Datasheet LTC3606B (Analog Devices) - 7

LTC3606B
OPERATION
The LTC3606B uses a constant-frequency, current mode at constant-frequency down to very low currents, where it architecture. The operating frequency is set at 2.25MHz. will begin skipping pulses to maintain output regulation. This mode of operation exhibits low output ripple as well The output voltage is set by an external resistor divider as low audio noise and reduced RF interference while returned to the VFB pins. An error amplifi er compares the providing reasonable low current effi ciency. divided output voltage with a reference voltage of 0.6V and regulates the peak inductor current accordingly.
Dropout Operation
The LTC3606B continuously monitors the input current When the input supply voltage decreases toward the via the voltage drop across the RDS(ON) of the internal output voltage the duty cycle increases to 100%, which P-channel MOSFET. When the input current exceeds the is the dropout condition. In dropout, the PMOS switch is programmed input current limit set by an external resistor, turned on continuously with the output voltage being equal RLIM, the regulator’s input current is limited. The regulator to the input voltage minus the voltage drops across the output voltage will drop to meet output current demand internal P-channel MOSFET and the inductor. and to maintain constant input current. 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 Characteristics MOSFET) is turned on at the beginning of a clock cycle section). Therefore, the user should calculate the worst- when the V case power dissipation when the LTC3606B is used at FB voltage is below the reference voltage. The current into the inductor and the load increases until the 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
Soft-Start
stored in the inductor is discharged through the bottom switch (N-channel MOSFET) into the load until the next In order to minimize the inrush current on the input by- clock cycle begins, or until the inductor current begins to pass capacitor, the LTC3606B slowly ramps up the output reverse (sensed by the IRCMP comparator). voltage during start-up. Whenever the RUN pin is pulled high, the corresponding output will ramp from zero to The peak inductor current is controlled by the internally full-scale over a time period of approximately 950μs. This compensated ITH voltage, which is the output of the error prevents the LTC3606B from having to quickly charge the amplifi er. This amplifi er regulates the VFB pin to the internal output capacitor and thus supplying an excessive amount 0.6V reference by adjusting the peak inductor current of instantaneous current. accordingly. When the output is loaded heavily, for example, with mil- When the input current limit is engaged, the peak inductor lifarad of capacitance, it may take longer than 950μs to current will be lowered, which then reduces the switching charge the output to regulation. If the output is still low duty cycle and VOUT. This allows the input voltage to stay after the soft-start time, the LTC3606B will try to quickly regulated when its programmed current output capability charge the output capacitor. In this case, the input current is met. limit (after it engages) can prevent excessive amount of
Light Load Operation
instantaneous current that is required to quickly charge the output. See the Start-Up from Shutdown curve The LTC3606B will automatically transition from continuous (CL = 4.4mF)in the Typical Performance Characteristics operation to the pulse-skipping operation when the load section. After input current limit is engaged, the output current is low. The inductor current is not fi xed during the slowly ramps up to regulation while limited by its 500mA pulse-skipping mode which allows the LTC3606B to switch of input current. 3606bfb 7