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LTC3606BEDD Datasheet(PDF) 7 Page - Linear Technology |
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LTC3606BEDD Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LTC3606B 7 3606bfa OPERATION The LTC3606B uses a constant-frequency, current mode architecture. The operating frequency is set at 2.25MHz. The output voltage is set by an external resistor divider returned to the VFB pins. An error amplifier compares the divided output voltage with a reference voltage of 0.6V and regulates the peak inductor current accordingly. The LTC3606B continuously monitors the input current via the voltage drop across the RDS(ON) of the internal P-channel MOSFET. When the input current exceeds the programmed input current limit set by an external resistor, RLIM, the regulator’s input current is limited. The regulator output voltage will drop to meet output current demand and to maintain constant input current. Main Control Loop During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle when the VFB voltage is below the reference voltage. The current into the inductor and the load increases until the peak inductor current (controlled by ITH) is reached. The RS latch turns off the synchronous switch and energy stored in the inductor is discharged through the bottom switch (N-channel MOSFET) into the load until the next clock cycle begins, or until the inductor current begins to reverse (sensed by the IRCMP comparator). The peak inductor current is controlled by the internally compensated ITH voltage, which is the output of the error amplifier. This amplifier regulates the VFB pin to the internal 0.6V reference by adjusting the peak inductor current accordingly. When the input current limit is engaged, the peak inductor current will be lowered, which then reduces the switching duty cycle and VOUT. This allows the input voltage to stay regulated when its programmed current output capability is met. Light Load Operation The LTC3606B will automatically transition from continuous operation to the pulse-skipping operation when the load current is low. The inductor current is not fixed during the pulse-skipping mode which allows the LTC3606B to switch at constant-frequency down to very low currents, where it will begin skipping pulses to maintain output regulation. This mode of operation exhibits low output ripple as well as low audio noise and reduced RF interference while providing reasonable low current efficiency. 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 turned on continuously with the output voltage being equal to the input voltage minus the voltage drops across the internal P-channel MOSFET and the inductor. An important design consideration is that the RDS(ON) of the P-channel switch increases with decreasing input supply voltage (see the Typical Performance Characteristics section). Therefore, the user should calculate the worst- case power dissipation when the LTC3606B is used at 100% duty cycle with low input voltage (see Thermal Considerations in the Applications Information section). Soft-Start In order to minimize the inrush current on the input bypass capacitor, the LTC3606B slowly ramps up the output voltage during start-up. Whenever the RUN pin is pulled high, the corresponding output will ramp from zero to full-scale over a time period of approximately 750μs. This prevents the LTC3606B from having to quickly charge the output capacitor and thus supplying an excessive amount of instantaneous current. When the output is loaded heavily, for example, with millifarad of capacitance, it may take longer than 750μs to charge the output to regulation. If the output is still low after the soft-start time, the LTC3606B will try to quickly charge the output capacitor. In this case, the input current limit (after it engages) can prevent excessive amount of instantaneous current that is required to quickly charge the output. See the Start-Up from Shutdown curve (CL = 4.4mF)in the Typical Performance Characteristics section. After input current limit is engaged, the output slowly ramps up to regulation while limited by its 500mA of input current. |
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