Datasheet LTC3415 (Analog Devices) - 8

LTC3415
OPERATION Main Control Loop
drops when the inductor’s average current is greater than the load requirement. As the I The LTC3415 is a constant frequency, current mode, TH voltage drops below 0.2V, the BURST comparator trips, causing the internal sleep monolithic step down regulator. In normal operation, line to go high and turn off both power MOSFETs. the internal top P-channel power MOSFET turns on each cycle when the oscillator sets the RS latch, and turns In sleep mode, the internal circuitry is partially turned off, off when the current comparator ICOMP resets the RS reducing the quiescent current to about 450μA. The load latch. The peak inductor current at which ICOMP resets current is now being supplied from the output capacitor. the RS latch is controlled by the voltage on the ITH pin, When the output voltage drops, causing ITH to rise above which is the output of error amplifi er EA. The FB pin 0.25V, the internal sleep line goes low, and the LTC3415 allows EA to receive an output feedback voltage from resumes normal operation. The next oscillator cycle will an external resistive divider. When the load current in- turn on the top power MOSFET and the switching cycle creases, it causes a slight decrease in the feedback volt- repeats. age relative to the 0.596V reference, which in turn causes I
Pulse-Skipping Mode Operation
TH voltage to increase until the average inductor current matches the new load current. While the top P-channel In applications where fi xed frequency operation, low power MOSFET is off, the bottom N-Channel power MOSFET output ripple and high effi ciency at intermediate current is turned on until either the inductor current starts to re- is desired, pulse-skipping mode should be used. Pulse- verse, as indicated by the current reversal comparator skipping operation allows the LTC3415 to skip cycles at IRCMP, or the beginning of the next cycle. low output loads, thus increasing effi ciency by reducing The main control loop is shut down by pulling the RUN pin switching current. Tying the MODE pin to VIN/2 enables pulse-skipping operation. This allows discontinuous below 1.5V (VTRACK = SVIN or VTRACK < 0.18V). Tying RUN conduction mode (DCM) operation down to near the limit higher than 1.5V allows operation to begin. To control the defi ned by the chip’s minimum on-time (about 100ns). rise time of the output, a voltage ramp can be applied to Below this output current level, the converter will begin the TRACK pin. The FB voltage will servo to the TRACK to skip cycles in order to maintain output regulation. voltage until TRACK goes above 0.57V, which is when Increasing the output load current slightly, above the PGOOD is high and the output is in normal regulation. minimum required for discontinuous conduction mode, If TRACK is not used (tied high), then an internal 100μs allows constant frequency PWM. soft-start will ramp up the output.
Forced Continuous Operation Burst Mode Operation
In applications where fi xed frequency operation is more The LTC3415 is capable of Burst Mode operation in which critical than low current effi ciency, and where the lowest the power MOSFETs operate intermittently based on load output ripple is desired, forced continuous operation should demand, thus saving quiescent current. For applications be used. Forced continuous operation can be enabled by where maximizing the effi ciency at very light loads is a tying the MODE pin to GND. In this mode, inductor cur- high priority, Burst Mode operation should be applied. To rent is allowed to reverse during low output loads, the ITH enable Burst Mode operation, simply tie the MODE pin to voltage is in control of the current comparator threshold VIN. During this operation, the peak current of the inductor throughout, and the top MOSFET always turns on with each is set to approximately 20% of the maximum peak current oscillator pulse. During start-up, forced continuous mode is value in normal operation even though the voltage at the disabled and inductor current is prevented from reversing ITH pin indicates a lower value. The voltage at the ITH pin until the LTC3415’s output voltage is in regulation. 3415fa 8