NCP1207A ON Semiconductor, NCP1207A Datasheet - Page 13

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

•
Again, adding some copper area around the PCB footprint
will help decrease this number: 12 mm
R
or 6.5 mm
As one can see, we do not recommend using the SO−8
package and the DSS if the part operates at high switching
frequencies. In that case, an auxiliary winding is the best
solution.
Overload Operation
controlled (e.g. wall adapters delivering raw DC level), it is
interesting to implement a true short−circuit protection. A
short−circuit actually forces the output voltage to be at a low
level, preventing a bias current to circulate in the
Optocoupler LED. As a result, the FB pin level is pulled up
to 4.2 V, as internally imposed by the IC. The peak current
setpoint goes to the maximum and the supply delivers a
rather high power with all the associated effects. Please note
that this can also happen in case of feedback loss, e.g. a
broken Optocoupler. To account for this situation,
Soft−Start
soften the constraints occurring in the power supply during
startup. It is activated during the power on sequence. As
soon as V
increased from nearly zero up to the maximum clamping
qJA
The SOIC package offers a 178°C/W thermal resistor.
In applications where the output current is purposely not
The NCP1207A features an internal 1 ms soft−start to
If the power consumption budget is really too high for the
DSS alone, connect a diode between the auxiliary
winding and the V
operation (V
down to 100°C/W with 35 mm copper thickness (1 oz)
FAULT FLAG
5.3 V
12 V
10 V
INTERNAL
CC
DRV
V
STARTUP PHASE
reaches VCC
CC
6.5 mm with 70 mm copper thickness (2 oz).
CC
u 10 V).
CC
OCCURS HERE
REGULATION
OFF
pin which will disable the DSS
, the peak current is gradually
LATCHOFF
FAULT OCCURS HERE
PULSES
DRIVER
PHASE
12 mm to drop
http://onsemi.com
FAULT IS
RELAXED
Figure 24.
13
NCP1207A hosts a dedicated overload detection circuitry.
Once activated, this circuitry imposes to deliver pulses in a
burst manner with a low duty−cycle. The system recovers
when the fault condition disappears.
maximum until the output voltage reaches its target and the
feedback loop takes over. This period of time depends on
normal output load conditions and the maximum peak
current allowed by the system. The time−out used by this IC
works with the V
V
device internally watches for an overload current situation.
If this condition is still present when the VCC
reached, the controller stops the driving pulses, prevents the
self−supply current source to restart and puts all the circuitry
in standby, consuming as little as 330 mA typical (I
parameter). As a result, the V
toward 0. When this level crosses 5.3 V typical, the
controller enters a new startup phase by turning the current
source on: V
pulses at the VCC
has been removed before VCC
continues its normal operation. Otherwise, a new fault cycle
takes place. Figure 24 shows the evolution of the signals in
presence of a fault.
level (e.g. 1.0 V). The soft−start is also activated during the
overcurrent burst (OCP) sequence. Every restart attempt is
followed by a soft−start activation. Generally speaking, the
soft−start will be activated when V
zero (fresh power−on sequence) or 5.3 V, the latchoff
voltage occurring during OCP.
CC
During the startup phase, the peak current is pushed to the
TIME
TIME
TIME
decreases from the VCC
CC
rises toward 12 V and again delivers output
CC
OFF
If the fault is relaxed during the Vcc
natural fall down sequence, the IC
automatically resumes.
If the fault still persists when Vcc
reached VCC
cuts everything off until recovery.
decoupling capacitor: as soon as the
crossing point. If the fault condition
OFF
ON
CC
ON
, then the controller
level (typically 12 V) the
CC
level slowly discharges
approaches, then the IC
ramps up either from
ON
level is
CC3