Datasheet LTC3411A (Analog Devices) - 9

LTC3411A
operaTion
The LTC3411A uses a constant frequency, current mode switch from continuous operation to the selected mode architecture. The operating frequency is determined by the when the load current is low. value of the RT resistor or can be synchronized to an external To optimize efficiency, the Burst Mode operation can be oscillator. To suit a variety of applications, the selectable selected. When the load is relatively light, the LTC3411A MODE pin allows the user to trade-off noise for efficiency. automatically switches into Burst Mode operation in which The output voltage is set by an external divider returned the PMOS switch operates intermittently based on load to the VFB pin. An error amplifier compares the divided demand. By running cycles periodically, the switching output voltage with the reference voltage of 0.8V and ad- losses which are dominated by the gate charge losses justs the peak inductor current accordingly. Overvoltage of the power MOSFETs are minimized. The main control and undervoltage comparators will pull the PGOOD output loop is interrupted when the output voltage reaches the low if the output voltage is not within ±7% of its regulated desired regulated value. The burst comparator trips when value. A tripping delay of 40µs and untripping delay of ITH is below approximately 0.5V, shutting off the switch 105µs ensures PGOOD will not glitch due to transient and reducing the power. The output capacitor and the spikes on VOUT. inductor supply the power to the load until ITH rises above approximately 0.5V, turning on the switch and the main
Main Control Loop
control loop which 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 LTC3411A Current flows through this switch into the inductor and continues to switch at a constant frequency down to the load, increasing until the peak inductor current reaches very low currents, where it will eventually begin skipping the limit set by the voltage on the ITH pin. Then the top pulses. switch is turned off, the bottom switch is turned on, and the energy stored in the inductor forces the current to flow Finally, in forced continuous mode, the inductor current through the bottom switch and the inductor, out into the is constantly cycled which creates a fixed output voltage load until the next clock cycle. ripple at all output current levels. This feature is desirable in telecommunications since the noise is at a constant fre- The peak inductor current is controlled by the voltage quency and is thus, easy to filter out. Another advantage of on the ITH pin, which is the output of the error amplifier. this mode is that the regulator is capable of both sourcing The output is developed by the error amplifier comparing current into a load and sinking current from the output. the feedback voltage, VFB, to the 0.8V reference voltage. When the load current increases, the output voltage and
Dropout Operation
VFB decrease slightly. This decrease in VFB causes the er- When the input supply voltage decreases toward the ror amplifier to increase the ITH voltage until the average output voltage, the duty cycle increases to 100% which inductor current matches the new load current. is the dropout condition. In dropout, the PMOS switch The main control loop is shut down by pulling the SHDN/RT is turned on continuously with the output voltage being pin to SVIN, resetting the internal soft-start. Re-enabling the equal to the input voltage minus the voltage drop across main control loop by releasing the SHDN/RT pin activates the the internal P-channel MOSFET and the inductor. internal soft-start, which slowly ramps the output voltage over approximately 0.8ms until it reaches regulation.
Low Supply Operation
The LTC3411A incorporates an undervoltage lockout circuit
Low Current Operation
which shuts down the part when the input voltage drops Three modes are available to control the operation of the below about 2.1V to prevent unstable operation. LTC3411A at low currents. All three modes automatically 3411afd For more information www.linear.com/LTC3411A 9 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 Revision History Typical Application Related Parts