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LTC3542IS6 Datasheet(PDF) 7 Page - Linear Integrated Systems |
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LTC3542IS6 Datasheet(HTML) 7 Page - Linear Integrated Systems |
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7 / 16 page  LTC3542 7 3542fa The LTC3542 uses a constant frequency, current mode, step-down architecture. The operating frequency is set at 2.25MHz and can be synchronized to an external oscillator. To suit a variety of applications, the selectable MODE/SYNC pin allows the user to trade-off noise for efficiency. The output voltage is set by an external divider returned to the VFB pin. An error amplifier compares the divided output voltage with a reference voltage of 0.6V and adjusts the peak inductor current accordingly. 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 flows 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 output of the error amplifier. When the load current increases, the VFB voltage decreases slightly below the reference. This decrease causes the error amplifier to increase its output voltage until the average inductor cur- rent matches the new load current. The main control loop is shut down by pulling the RUN pin to ground. Low Load Current Operation By selecting MODE/SYNC pin, two modes are available to control the operation of the LTC3542 at low load 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 converter is in Burst Mode operation, the peak current of the inductor is set to approximately 60mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 26μA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage drops, the EA amplifier’s output rises above the sleep threshold and turns the top MOSFET on. This process repeats at a rate that is dependent on the load demand. By running cycles periodically, the switching losses which are dominated by the gate charge losses of the power MOSFETs are minimized. For lower ripple noise at low load currents, the pulse skip mode can be used. In this mode, the regulator continues to switch at a constant frequency down to very low load currents, where it will begin skipping pulses. 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 LTC3542 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information Section). Low Supply Operation To prevent unstable operation, the LTC3542 incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops below about 2V. Internal Soft-Start At start-up when the RUN pin is brought high, the internal reference is linearly ramped from 0V to 0.6V in about 1ms. The regulated feedback voltage follows this ramp resulting in the output voltage ramping from 0% to 100% in 1ms. The current in the inductor during soft-start is defined by the combination of the current needed to charge the output capacitance and the current provided to the load as the output voltage ramps up. The start-up waveform, shown in the Typical Performance Characteristics, shows the output voltage start-up from 0V to 1.8V with a 500mA load and VIN = 3.6V (refer to Figure 3a). OPERATION |
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