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LTC3565IMSETRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3565IMSETRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 22 page  LTC3565 0 3565fa operation The LTC3565 uses a constant frequency, current mode architecture. The operating frequency is determined by the value of the RT resistor or can be synchronized to an external oscillator. To suit a variety of applications, the selectable MODE pin allows the user to trade-off noise for efficiency. The output voltage is set by an external divider returned to the VFB pin. An error amplifier compares the divided output voltage with the reference voltage of 0.6V and ad- justs the peak inductor current accordingly. Overvoltage and undervoltage comparators will pull the PGOOD output low if the output voltage is not within ±7% of its regulated value. A tripping delay of 40µs and untripping delay of 105µs ensures PGOOD will not glitch due to transient spikes on VOUT. Main Control Loop During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle. Current flows through this switch into the inductor and theload,increasinguntilthepeakinductorcurrentreaches the limit set by the voltage on the ITH pin. Then, the top switch is turned off, the bottom switch is turned on, and the energy stored in the inductor forces the current to flow through the bottom switch and the inductor out into the load until the next clock cycle. The peak inductor current is controlled by the voltage on the ITH pin, which is the output of the error amplifier. The output is developed by the error amplifier comparing the feedback voltage, VFB, to the 0.6V reference voltage. When the load current increases, the output voltage and VFB decrease slightly. This decrease in VFB causes the er- ror amplifier to increase the ITH voltage until the average inductor current matches the new load current. The main control loop is shut down by grounding the RUN pin, resetting the internal soft-start. Re-enabling the main control loop by pulling RUN high activates the internal soft-start, which slowly ramps the output voltage over approximately 0.9ms until it reaches regulation. Low Current Operation Three modes are available to control the operation of the LTC3565 at low currents. All three modes automatically switch from continuous operation to the selected mode when the load current is low. To optimize efficiency, the Burst Mode operation can be selected. When the load is relatively light, the LTC3565 automaticallyswitchesintoBurstModeoperationinwhich the PMOS switch operates intermittently based on load demand. By running cycles periodically, the switching losses which are dominated by the gate charge losses of the power MOSFETs are minimized. The main control loop is interrupted when the output voltage reaches the desired regulated value. The burst comparator trips when ITH is below approximately 0.5V, shutting off the switch and reducing the power. The output capacitor and the in- ductor supply the power to the load until ITH rises above approximately 0.5V, turning on the switch and the main control loop which starts another cycle. For lower output voltage ripple at low currents, pulse skipping mode can be used. In this mode, the LTC3565 continues to switch at a constant frequency down to very low currents, where it will eventually begin skipping pulses. Finally, in forced continuous mode, the inductor current is constantly cycled which creates a fixed output voltage ripple at all output current levels. This feature is desirable in telecommunications since the noise is at a constant fre- quency and is thus easy to filter out. Another advantage of this mode is that the regulator is capable of both sourcing current into a load and sinking current from the output. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases to 100% which is the dropout condition. In dropout, the PMOS switch is turnedoncontinuouslywiththeoutputvoltagebeingequal to the input voltage minus the voltage drops across the internal P-channel MOSFET and the inductor. Low Supply Operation The LTC3565 incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops below about 1.9V to prevent unstable operation. |
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