LTC3104 OPERATION inductor current where the comparator trips is controlled value. Typical curves for Burst Mode entry threshold are by the voltage on the output of the error amplifier. The FB provided in the Typical Performance Characteristics sec- pin allows the internally compensated error amplifier to tion of this data sheet. receive an output feedback voltage from an external resis- tive divider from VOUT . When the load current increases, Soft-Start the output begins to fall causing a slight decrease in the The converter has an internal closed-loop soft-start circuit feedback voltage relative to the 0.6V reference, this in turn, with a nominal duration of 1.4ms. The converter remains causes the control voltage to increase until the average in regulation during soft-start and will therefore respond inductor current matches the new load current. While the to output load transients that occur during this time. In top MOSFET is off, the bottom MOSFET is turned on until addition, the output voltage rise time has minimal depen- either the inductor current starts to reverse as indicated by dency on the size of the output capacitor or load current. the current reversal comparator, IZERO, or the beginning of the next clock cycle. IZERO is set to 40mA (typical) in Thermal Shutdown automatic Burst Mode operation and –110mA (typical) in forced continuous mode. If the die temperature exceeds 150°C (typical) the con- verter and LDO will be disabled. All power devices will be Forced Continuous Mode turned off and the switch node will be forced into a high impedance state. The soft-start circuit is reset during Grounding MODE enables forced continuous operation thermal shutdown to provide a smooth recovery once the and disables Burst Mode operation. At light loads, forced overtemperature condition is eliminated. If enabled, the continuous mode minimizes output voltage ripple and converter and the LDO will restart when the die temperature noise but is less efficient than Burst Mode operation. drops to approximately 130°C. Forced continuous operation may be desirable for use in applications that are sensitive to the Burst Mode output Power Good Status Output voltage ripple or its harmonics. The LTC3104 offers a broad range of possible step down ratios without pulse skipping The PGOOD pin is an open-drain output which indicates but for very small step-down ratios, the minimum on-time the output voltage status of the step-down converter. If the of the main switch will be reached and the converter will output voltage falls 10% below the regulation voltage, the begin turning off for multiple cycles in order to maintain PGOOD open-drain output will pull low. A built-in deglitch- regulation. ing delay prevents false trips due to voltage transients on load steps. The output voltage must rise 2% above the Burst Mode Operation falling threshold before the pull-down will turn off. The PGOOD output will also pull low during overtemperature Holding the MODE pin above 1.2V will enable automatic shutdown and undervoltage lockout to indicate these fault Burst Mode operation and disable forced continuous op- conditions. The PGOOD output is valid 1ms after the buck eration. As the load current increases, the converter will converter is enabled. When the converter is disabled the automatically transition between Burst Mode and PWM open-drain device is forced on into a low impedance state. operation. Conversely the converter will automatically The PGOOD pull-up voltage must be below the 6V absolute transition from PWM operation to Burst Mode operation maximum voltage rating of the pin. as the load decreases. Between bursts the converter is not active (i.e., both switches are off) and most of the internal Current Limit circuitry is disabled to reduce the quiescent current to 2.6µA. Burst Mode entry and exit is determined by the peak The peak inductor current limit comparator shuts off the inductor current and therefore the load current at which buck switch once the internal limit threshold is reached. Burst Mode operation will be entered or exited depends Peak switch current is no less than 400mA. on the input voltage, the output voltage and the inductor 3104fa 10 For more information www.linear.com/LTC3104 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts
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