NCP1207A ON Semiconductor, NCP1207A Datasheet - Page 14

• Manufacturer Part Number: NCP1207A
• Description: PWM Current-Mode Controller
• Manufacturer: ON Semiconductor

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, one instance can occur:
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.
the bulk capacitor and the V
Operation Shots
V
primary inductance.
CC
CC
in
As the above section describes, the fall down sequence
When the user unplugs a power supply built with a QR
A negative ringing can take place on pin8 due to a
For this reason, we recommend adding a resistor between
Below are some oscilloscope shots captured at
= 120 VDC with a transformer featuring a 800 mH
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
Dt + DV @ C
reaches 5.3 V: we are in latchoff phase. Again,
i
CC
, with DV = 2.0 V. Then for a wanted
level: how long does it take for the
CC
CC
= 10 V and it remains stuck
pin.
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14
operating points:
current creates the skip cycle level.
This plot gathers waveforms captured at three different
This picture explains how the 200 mA internal offset
1
P
2
and selects the second valley
3
and will further expand the burst as P
st
nd
rd
out
lowest plot: the skip slices the second valley pattern
middle plot: min T
upper plot: free run, valley switching operation,
= 26 W
Current Sense Pin (200 mV/div)
V
GATE
200 mA X R
(5 V/div)
Figure 25.
Figure 26.
off
clamps the switching frequency
SKIP
V
Rsense
out
(200 mV/div)
goes low