LTC3409A OPERATIONBurst Mode Operation Near DropoutSlope Compensation With a light load the part will transition from Burst Mode Slope compensation provides stability in constant fre- operation to 100% duty cycle as (VIN to VOUT) approaches quency architectures by preventing subharmonic oscil- ILOAD • (RMAINSWITCH + RINDUCTOR). When (VIN to VOUT) lations at high duty cycles. It is accomplished internally results in near 100% duty cycle the inductor up slope will by adding a compensating ramp to the inductor current be quite shallow compared to the inductor down slope, signal at duty cycles in excess of 40%. with low peak currents. User Controlled Switching Frequency The LTC3409A is a micropower part and the speed of the comparator controlling the synchronous switch may allow The internal oscillator of the LTC3409A can be synchronized the inductor current to reverse in a low (V to a user-supplied external clock applied to the SYNC pin. IN to VOUT) situa- tion, resulting in CCM operation. Transition from CCM back Alternately, when this pin is held at a fi xed high or low to Burst Mode operation will occur when (V level for more than 30μs, the internal oscillator will revert IN to VOUT) is suffi cient for the average inductor current to exceed the to fi xed-frequency operation; where the frequency may load current. This occurs when the average positive cur- be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC rent in the inductor exceeds the average reverse current High). caused by synchronous switch propagation delay. The CCM to Burst Mode operation re-entry transition point will Internal Soft-Start be a function of VIN, VOUT, ILOAD, COUT and the inductor At start-up when the RUN pin is brought high, the internal used in the application. reference is linearly ramped from 0V to 0.612V in 1ms. The regulated feedback voltage will follow this ramp resulting Short-Circuit Protection in the output voltage ramping from 0% to 100% in 1ms. When the output is shorted to ground the LTC3409A limits The current in the inductor during soft-start will be defi ned the synchronous switch current to 1.5A. If this limit is by the combination of the current needed to charge the exceeded, the top power MOSFET is inhibited from turn- output capacitance and the current provided to the load ing on until the current in the synchronous switch falls as the output voltage ramps up. The start-up waveform, below 1.5A. shown in the Typical Performance Characteristics, shows the output voltage start-up from 0V to 1.8V with a 1kΩ Dropout Operation load and VIN = 3.6V. The 1kΩ load results in an output of 1.8mA at 1.8V. 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 until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. 3409af 9
