NCP1562B ON Semiconductor, NCP1562B Datasheet - Page 19

• Manufacturer Part Number: NCP1562B
• Description: High Performance Active Clamp/Reset PWM Controller
• Manufacturer: ON Semiconductor

(patent pending). This architecture allows both the UV and
OV levels to be set independently. Both the UV and OV
detectors have a 100 mV hysteresis.
shown in Figure 42.
V
UV turn threshold. Once the UVOV voltage exceeds 2.5 V,
an internal current source (I
the UVOV pin. This will clamp the UVOV voltage at 2.5 V
while the current across R1 is less than I
input voltage continues to increase, the 50 mA source will
be overridden and the voltage at the UVOV pin will
increase. An OV condition exists if the UVOV voltage
exceeds V
relationship between UVOV and V
voltage is solely determined by the ratio of R1 and R2. The
input voltage at which the converter turns ON is given by
UV
The line voltage is sampled using a resistor divider as
A UV condition exists if the UVOV voltage is below
While the internal current source is disabled, the UVOV
, typically 2.0 V. The ratio of R1 and R2 determines the
Figure 43. UVOV Detectors Typical Waveforms
V
V
R1
R2
UVOV
UVCOMP
V
in
Figure 42. Line UVOV Detectors
OV
, typically 3.0 V. Figure 43 shows the
C
UVOV
UVOV
Time
V
OVCOMP
offset(UVOV)
I
offset(UVOV)
+
−
+
−
+
−
3.0 V
2.0 V
2.5 V
+
−
+
−
+
−
in
.
OV Comparator
UV Comparator
) sinks 50 mA into
offset(UVOV)
V
V
OVCOMP
UVCOMP
http://onsemi.com
. If the
19
Equation 1. Once the internal current source is enabled, the
absolute value of R1 together with the ratio of R1 and R2
determine the turn OFF threshold as shown in Equation 2.
manufacturing to obtain "3% accuracy allowing a tighter
power stage design.
V
soft−start sequence commences. If a UV or OV fault is
detected afterwards, the converter enters a soft−stop mode.
UVOV pin to GND to prevent oscillation of the UVOV pin
and filter line transients.
Line Feedforward
limit the maximum volt−second product. It is the line
voltage times the ON time. This limit prevent saturation of
the transformer in forward and flyback topologies. Another
advantage of feedforward is a controller frequency gain
independent of line voltage. A constant gain facilitates
frequency compensation of the converter.
proportional to V
error signal solely controls the duty cycle while the input
voltage is fixed. If the line voltage changes, the FF Ramp
slope changes and duty cycle is immediately adjusted instead
of waiting for the change to propagate around the feedback
loop and be reflected back on the error signal.
from the input line as shown in Figure 44. The divider is
selected such that the FF Ramp reaches 3.0 V in the desired
maximum ON time. The FF Ramp terminates by
effectively grounding C
This can be triggered by the FF Ramp reaching 3.0 V, or
any other condition that limits the duty cycle.
during standby mode to prevent the FF pin from charging
up to V
V in(OV) + V OV
AUX
The
Once the line voltage is within the operating range, and
A small capacitor is required (>1000 pF) from the
The NCP1562 incorporates line feedforward (FF) to
Feedforward is implemented by generating a ramp
The FF Ramp is generated with an R−C (R
The FF pin is effectively grounded during power or
FF Reset
Figure 44. Feed Forward Ramp Generation
reaches V
in
undervoltage
To PWM and VS
.
Comparators
V in(UV) + V UV
(R 1 ) R 2 )
in
AUX(on)
and comparing it to the error signal. The
R 2
I
FF(D)
FF
threshold is
, the outputs are enabled and a
during the converter OFF time.
) (I offset(UVOV)
R
C
FF
(R 1 ) R 2 )
FF
FF
V
R 2
in
trimmed
I
RFF
t
FF
on
T
C
FF
R1)
) divider
0 V
(eq. 1)
during
3 V
(eq. 2)