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LTC3528-2 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3528-2 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3528-2 10 35282f Zero Current Comparator The zero current comparator monitors the inductor cur- rent to the output and shuts off the synchronous rectifier when this current reduces to approximately 20mA. This prevents the inductor current from reversing in polarity, improving efficiency at light loads. Synchronous Rectifier To control inrush current and to prevent the inductor current from running away when VOUT is close to VIN, the P-channel MOSFET synchronous rectifier is only enabled when VOUT > (VIN + 0.24V). Anti-Ringing Control The anti-ringing control connects a resistor across the inductor to prevent high frequency ringing on the SW pin during discontinuous current mode operation. The ringing of the resonant circuit formed by L and CSW (capacitance on SW pin) is low energy, but can cause EMI radiation. Output Disconnect The LTC3528-2 is designed to allow true output discon- nect by eliminating body diode conduction of the internal P-channel MOSFET rectifier. This allows for VOUT to go to zero volts during shutdown, drawing no current from the input source. It also enables inrush current limiting at turn- on, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, a Schottky diode cannot be connected between SW and VOUT. The output disconnect feature also allows VOUT to be forced above the programmed regulation voltage, without any reverse current into the input power source. Thermal Shutdown If the die temperature exceeds 160°C, the LTC3528-2 enters thermal shutdown. All switches will be turned off and the soft-start capacitor will be discharged. The device will be enabled again when the die temperature drops by approximately 15°C. Burst Mode OPERATION The LTC3528-2 will automatically enter Burst Mode op- eration at light load current and return to fixed frequency PWM mode when the load increases. Refer to the Typical Performance Characteristics to see the output load Burst Mode threshold vs VIN. The load at which Burst Mode operation is entered can be changed by adjusting the inductor value. Raising the inductor value will lower the load current at which Burst Mode operation is entered. In Burst Mode operation, the LTC3528-2 continues switch- ing at a fixed frequency of 2MHz, using the same error amplifier and loop compensation for peak current mode control. This control method minimizes output transients when switching between modes. In Burst Mode opera- tion, energy is delivered to the output until it reaches the nominal regulated value, then the LTC3528-2 transitions to sleep mode where the outputs are off and the LTC3528-2 consumes only 12μA of quiescent current from VOUT. Once the output voltage has drooped slightly, switching resumes again. This maximizes efficiency at very light loads by minimizing switching and quiescent current losses. Burst Mode output ripple, which is typically 1% peak-to-peak, can be reduced by using more output capacitance (10μF or greater). As the load current increases, the LTC3528-2 automatically leaves Burst Mode operation. Note that larger output ca- pacitor values may cause this transition to occur at lighter loads. The regulator will also leave Burst Mode operation if a load transient occurs which causes the inductor current to repeatedly reach current limit. Once the LTC3528-2 has left Burst Mode operation and returned to normal opera- tion, it will remain there until the output load is reduced below the Burst threshold. Burst Mode operation is inhibited during start-up and until soft-start is done and VOUT is at least 0.24V greater than VIN. Single Cell to 5V Step-Up Applications Due to the high inductor current slew rate in applications boosting to 5V from a single cell (alkaline, NiCd or NiMH), the LTC3528-2 may not enter Burst Mode operation at in- put voltages below 1.5V. For single cell to 5V applications requiring Burst Mode operation, the 1MHz LTC3528 is recommended. Refer to the Typical Performance Charac- teristics for the Burst Mode thresholds for different input and output voltages. OPERATION (Refer to Block Diagram) |
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Similar Description - LTC3528-2 |
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