Datasheet LTC3409 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 600mA Low VIN Buck Regulator in 3mm × 3mm DFN |
| Pages / Page | 16 / 9 — OPERATION. Slope Compensation. User Controlled Switching Frequency. … |
| File Format / Size | PDF / 295 Kb |
| Document Language | English |
OPERATION. Slope Compensation. User Controlled Switching Frequency. Short-Circuit Protection. Internal Soft-Start
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LTC3409
OPERATION
to audio circuitry. When the converter is in Burst Mode
Slope Compensation
operation, the minimum peak current of the inductor is Slope compensation provides stability in constant fre- set to approximately 200mA regardless of the output load. quency architectures by preventing subharmonic oscil- Each burst event can last from a few cycles at light loads lations at high duty cycles. It is accomplished internally to almost continuously cycling with short sleep intervals by adding a compensating ramp to the inductor current at moderate loads. In between these burst events, the signal at duty cycles in excess of 40%. power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 65μA. In this sleep
User Controlled Switching Frequency
state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA The internal oscillator of the LTC3409 can be synchronized amplifi er’s output rises above the sleep threshold signaling to a user-supplied external clock applied to the SYNC pin. the BURST comparator to trip and turn the top MOSFET Alternately, when this pin is held at a fi xed High or Low on. This process repeats at a rate that is dependent on level for more than 30μs, the internal oscillator will revert the load demand. to fi xed-frequency operation; where the frequency may be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC
Short-Circuit Protection
High). When the output is shorted to ground the LTC3409 limits
Internal Soft-Start
the synchronous switch current to 1.5A. If this limit is exceeded, the top power MOSFET is inhibited from turn- At start-up when the RUN pin is brought high, the internal ing on until the current in the synchronous switch falls reference is linearly ramped from 0V to 0.613V in 1ms. The below 1.5A. regulated feedback voltage will follow this ramp resulting in the output voltage ramping from 0% to 100% in 1ms.
Dropout Operation
The current in the inductor during soft-start will be defi ned by the combination of the current needed to charge the As the input supply voltage decreases to a value ap- output capacitance and the current provided to the load proaching the output voltage, the duty cycle increases as the output voltage ramps up. The start-up waveform, toward the maximum on-time. Further reduction of the shown in the Typical Performance Characteristics, shows supply voltage forces the main switch to remain on for the output voltage start-up from 0V to 1.5V with a 2.5Ω more than one cycle. load and VIN = 2.2V. The 2.5Ω load results in an output of 600mA at 1.5V. 3409fc 9
OPERATION
to audio circuitry. When the converter is in Burst Mode
Slope Compensation
operation, the minimum peak current of the inductor is Slope compensation provides stability in constant fre- set to approximately 200mA regardless of the output load. quency architectures by preventing subharmonic oscil- Each burst event can last from a few cycles at light loads lations at high duty cycles. It is accomplished internally to almost continuously cycling with short sleep intervals by adding a compensating ramp to the inductor current at moderate loads. In between these burst events, the signal at duty cycles in excess of 40%. power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 65μA. In this sleep
User Controlled Switching Frequency
state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA The internal oscillator of the LTC3409 can be synchronized amplifi er’s output rises above the sleep threshold signaling to a user-supplied external clock applied to the SYNC pin. the BURST comparator to trip and turn the top MOSFET Alternately, when this pin is held at a fi xed High or Low on. This process repeats at a rate that is dependent on level for more than 30μs, the internal oscillator will revert the load demand. to fi xed-frequency operation; where the frequency may be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC
Short-Circuit Protection
High). When the output is shorted to ground the LTC3409 limits
Internal Soft-Start
the synchronous switch current to 1.5A. If this limit is exceeded, the top power MOSFET is inhibited from turn- At start-up when the RUN pin is brought high, the internal ing on until the current in the synchronous switch falls reference is linearly ramped from 0V to 0.613V in 1ms. The below 1.5A. regulated feedback voltage will follow this ramp resulting in the output voltage ramping from 0% to 100% in 1ms.
Dropout Operation
The current in the inductor during soft-start will be defi ned by the combination of the current needed to charge the As the input supply voltage decreases to a value ap- output capacitance and the current provided to the load proaching the output voltage, the duty cycle increases as the output voltage ramps up. The start-up waveform, toward the maximum on-time. Further reduction of the shown in the Typical Performance Characteristics, shows supply voltage forces the main switch to remain on for the output voltage start-up from 0V to 1.5V with a 2.5Ω more than one cycle. load and VIN = 2.2V. The 2.5Ω load results in an output of 600mA at 1.5V. 3409fc 9
