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LTC3548-2 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3548-2 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC3548-2 7 35482fb OPERATION The LTC3548-2 uses a constant-frequency, current mode architecture. The operating frequency is set at 2.25MHz and can be synchronized to an external oscillator. Both channels share the same clock and run in-phase. To suit a variety of applications, the selectable MODE pin allows the user to choose between low noise and high efficiency. The output voltage for channel 1 is set by an internal divider returned to the VOUT1 pin. The output voltage for channel 2 is set by an external divider returned to the VFB2 pin. An error amplifier compares the feedback output voltage with a reference voltage of 0.6V and adjusts the peak inductor current accordingly. An undervoltage comparator will pull the POR output low if the output voltage is not above –8.5% of the reference voltage. The POR output will go high after 262,144 clock cycles (about 117ms in pulse- skipping mode) of achieving regulation. 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 (see the Block Diagram) is below the the reference voltage. The current into the inductor and the load increases until the current limit is reached. The switch turns off and energy stored in the inductor flows through the bottom switch (N-channel MOSFET) into the load until the next clock cycle. The peak inductor current is controlled by the internally compensated ITH voltage, which is the output of the er- ror amplifier. This amplifier compares the VFB to the 0.6V reference. When the load current increases, the VFB volt- age decreases slightly below the reference. This decrease causes the error amplifier to increase the ITH voltage until the average inductor current matches the new load cur- rent. The main control loop is shut down by pulling the RUN pin to ground. Low Current Operation By selecting MODE/SYNC (Pin 6), two modes are available to control the operation of the LTC3548-2 at low currents. Both modes automatically switch from continuous opera- tion to the selected mode when the load current is low. To optimize efficiency, the Burst Mode operation can be selected. When the load is relatively light, the LTC3548-2 automatically switches into Burst Mode operation in which the PMOS switch operates intermittently based on load demand with a fixed peak inductor current. By running cycles periodically, the switching losses which are domi- nated by the gate charge losses of the power MOSFETs are minimized. The main control loop is interrupted when the output voltage reaches the desired regulated value. A voltage comparator trips when ITH is below 0.35V, shutting off the switch and reducing the power. The output capaci- tor and the inductor supply the power to the load until ITH exceeds 0.65V, turning on the switch and the main control loop which starts another cycle. For lower ripple noise at low currents, the pulse-skipping mode can be used. In this mode, the LTC3548-2 continues to switch at a constant frequency down to very low cur- rents, where it will begin skipping pulses. The efficiency in pulse-skipping mode can be improved slightly by connect- ing the SW node to the MODE/SYNC input which reduces the clock frequency by approximately 30%. 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 Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3548-2 is used at 100% duty cycle with low input voltage (see Thermal Considerations in the Applica- tions Information section). Low Supply Operation To prevent unstable operation, the LTC3548-2 incorporates an undervoltage lockout circuit that shuts down the part when the input voltage drops below about 1.65V. |
Similar Part No. - LTC3548-2_15 |
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Similar Description - LTC3548-2_15 |
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