Datasheet LTC3561A (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | 1A, 4MHz, Synchronous Step-Down DC/DC Converter |
| Pages / Page | 20 / 8 — OPERATION. Main Control Loop. Dropout Operation. Low Supply Operation |
| File Format / Size | PDF / 364 Kb |
| Document Language | English |
OPERATION. Main Control Loop. Dropout Operation. Low Supply Operation
Model Line for this Datasheet
Text Version of Document
LTC3561A
OPERATION
The LTC3561A uses a constant frequency, current mode VFB decrease slightly. This decrease in VFB causes the er- architecture. The operating frequency is determined by ror amplifier to increase the ITH voltage until the average the value of the RT resistor. inductor current matches the new load current. The output voltage is set by an external divider returned to The main control loop is shut down by pulling the SHDN/RT the VFB pin. An error amplifier compares the divided output pin to SVIN, resetting the internal soft-start. Re-enabling voltage with the reference voltage of 0.8V and adjusts the the main control loop by releasing the SHDN/RT pin peak inductor current accordingly. activates the internal soft-start, which slowly ramps the output voltage over approximately 0.8ms until it reaches
Main Control Loop
regulation. During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle.
Dropout Operation
Current flows through this switch into the inductor and When the input supply voltage decreases toward the the load, increasing until the peak inductor current reaches output voltage, the duty cycle increases to 100% which the limit set by the voltage on the ITH pin. Then the top is the dropout condition. In dropout, the PMOS switch switch is turned off, the bottom switch is turned on, and is turned on continuously with the output voltage being the energy stored in the inductor forces the current to flow equal to the input voltage minus the voltage drop across through the bottom switch, and the inductor, out into the the internal P-channel MOSFET and the inductor. load until the next clock cycle.
Low Supply Operation
The peak inductor current is controlled by the voltage on the ITH pin, which is the output of the error amplifier. The LTC3561A incorporates an undervoltage lockout circuit The output is developed by the error amplifier comparing which shuts down the part when the input voltage drops the feedback voltage, VFB, to the 0.8V reference voltage. below about 2.1V to prevent unstable operation. When the load current increases, the output voltage and 3561afa 8 For more information www.linear.com/LTC3561A
OPERATION
The LTC3561A uses a constant frequency, current mode VFB decrease slightly. This decrease in VFB causes the er- architecture. The operating frequency is determined by ror amplifier to increase the ITH voltage until the average the value of the RT resistor. inductor current matches the new load current. The output voltage is set by an external divider returned to The main control loop is shut down by pulling the SHDN/RT the VFB pin. An error amplifier compares the divided output pin to SVIN, resetting the internal soft-start. Re-enabling voltage with the reference voltage of 0.8V and adjusts the the main control loop by releasing the SHDN/RT pin peak inductor current accordingly. activates the internal soft-start, which slowly ramps the output voltage over approximately 0.8ms until it reaches
Main Control Loop
regulation. During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle.
Dropout Operation
Current flows through this switch into the inductor and When the input supply voltage decreases toward the the load, increasing until the peak inductor current reaches output voltage, the duty cycle increases to 100% which the limit set by the voltage on the ITH pin. Then the top is the dropout condition. In dropout, the PMOS switch switch is turned off, the bottom switch is turned on, and is turned on continuously with the output voltage being the energy stored in the inductor forces the current to flow equal to the input voltage minus the voltage drop across through the bottom switch, and the inductor, out into the the internal P-channel MOSFET and the inductor. load until the next clock cycle.
Low Supply Operation
The peak inductor current is controlled by the voltage on the ITH pin, which is the output of the error amplifier. The LTC3561A incorporates an undervoltage lockout circuit The output is developed by the error amplifier comparing which shuts down the part when the input voltage drops the feedback voltage, VFB, to the 0.8V reference voltage. below about 2.1V to prevent unstable operation. When the load current increases, the output voltage and 3561afa 8 For more information www.linear.com/LTC3561A
