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NCP1030, NCP1031
be limited by the EA sink current, typically 110 m A.
Optimum transient response is obtained if the compensation
is discharging, guaranteeing a maximum duty cycle of 75 %
as shown in Figure 35.
components allow COMP to swing across its operating
range in 1 cycle.
Line Under and Overvoltage Detector
C T Ramp
COMP
Enabled
C T Charge
Signal
The NCP103x incorporates individual line undervoltage
(UV) and overvoltage (OV) shutdown circuits. The UV and
OV thresholds are 2.5 V. A fault is present if the UV is below
2.5 V or if the OV voltage is above 2.5 V. The UV/OV
detectors incorporate 175 mV hysteresis to prevent noise
from triggering the shutdown circuits.
The UV/OV circuits can be biased using an external
resistor divider from the input line as shown in Figure 34.
The UV/OV pins should be bypassed using a capacitor to
prevent triggering the UV or OV circuits during normal
switching operation.
V in
R 1
+
R 2
+ V UV
V OV R 3
-- --
Figure 34. UV/OV Resistor Divider
from the Input Line
The resistor divider must be sized to enable the controller
Power Switch Max
Duty Cycle
75%
25 %
Figure 35. Maximum Duty Cycle vs COMP
Figure 18 shows the relationship between the operating
frequency and C T . If an UV fault is present, both I CT(C) and
I CT(D) are reduced by a factor of 7, thus reducing the
operating frequency by the same factor.
The oscillator can be synchronized to a higher frequency
by capacitively coupling a synchronization pulse into the C T
pin. In sync mode, the voltage on the C T pin needs to be
driven above 3.5 V to trigger the internal comparator and
complete the C T charging period. However, pulsing the C T
pin before it reaches 3.5 V will reduce the p--p amplitude of
the C T Ramp as shown in Figure 36.
3.0 V/3.5 V
Sync Pulse Comparator
Reset
T2 (f2)
3.5 V
once V in is within the required operating range. While a UV
or OV fault is present, switching is not allowed and the
COMP pin is effectively grounded.
C T
Ramp
T1 (f1)
T2 (f2)
C T Voltage
Range in Sync
Either of these comparators can be used for a different
function if UV or OV functions are not needed. For example,
the UV/OV detectors can be used to implement an enable or
3.0 V
Free Running
Mode
Sync Mode
disable function. If positive logic is used, the enable signal
is applied to the UV pin while the OV pin is grounded. If
negative logic is used, the disable signal is applied to the OV
pin while biasing the UV pin from V CC using a resistor
divider.
Oscillator
Figure 36. External Frequency Synchronization
Waveforms
The oscillator frequency should be set no more that 25%
below the target sync frequency to maintain an adequate
voltage range and provide good noise immunity. A possible
circuit to synchronize the oscillator is shown in Figure 37.
The oscillator is optimized for operation up to 1 MHz and
its frequency is set by the external timing capacitor (C T )
connected to the C T pin. The oscillator has two modes of
operation, free running and synchronized (sync). While in
free running mode, an internal current source sequentially
charges and discharges C T generating a voltage ramp
between 3.0 V and 3.5 V. Under normal operating
conditions, the charge (I CT(C) ) and discharge (I CT(D) )
currents are typically 215 m A and 645 m A, respectively. The
5V
R1
C1
C T
R2
C T
2
charge:discharge current ratio of 1:3 discharges C T in 25 %
of the total period. The Power Switch is disabled while C T
Figure 37. External Frequency Synchronization
Circuit.
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