Datasheet LTC3542 (Analog Devices) - 7
| Fabricante | Analog Devices |
| Descripción | 500mA, 2.25MHz Synchronous Step-Down DC/DC Converter |
| Páginas / Página | 16 / 7 — OPERATION. Main Control Loop. Dropout Operation. Low Supply Operation. … |
| Formato / tamaño de archivo | PDF / 222 Kb |
| Idioma del documento | Inglés |
OPERATION. Main Control Loop. Dropout Operation. Low Supply Operation. Low Load Current Operation. Internal Soft-Start
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LTC3542
OPERATION
The LTC3542 uses a constant frequency, current mode, the sleep threshold and turns the top MOSFET on. This step-down architecture. The operating frequency is set at process repeats at a rate that is dependent on the load 2.25MHz and can be synchronized to an external oscillator. demand. By running cycles periodically, the switching To suit a variety of applications, the selectable MODE/SYNC losses which are dominated by the gate charge losses of pin allows the user to trade-off noise for effi ciency. the power MOSFETs are minimized. The output voltage is set by an external divider returned For lower ripple noise at low load currents, the pulse skip to the VFB pin. An error amplifi er compares the divided mode can be used. In this mode, the regulator continues output voltage with a reference voltage of 0.6V and adjusts to switch at a constant frequency down to very low load the peak inductor current accordingly. currents, where it will begin skipping pulses.
Main Control Loop Dropout Operation
During normal operation, the top power switch (P-channel When the input supply voltage decreases toward the output MOSFET) is turned on at the beginning of a clock cycle voltage, the duty cycle increases to 100%, which is the when the VFB voltage is below the reference voltage. The dropout condition. In dropout, the PMOS switch is turned current fl ows into the inductor and the load increases until on continuously with the output voltage being equal to the the current limit is reached. The switch turns off and energy input voltage minus the voltage drops across the internal stored in the inductor fl ows through the bottom switch P-channel MOSFET and the inductor. An important design (N-channel MOSFET) into the load until the next clock cycle. consideration is that the RDS(ON) of the P-channel switch The peak inductor current is controlled by the internally increases with decreasing input supply voltage (See Typical compensated output of the error amplifi er. When the load Performance Characteristics). Therefore, the user should current increases, the VFB voltage decreases slightly below calculate the power dissipation when the LTC3542 is used the reference. This decrease causes the error amplifi er to at 100% duty cycle with low input voltage (See Thermal increase its output voltage until the average inductor cur- Considerations in the Applications Information Section). rent matches the new load current. The main control loop is shut down by pulling the RUN pin to ground.
Low Supply Operation
To prevent unstable operation, the LTC3542 incorporates
Low Load Current Operation
an undervoltage lockout circuit which shuts down the part By selecting MODE/SYNC pin, two modes are available to when the input voltage drops below about 2V. control the operation of the LTC3542 at low load currents. Both modes automatically switch from continuous opera-
Internal Soft-Start
tion to the selected mode when the load current is low. At start-up when the RUN pin is brought high, the internal To optimize effi ciency, the Burst Mode operation can be reference is linearly ramped from 0V to 0.6V in about 1ms. selected. When the converter is in Burst Mode operation, The regulated feedback voltage follows this ramp resulting the peak current of the inductor is set to approximately in the output voltage ramping from 0% to 100% in 1ms. 60mA regardless of the output load. Each burst event can The current in the inductor during soft-start is defi ned last from a few cycles at light loads to almost continuously by the combination of the current needed to charge the cycling with short sleep intervals at moderate loads. In output capacitance and the current provided to the load between these burst events, the power MOSFETs and any as the output voltage ramps up. The start-up waveform, unneeded circuitry are turned off, reducing the quiescent shown in the Typical Performance Characteristics, shows current to 26μA. In this sleep state, the load current is the output voltage start-up from 0V to 1.8V with a 500mA being supplied solely from the output capacitor. As the load and VIN = 3.6V (refer to Figure 3a). output voltage drops, the EA amplifi er’s output rises above 3542fa 7
OPERATION
The LTC3542 uses a constant frequency, current mode, the sleep threshold and turns the top MOSFET on. This step-down architecture. The operating frequency is set at process repeats at a rate that is dependent on the load 2.25MHz and can be synchronized to an external oscillator. demand. By running cycles periodically, the switching To suit a variety of applications, the selectable MODE/SYNC losses which are dominated by the gate charge losses of pin allows the user to trade-off noise for effi ciency. the power MOSFETs are minimized. The output voltage is set by an external divider returned For lower ripple noise at low load currents, the pulse skip to the VFB pin. An error amplifi er compares the divided mode can be used. In this mode, the regulator continues output voltage with a reference voltage of 0.6V and adjusts to switch at a constant frequency down to very low load the peak inductor current accordingly. currents, where it will begin skipping pulses.
Main Control Loop Dropout Operation
During normal operation, the top power switch (P-channel When the input supply voltage decreases toward the output MOSFET) is turned on at the beginning of a clock cycle voltage, the duty cycle increases to 100%, which is the when the VFB voltage is below the reference voltage. The dropout condition. In dropout, the PMOS switch is turned current fl ows into the inductor and the load increases until on continuously with the output voltage being equal to the the current limit is reached. The switch turns off and energy input voltage minus the voltage drops across the internal stored in the inductor fl ows through the bottom switch P-channel MOSFET and the inductor. An important design (N-channel MOSFET) into the load until the next clock cycle. consideration is that the RDS(ON) of the P-channel switch The peak inductor current is controlled by the internally increases with decreasing input supply voltage (See Typical compensated output of the error amplifi er. When the load Performance Characteristics). Therefore, the user should current increases, the VFB voltage decreases slightly below calculate the power dissipation when the LTC3542 is used the reference. This decrease causes the error amplifi er to at 100% duty cycle with low input voltage (See Thermal increase its output voltage until the average inductor cur- Considerations in the Applications Information Section). rent matches the new load current. The main control loop is shut down by pulling the RUN pin to ground.
Low Supply Operation
To prevent unstable operation, the LTC3542 incorporates
Low Load Current Operation
an undervoltage lockout circuit which shuts down the part By selecting MODE/SYNC pin, two modes are available to when the input voltage drops below about 2V. control the operation of the LTC3542 at low load currents. Both modes automatically switch from continuous opera-
Internal Soft-Start
tion to the selected mode when the load current is low. At start-up when the RUN pin is brought high, the internal To optimize effi ciency, the Burst Mode operation can be reference is linearly ramped from 0V to 0.6V in about 1ms. selected. When the converter is in Burst Mode operation, The regulated feedback voltage follows this ramp resulting the peak current of the inductor is set to approximately in the output voltage ramping from 0% to 100% in 1ms. 60mA regardless of the output load. Each burst event can The current in the inductor during soft-start is defi ned last from a few cycles at light loads to almost continuously by the combination of the current needed to charge the cycling with short sleep intervals at moderate loads. In output capacitance and the current provided to the load between these burst events, the power MOSFETs and any as the output voltage ramps up. The start-up waveform, unneeded circuitry are turned off, reducing the quiescent shown in the Typical Performance Characteristics, shows current to 26μA. In this sleep state, the load current is the output voltage start-up from 0V to 1.8V with a 500mA being supplied solely from the output capacitor. As the load and VIN = 3.6V (refer to Figure 3a). output voltage drops, the EA amplifi er’s output rises above 3542fa 7