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LTC3409EDD Datasheet(PDF) 9 Page - Linear Technology |
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LTC3409EDD Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC3409 3409f skipping mode is lower output ripple and less interference to audio circuitry. When the converter is in Burst Mode operation, the minimum peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 65µA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier’s output rises above the sleep threshold signal- ing the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. Short-Circuit Protection When the output is shorted to ground the LTC3409 limits the synchronous switch current to 1.5A. If this limit is exceeded, the top power MOSFET is inhibited from turn- ing on until the current in the synchronous switch falls below 1.5A. Dropout Operation As the input supply voltage decreases to a value ap- proaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle. OPERATIO Slope Compensation Slope compensation provides stability in constant fre- quency architectures by preventing subharmonic oscilla- tions at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. User Controlled Switching Frequency The internal oscillator of the LTC3409 can be synchronized to a user-supplied external clock applied to the SYNC pin. Alternately, when this pin is held at a fixed High or Low level for more than 30µs, the internal oscillator will revert to fixed-frequency operation; where the frequency may be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC High). Internal Soft-Start At start-up when the RUN pin is brought high, the internal reference is linearly ramped from 0V to 0.613V in 1ms. The regulated feedback voltage will follow this ramp resulting in the output voltage ramping from 0% to 100% in 1ms. The current in the inductor during soft-start will be defined by the combination of the current needed to charge the output capacitance and the current provided to the load as the output voltage ramps up. The start-up waveform, shown in the Typical Performance Characteristics, shows the output voltage start-up from 0V to 1.5V with a 2.5Ω load and VIN = 2.2V. The 2.5Ω load results in an output of 600mA at 1.5V. |
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