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NCP81243 Datasheet(PDF) 21 Page - ON Semiconductor |
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NCP81243 Datasheet(HTML) 21 Page - ON Semiconductor |
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21 / 26 page  NCP81243 www.onsemi.com 21 Rcomp2 8.2K RNTC 100K Cfilter 0.1uF AGND AGND Rcomp1 0.0 TSENSE Precision Oscillator A programmable precision oscillator is provided. The clock oscillator serves as the master clock to the ramp generator circuit. This oscillator is programmed by a resistor to ground on the ROSC pin. The oscillator frequency range is between 300 KHz/phase to 1.4 MHz/phase. The ROSC pin provides approximately 2 V out and the source current is mirrored into the internal ramp oscillator. The oscillator frequency is approximately proportional to the current flowing in the ROSC resistor. Figure 7. NCP81243 Operating Frequency vs. Rosc The oscillator generates triangle ramps that are 1.3~2.5 V in amplitude depending on the VRMP pin voltage to provide input voltage feed forward compensation. The ramps are equally spaced out of phase with respect to each other and the signal phase rail is set half way between phases 1 and 2 of the multi phase rail for minimum input ripple current. For use with On Semiconductors phase doubler the NCP81243 offers the user the ability to double the frequency of each rail independently or simultaneously. This will allow the rail that is being doubled to maintain a higher system switching frequency. Programming the Ramp Feed−Forward Circuit The ramp generator circuit provides the ramp used by the PWM comparators. The ramp generator provides voltage feed−forward control by varying the ramp magnitude with respect to the VRMP pin voltage. The VRMP pin also has a 4V UVLO function. The VRMP UVLO is only active after the controller is enabled. The VRMP pin is high impedance input when the controller is disabled. The PWM ramp time is changed according to the following, V RAMPpk +pk PP + 0.1 @ V VRMP Vin Comp −IL Duty Vramp _pp |
Similar Part No. - NCP81243_17 |
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Similar Description - NCP81243_17 |
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