ncp1282 ON Semiconductor, ncp1282 Datasheet - Page 16

• Manufacturer Part Number: ncp1282
• Description: High-performance Active Clamp/reset Pwm Controller Featuring Hv Start-up
• Manufacturer: ON Semiconductor

V
threshold of the latch. The external latch is cleared by
bringing the UVOV voltage below V
output buffer from ON Semiconductor's MiniGatet
family. The enable (OE) and output (OUTY) terminals are
connected to CSKIP and the V
connected to V
impedance mode when OE is low. Once a continuous
current limit condition is detected, the CSKIP timer is
enabled and CSKIP begins charging. Once the voltage on
CSKIP reaches the enable threshold of the buffer, the
output of the buffer is pulled to V
timer. The OE threshold of the buffer is typically 1.5 V.
MOSFETs is shown in Figure 39. The latch is enabled once
the CSKIP voltage reaches the threshold of M1. Once M1
turns on, it pulls low the gate of M2. CSKIP is then pulled
to V
R
charges. This will cause the controller to latch during initial
power-up. In this particular implementation the turn on
threshold of M1 is 2 V and R
Leading Edge Blanking
caused by the power switch transitions. The current signal
CSKIP(valley)
pull-up
The latch in Figure 38 consists of a TTL level tri-state
A latch implemented using discrete N and P-channel
The current sense signal is prone to leading edge spikes
V
REF
CSKIP
CSKIP
V
REF
Figure 38. External Latch Implemented using
Figure 39. External Latch Implemented using
REF
ON Semiconductor's MiniGatet Buffer
by M2. It is important to size R
is too big, it will not keep M2 off while V
Discrete N and P-Channel MOSFETs
C
C
REF
REF
once the CSKIP voltage reaches the turn on
REF
BSS84L
M2
. The output of the buffer is in a high
C
CSKIP
C
OUTY
CSKIP
24.9 kW
pull-up
V
OE
CC
REF
M1
CC
UV
is sized to 24.9 kΩ.
, latching the CSKIP
and INA pins are
MC74VHC1GT126
and disabling V
R
pull-up
pull-up
INA
correctly. If
2N7002L
http://onsemi.com
REF
REF
NCP1282
.
16
is usually filtered using an RC low–pass filter to avoid
premature triggering of the current limit circuit. However,
the low pass filter will inevitably change the shape of the
current pulse and also add cost and complexity. The
NCP1282 uses LEB circuitry that blocks out the first 70 ns
(typ) of each current pulse. This removes the leading edge
spikes without altering the current waveform. The blanking
period is disabled during soft-start as the blanking period
may be longer than the startup duty cycle. It is also disabled
if the output of the Saturation Comparator is low, indicating
that the output is not yet in regulation. This occurs during
power up or during an output overload condition.
Supply Voltage and Startup Circuit
need for external startup components. In addition, this
regulator increases the efficiency of the supply as it uses no
power when in the normal mode of operation, but instead
uses power supplied by an auxiliary winding. The
NCP1282 incorporates an optimized startup circuit that
reduces the requirement of the supply capacitor,
particularly important in size constrained applications.
optimized for driving high voltage MOSFETs. It is not
uncommon for high voltage MOSFETs to have a gate
plateau voltage of 6 V. In addition, high voltage
applications may require a high side drive circuit with a
voltage drop of up to two diodes. If the minimum supply
voltage is too low, there may not be enough voltage for
driving the external MOSFETs causing the system to
malfunction. The NCP1282 eliminates this problem with a
minimum supply voltage of 8.5 V.
source that supplies current from the input line voltage
(V
startup current (I
regulator is disabled and the outputs are enabled if there are
no UV, OV, CSKIP or thermal shutdown faults. The startup
regulator remains disabled until the lower voltage
threshold (V
the startup circuit is enabled and a soft-stop event is
initiated. If the bias current requirement out of C
greater than the startup current, V
reaching the lower voltage threshold (V
Upon reaching V
the outputs are disabled, the bias current of the IC is
reduced, allowing V
operation allows a dramatic reduction in the size of C
as not all the power required for startup needs to be stored
by C
Self Supply (DSS). Figure 40 shows the relationship
between V
shown in Figure 40, the outputs are not enabled until the
UV fault is removed and V
The NCP1282 internal startup regulator eliminates the
The minimum supply voltage of the NCP1282 is
The startup regulator consists of a constant current
Once C
in
) to the supply capacitor on the V
AUX
. This mode of operation is known as Dynamic
AUX
AUX(on)
AUX(off1)
is charged to 11.0 V (V
start
AUX(off2)
, V
) is typically 10 mA.
AUX
) of 9.5 V is reached. Once reached,
AUX(off1)
to charge back up. This mode of
, the outputs are disabled. Once
AUX
, V
reaches V
AUX
AUX(off2)
AUX
AUX(on)
will discharge until
AUX(off2)
pin (C
AUX(on)
and UV. As
), the startup
AUX
) of 8.5 V.
.
AUX
). The
AUX
is