Datasheet LTC3568 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | 1.8A, 4MHz, Synchronous Step-Down DC/DC Converter |
| Pages / Page | 18 / 7 — Main Control Loop. Dropout Operation. Low Supply Operation. … |
| File Format / Size | PDF / 313 Kb |
| Document Language | English |
Main Control Loop. Dropout Operation. Low Supply Operation. Low Current Operation
Model Line for this Datasheet
Text Version of Document
LTC3568 operaTion The LTC3568 uses a constant frequency, current mode To optimize efficiency, the Burst Mode operation can be architecture. The operating frequency is determined by selected. When the load is relatively light, the LTC3568 the value of the RT resistor or can be synchronized to an automatically switches into Burst Mode operation in which external oscillator. To suit a variety of applications, the the PMOS switch operates intermittently based on load selectable Mode pin, allows the user to trade-off noise demand. By running cycles periodically, the switching for efficiency. losses which are dominated by the gate charge losses The output voltage is set by an external divider returned of the power MOSFETs are minimized. The main control to the V loop is interrupted when the output voltage reaches the FB pin. An error amplfier compares the divided output voltage with a reference voltage of 0.8V and adjusts desired regulated value. The hysteretic voltage comparator the peak inductor current accordingly. Overvoltage and B trips when ITH is below 0.24V, shutting off the switch and undervoltage comparators will pull the PGOOD output reducing the power. The output capacitor and the inductor low if the output voltage is not within ±7.5%. supply the power to the load until ITH/RUN exceeds 0.31V, turning on the switch and the main control loop which
Main Control Loop
starts another cycle. During normal operation, the top power switch (P-channel For lower output voltage ripple at low currents, pulse MOSFET) is turned on at the beginning of a clock cycle skipping mode can be used. In this mode, the LTC3568 when the V continues to switch at a constant frequency down to FB voltage is below the the reference voltage. The current into the inductor and the load increases until very low currents, where it will eventually begin skipping the current limit is reached. The switch turns off and pulses. energy stored in the inductor flows through the bottom Finally, in forced continuous mode, the inductor current switch (N-channel MOSFET) into the load until the next is constantly cycled which creates a fixed output voltage clock cycle. ripple at all output current levels. This feature is desirable The peak inductor current is controlled by the voltage on in telecommunications since the noise is at a constant the I frequency and is thus easy to filter out. Another advan- TH pin, which is the output of the error amplifier.This amplifier compares the V tage of this mode is that the regulator is capable of both FB pin to the 0.8V reference. When the load current increases, the V sourcing current into a load and sinking some current FB voltage decreases slightly below the reference. This decrease causes the er- from the output. ror amplifier to increase the ITH voltage until the average inductor current matches the new load current.
Dropout Operation
The main control loop is shut down by pul ing the SHDN/R When the input supply voltage decreases toward the T pin to SV output voltage, the duty cycle increases to 100% which IN. A digital soft-start is enabled after shutdown, which will slowly ramp the peak inductor current up over is the dropout condition. In dropout, the PMOS switch is 1024 clock cycles or until the output reaches regulation, turned on continuously with the output voltage being equal whichever is first. Soft-start can be lengthened by ramping to the input voltage minus the voltage drops across the the voltage on the I internal P-channel MOSFET and the inductor. TH pin (see Applications Information section).
Low Supply Operation Low Current Operation
The LTC3568 incorporates an undervoltage lockout circuit Three modes are available to control the operation of the which shuts down the part when the input voltage drops LTC3568 at low currents. All three modes automatically below about 2V. switch from continuous operation to to the selected mode when the load current is low. 3568fa 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 Typical Applications Package Description Revision History Typical Application Related Parts
Main Control Loop
starts another cycle. During normal operation, the top power switch (P-channel For lower output voltage ripple at low currents, pulse MOSFET) is turned on at the beginning of a clock cycle skipping mode can be used. In this mode, the LTC3568 when the V continues to switch at a constant frequency down to FB voltage is below the the reference voltage. The current into the inductor and the load increases until very low currents, where it will eventually begin skipping the current limit is reached. The switch turns off and pulses. energy stored in the inductor flows through the bottom Finally, in forced continuous mode, the inductor current switch (N-channel MOSFET) into the load until the next is constantly cycled which creates a fixed output voltage clock cycle. ripple at all output current levels. This feature is desirable The peak inductor current is controlled by the voltage on in telecommunications since the noise is at a constant the I frequency and is thus easy to filter out. Another advan- TH pin, which is the output of the error amplifier.This amplifier compares the V tage of this mode is that the regulator is capable of both FB pin to the 0.8V reference. When the load current increases, the V sourcing current into a load and sinking some current FB voltage decreases slightly below the reference. This decrease causes the er- from the output. ror amplifier to increase the ITH voltage until the average inductor current matches the new load current.
Dropout Operation
The main control loop is shut down by pul ing the SHDN/R When the input supply voltage decreases toward the T pin to SV output voltage, the duty cycle increases to 100% which IN. A digital soft-start is enabled after shutdown, which will slowly ramp the peak inductor current up over is the dropout condition. In dropout, the PMOS switch is 1024 clock cycles or until the output reaches regulation, turned on continuously with the output voltage being equal whichever is first. Soft-start can be lengthened by ramping to the input voltage minus the voltage drops across the the voltage on the I internal P-channel MOSFET and the inductor. TH pin (see Applications Information section).
Low Supply Operation Low Current Operation
The LTC3568 incorporates an undervoltage lockout circuit Three modes are available to control the operation of the which shuts down the part when the input voltage drops LTC3568 at low currents. All three modes automatically below about 2V. switch from continuous operation to to the selected mode when the load current is low. 3568fa 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 Typical Applications Package Description Revision History Typical Application Related Parts
