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NCP3065 Datasheet(PDF) 7 Page - ON Semiconductor |
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NCP3065 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 18 page NCP3065, NCV3065 http://onsemi.com 7 INTRODUCTION The NCP3065 is a monolithic power switching regulator optimized for LED Driver applications. Its flexible architecture enables the system designer to directly implement a step-up or step-down topology with a minimum number of external components for driving LEDs. A representative block diagram is shown in Figure 4. OPERATING DESCRIPTION The NCP3065 operates as a fixed oscillator frequency output voltage ripple gated regulator. In general, this mode of operation is somewhat analogous to a capacitor charge pump and does not require dominant pole loop compensation for converter stability. The typical operating waveforms are shown in Figure 14. The output voltage waveform shown is for a step-down converter with the ripple and phasing exaggerated for clarity. During initial converter startup, the feedback comparator senses that the output voltage level is below nominal. This causes the output switch to turn on and off at a frequency and duty cycle controlled by the oscillator, thus pumping up the output filter capacitor. When the feedback voltage level reaches nominal comparator value, the output switch cycle is inhibited. When the load current causes the output voltage to fall below the nominal value feedback comparator enables switching immediately. Under these conditions, the output switch conduction can be enabled for a partial oscillator cycle, a partial cycle plus a complete cycle, multiple cycles, or a partial cycle plus multiple cycles. Oscillator The oscillator frequency and off-time of the output switch are programmed by the value of the timing capacitor CT. Capacitor CT is charged and discharged by a 1 to 6 ratio internal current source and sink, generating a positive going sawtooth waveform at Pin 3. This ratio sets the maximum tON/(tON+tOFF) of the switching converter as 6/(6+1) or 85.7% (typical). The oscillator peak and valley voltage difference is 500 mV typically. To calculate the CT capacitor value for required oscillator frequency, use the equations found in Figure 22. An online NCP3065 design tool can be found at www.onsemi.com, which adds in selecting component values. Figure 14. Typical Operating Waveforms 1 0 Output Switch 1 0 On Off Feedback Comparator Output Nominal Output Voltage Level Startup Operation Output Voltage Timing Capacitor, CT IPK Comparator Output |
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