Datasheet LTC3621, LTC3621-2 (Analog Devices) - 8

LTC3621/LTC3621-2
operaTion
The LTC3621 uses a constant-frequency, peak current To optimize efficiency, Burst Mode operation can be selected mode architecture. It operates through a wide VIN range by tying the MODE/SYNC pin to INTVCC. In Burst Mode and regulates with ultralow quiescent current. The opera- operation, the peak inductor current is set to be at least tion frequency is set at either 2.25MHz or 1MHz and can 400mA, even if the output of the error amplifier demands be synchronized to an external oscillator ±40% of the less. Thus, when the switcher is on at relatively light output inherent frequency. To suit a variety of applications, the loads, FB voltage will rise and cause the ITH voltage to selectable MODE/SYNC pin allows the user to trade off drop. Once the ITH voltage goes below 0.2V, the switcher output ripple for efficiency. goes into its sleep mode with both power switches off. The output voltage is set by an external divider returned to The switcher remains in this sleep state until the external the FB pin. An error amplifier compares the divided output load pulls the output voltage below its regulation point. voltage with a reference voltage of 0.6V and adjusts the During sleep mode, the part draws an ultralow 3.5µA of peak inductor current accordingly. In the MS8E package, quiescent current from VIN. overvoltage and undervoltage comparators will pull the To minimize VOUT ripple, pulse-skipping mode can be se- PGOOD output low if the output voltage is not within 7.5% lected by grounding the MODE/SYNC pin. In the LTC3621, of the programmed value. The PGOOD output will go high pulse-skipping mode is implemented similarly to Burst immediately after achieving regulation and will go low 32 Mode operation with the peak inductor current set to be clock cycles after falling out of regulation. at about 66mA. This results in lower output voltage ripple than in Burst Mode operation with the trade-off being
Main Control Loop
slightly lower efficiency. During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle.
Forced Continuous Mode Operation
The inductor current is allowed to ramp up to a peak level. Aside from the two discontinuous-conduction modes, Once that level is reached, the top power switch is turned the LTC3621 also has the ability to operate in the forced off and the bottom switch (N-channel MOSFET) is turned continuous mode by setting the MODE/SYNC voltage on until the next clock cycle. The peak current level is con- between 1V and VINTVCC – 1V. In forced continuous mode, trolled by the internally compensated ITH voltage, which is the switcher will switch cycle by cycle regardless of what the output of the error amplifier. This amplifier compares the output load current is. If forced continuous mode is the FB voltage to the 0.6V internal reference. When the selected, the minimum peak current is set to be –133mA load current increases, the FB voltage decreases slightly in order to ensure that the part can operate continuously below the reference, which causes the error amplifier to at zero output load. increase the ITH voltage until the average inductor current matches the new load current.
High Duty Cycle/Dropout Operation
The main control loop is shut down by pulling the RUN When the input supply voltage decreases towards the output pin to ground. voltage, the duty cycle increases and slope compensation is required to maintain the fixed switching frequency. The
Low Current Operation
LTC3621 has internal circuitry to accurately maintain the Two discontinuous-conduction modes (DCMs) are available peak current limit (ILIM) of 1.6A even at high duty cycles. to control the operation of the LTC3621 at low currents. As the duty cycle approaches 100%, the LTC3621 enters Both modes, Burst Mode operation and pulse-skipping, dropout operation. During dropout, if force continuous automatically switch from continuous operation to the mode is selected, the top PMOS switch is turned on selected mode when the load current is low. continuously, and all active circuitry is kept alive. How- ever, if Burst Mode operation or pulse-skipping mode is 3621fc 8 For more information www.linear.com/LTC3621 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts