NCP1379 ON Semiconductor, NCP1379 Datasheet - Page 11

• Manufacturer Part Number: NCP1379
• Description: Quasi-Resonant Current-Mode Controller
• Manufacturer: ON Semiconductor

operation and VCO operation for the frequency foldback.
portrayed by Figure 22:
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•
the auxiliary winding of the transformer. A valley is detected
when the voltage on pin 1 crosses down the 55 mV internal
NCP1379 has two operating mode: quasi−resonant
The operating mode is fixed by the FB voltage as
The valley detection is done by monitoring the voltage of
Quasi−resonant operation occurs for FB voltage higher
than 0.8 V (FB decreasing) or higher than 1.4 V (FB
increasing) which correspond to high output power and
medium output power. The peak current is variable and
is set by the FB voltage divided by 4.
Frequency foldback or VCO mode occurs for FB
voltage lower than 0.8 V (FB decreasing) or lower than
1.4 V (FB increasing). This corresponds to low output
power.
Figure 22. Operating Valley According to FB Voltage
VALLEY DETECTION AND SELECTION
NCP1379 OPERATING MODES
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threshold. When a valley is detected, an internal counter is
incremented. The operating valley (1
determined by the FB voltage as shown by Figure 22.
During VCO mode, the peak current decreases down to
17.5% of its maximum value and is then frozen. The
switching frequency is variable and decreases as the
output load decreases.
The switching frequency is set by the end of charge of
the capacitor connected to the C
charged with a constant current source and the
capacitor voltage is compared to an internal threshold
fixed by FB voltage. When this capacitor voltage
reaches the threshold the capacitor is rapidly discharged
down to 0 V and a new period start.
T
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pin. This capacitor is
st
, 2
nd
, 3
rd
or 4
th
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