Datasheet LTC3619B (Analog Devices) - 8

LTC3619B
operaTion
The LTC3619B uses a constant-frequency, current mode will begin skipping pulses to maintain output regulation. architecture. The operating frequency is set at 2.25MHz. This mode of operation exhibits low output ripple as well Both channels share the same clock and run in-phase. as low audio noise and reduced RF interference while The output voltage is set by an external resistor divider providing reasonable low current efficiency. returned to the VFB pins. An error amplifier compares the
Dropout Operation
divided output voltage with a reference voltage of 0.6V and regulates the peak inductor current accordingly. When the input supply voltage decreases toward the output voltage the duty cycle increases to 100%, which The LTC3619B continuously monitors the input current of both channels. When the sum of the currents of both is the dropout condition. In dropout, the PMOS switch is channels exceeds the programmed input current limit set turned on continuously with the output voltage being equal by an external resistor, R to the input voltage minus the voltage drops across the LIM, channel 2 is current limited while channel 1 remains regulated. internal P-channel MOSFET and the inductor. An important design consideration is that the RDS(ON)
Main Control Loop
of the P-channel switch increases with decreasing input During normal operation, the top power switch (P-channel supply voltage (see the Typical Performance Character- MOSFET) is turned on at the beginning of a clock cycle istics section). Therefore, the user should calculate the when the V worst-case power dissipation when the LTC3619B is used FB voltage is below the reference voltage. The current into the inductor and the load increases until the at 100% duty cycle with low input voltage (see Thermal peak inductor current (controlled by I Considerations in the Applications Information section). TH) is reached. The RS latch turns off the synchronous switch and energy stored in the inductor is discharged through the bottom
Soft-Start
switch (N-channel MOSFET) into the load until the next In order to minimize the inrush current on the input bypass clock cycle begins, or until the inductor current begins to capacitor, the LTC3619B slowly ramps up the output volt- reverse (sensed by the IRCMP comparator). age during start-up. Whenever the RUN1 or RUN2 pin is The peak inductor current is controlled by the internally pulled high, the corresponding output will ramp from zero compensated I to full-scale over a time period of approximately 950µs. This TH voltage, which is the output of the er- ror amplifier. This amplifier regulates the V prevents the LTC3619B from having to quickly charge the FB pin to the internal 0.6V reference by adjusting the peak inductor output capacitor and thus supplying an excessive amount current accordingly. of instantaneous current. When the input current limit is engaged, the peak inductor When the output is loaded heavily, for example, with milli- current will be lowered, which then reduces the switch- farad of capacitance, it may take longer than 950µs to ing duty cycle and V charge the output to regulation. If the output is still low OUT. This allows the input voltage to stay regulated when its programmed current output after the soft-start time, the LTC3619B will try to quickly capability is met. charge the output capacitor. In this case, the input current limit (after it engages) can prevent excessive amount of
Light Load Operation
instantaneous current that is required to quickly charge the output. See the Channel 2 Start-Up from Shutdown curve The LTC3619B will automatically transition from continu- in the Typical Performance Characteristics section. After ous operation to the pulse-skipping operation when the load input current limit is engaged, the output slowly ramps up current is low. The inductor current is not fixed during the to regulation while limited by its 500mA of input current. pulse-skipping mode which allows the LTC3619B to switch at constant-frequency down to very low currents, where it 3619bfb 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Package Description Revision History Typical Applications Related Parts