stays high since no reset occurs. This situation is
not desirable in many applications.
Dt + DV @ C
reaches 5.3 V: we are in latchoff phase. Again,
i
CC
CC
, with DV = 2.0 V. Then for a wanted
pin. When the bulk level collapses, it
level: how long does it take for the
CC
= 10 V and it remains stuck
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NCP1207A
13
Operation Shots
V
primary inductance.
operating points:
in
Below are some oscilloscope shots captured at
This plot gathers waveforms captured at three different
1
P
2
and selects the second valley
3
and will further expand the burst as P
+
Cbulk
st
nd
rd
= 120 VDC with a transformer featuring a 800 mH
out
lowest plot: the skip slices the second valley pattern
upper plot: free run, valley switching operation,
middle plot: min T
= 26 W
1
2
3
4
NCP1207A
Figure 25.
Figure 26.
off
8
7
6
5
HV
clamps the switching frequency
+
D1
1N4007
out
goes low
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