Datasheet LTC3103 (Analog Devices) - 9

LTC3103
OPERATION
The LTC3103 step-down DC/DC converter is capable of voltage ripple or its harmonics. The LTC3103 offers a broad supplying 300mA to the load. The output voltage is adjust- range of possible step-down ratios without pulse skipping able over a broad range and can be set as low as 0.6V. but for very small step-down ratios, the minimum on-time Both the power and the synchronous rectifier switches of the main switch will be reached and the converter will are internal N-channel MOSFETs. The converter uses a begin turning off for multiple cycles in order to maintain constant-frequency, current mode architecture and may regulation. be configured using automatic Burst Mode operation for highly efficient light load operation or configured for low
Burst Mode Operation
noise forced conduction continuous operation where the Holding the MODE pin above 1.2V will enable automatic converter is optimized to operate over a broad range of Burst Mode operation and disable forced continuous step-down ratios without pulse skipping. With the auto- operation. As the load transitions current increases the matic Burst Mode feature enabled the typical DC supply converter will automatically transition between Burst current drops to only 1.8µA with no load. Mode and PWM operation. Conversely the converter will automatically transition from PWM operation to Burst
Main Control Loop
Mode operation as the load decreases. Between bursts During normal operation, the internal top power MOSFET the converter is not active (i.e., both switches are off) is turned on at the beginning of each cycle and turned off and most of the internal circuitry is disabled to reduce the when the PWM current comparator trips. The peak induc- quiescent current to 1.8µA. Burst Mode entry and exit is tor current at which the comparator trips is controlled by determined by the peak inductor current and therefore the the voltage on the output of the error amplifier. The FB load current at which Burst Mode operation will be entered pin allows the internally compensated error amplifier to or exited depends on the input voltage, the output voltage receive an output feedback voltage from an external resis- and the inductor value. Typical curves for Burst Mode tive divider from VOUT . When the load current increases, entry threshold are provided in the Typical Performance the output begins to fall causing a slight decrease in the Characteristics section of this data sheet. feedback voltage relative to the 0.6V reference, this in turn causes the control voltage to increase until the average
Soft-Start
inductor current matches the new load current. While the The converter has an internal closed-loop soft-start circuit top MOSFET is off, the bottom MOSFET is turned on until with a nominal duration of 1.4ms. The converter remains either the inductor current starts to reverse as indicated by in regulation during soft-start and will therefore respond the current reversal comparator, IZERO, or the beginning to output load transients that occur during this time. In of the next clock cycle. IZERO is set to 40mA (typical) in addition, the output voltage rise time has minimal depen- automatic Burst Mode operation and –110mA (typical) in dency on the size of the output capacitor or load current. forced continuous mode.
Thermal Shutdown Forced Continuous Mode
If the die temperature exceeds 150°C (typical) the converter Grounding MODE enables forced continuous operation will be disabled. All power devices will be turned off and and disables Burst Mode operation. At light loads, forced the switch node will be forced into a high impedance state. continuous mode minimizes output voltage ripple and The soft-start circuit is reset during thermal shutdown noise but is less efficient than Burst Mode operation. to provide a smooth recovery once the overtemperature Forced continuous operation may be desirable for use in condition is eliminated. If enabled, the converter wil restart applications that are sensitive to the Burst Mode output when the die temperature drops to approximately 130°C. 3103fa For more information www.linear.com/LTC3103 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts