Datasheet LTC3619 (Analog Devices) - 9

LTC3619
operaTion
The LTC3619 uses a constant-frequency, current mode
Light Load Operation
architecture. The operating frequency is set at 2.25MHz. The LTC3619 wil automatical y transition from continuous Both channels share the same clock and run in-phase. operation to Burst Mode operation when the load current is The output voltage is set by an external resistor divider low. During relatively light loads, the peak inductor current returned to the VFB pins. An error amplifier compares the (as set by ITH) remains fixed at approximately 60mA and divided output voltage with a reference voltage of 0.6V and 120mA for channel 1 and channel 2, respectively. The PMOS regulates the peak inductor current accordingly. switch operates intermittently based on load demand. By The LTC3619 continuously monitors the input current running cycles periodically, the switching losses are mini- of both channels. When the sum of the currents of both mized. The duration of each burst event can range from a channels exceeds the programmed input current limit set few cycles at light load to almost continuous cycling with by an external resistor, R short sleep intervals at moderate loads. During the sleep LIM, channel 2 is current limited while channel 1 remains regulated. intervals, the load current is being supplied solely from the output capacitor. A majority of the internal circuitry
Main Control Loop
is shut off to conserve quiescent current. As the output voltage droops, the error amplifier output rises above the During normal operation, the top power switch (P-channel sleep threshold, signaling the burst comparator to un-trip MOSFET) is turned on at the beginning of a clock cycle and turns the top MOSFET on. This cycle repeats at a rate when the VFB voltage is below the reference voltage. The that is dependent on load demand. current into the inductor and the load increases until the peak inductor current (controlled by ITH) is reached. The
Dropout Operation
RS latch turns off the synchronous switch and energy stored in the inductor is discharged through the bottom When the input supply voltage decreases toward the switch (N-channel MOSFET) into the load until the next output voltage the duty cycle increases to 100%, which clock cycle begins, or until the inductor current begins to is the dropout condition. In dropout, the PMOS switch is reverse (sensed by the I turned on continuously with the output voltage being equal RCMP comparator). to the input voltage minus the voltage drops across the The peak inductor current is controlled by the internally internal P-channel MOSFET and the inductor. compensated ITH voltage, which is the output of the er- ror amplifier. This amplifier regulates the V An important design consideration is that the RDS(ON) FB pin to the internal 0.6V reference by adjusting the peak inductor of the P-channel switch increases with decreasing input current accordingly. supply voltage (see Typical Performance Characteristics). Therefore, the user should calculate the worst-case power When the input current limit is engaged, the peak inductor dissipation when the LTC3619 is used at 100% duty cycle current will be lowered, which then reduces the switch- with low input voltage (see Thermal Considerations in the ing duty cycle and VOUT. This allows the input voltage Applications Information section). to stay regulated when its programmed current output capability is met.
Soft-Start
In order to minimize the inrush current on the input bypass capacitor, the LTC3619 slowly ramps up the output volt- age during start-up. Whenever the RUN1 or RUN2 pin is pulled high, the corresponding output will ramp from zero to full-scale over a time period of approximately 950µs. This prevents the LTC3619 from having to quickly charge the output capacitor and thus supplying an excessive amount of instantaneous current. 3619fa 9 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Functional Diagram Operation Applications Information Package Description Typical Application Related Parts