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LTC3539 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3539 Datasheet(HTML) 8 Page - Linear Technology |
8 / 14 page LTC3539/LTC3539-2 8 35392fc For more information www.linear.com/3539/3539-2 OPERATION Zero Current Comparator: The zero current comparator monitors the inductor current to the output and shuts off the synchronous rectifier once this current reduces to ap- proximately40mA.Thispreventstheinductorcurrentfrom reversing in polarity, improving efficiency at light loads. Synchronous Rectifier: The P-channel MOSFET synchro- nous rectifier is disabled when VOUT is much less than VIN. This is to control inrush current and to prevent inductor current from running away 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 LTC3539 is designed to allow true outputdisconnectbyeliminatingbodydiodeconductionof the internal P-channel MOSFET rectifier. This allows VOUT to go to zero volts during shutdown, drawing no current from the input source. It also provides inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there must not be an external Schottky diode connected between the SW pin and VOUT. The output disconnect feature also allows VOUT to be pulled high, without any reverse current into a battery on VIN. Thermal Shutdown: If the die temperature exceeds 160°C, the device will go into thermal shutdown. All switches will be turned off and the internal soft-start capacitor will be discharged. The device will be enabled again when the die temperature drops by about 15°C. Burst Mode Operation When Burst Mode operation is enabled by bringing the MODE pin above 0.88V, the LTC3539 will automatically enter Burst Mode operation at light load current, then return to fixed frequency PWM mode when the load in- creases. Refer to the typical performance characteristics to see the output load Burst Mode threshold vs VIN. The load current 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 LTC3539 still switches at a fixed frequency, using the same error amplifier and loop compensation for peak current mode control. This control method eliminates any output transient when switching between modes. In Burst Mode operation, energy is deliv- ered to the output until it reaches the nominal regulation value, then the LTC3539 transitions to Sleep mode where the outputs are off and the LTC3539 consumes only 10µA of quiescent current from VOUT. When the output voltage droops slightly, switching resumes. This maximizes ef- ficiency at very light loads by minimizing switching and quiescent current losses. Burst Mode output voltage ripple, which is application sensitive and typically 1% to 2% peak-to-peak if data sheet recommendations are fol- lowed, can be reduced by using more output capacitance (47µF or greater). As the load increases, the LTC3539 will automatically leave Burst Mode operation. Once the LTC3539 has left Burst Mode operation and returned to normal operation, it will remain there until the output load is reduced below the burst threshold. Burst Mode operation is inhibited during start-up until soft-start is complete and VOUT is at least 0.24V greater than VIN. When the MODE pin is below 0.3V, the LTC3539 features continuous PWM operation. In this mode, at very light loads, the LTC3539 will exhibit pulse-skip operation. If the MODE pin voltage exceeds the greater of VIN or VOUT by 0.5V, the MODE pin will sink additional current. Due to the high inductor current slew rate in 5V output applications, the LTC3539-2 may not enter Burst Mode operation for input voltages less than 1.8V. For 5V applica- tions requiring Burst Mode operation, the 1MHz LTC3539 is recommended. Refer to the Typical Performance Characteristics curves for the Burst Mode thresholds for different input and output voltages. |
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Similar Description - LTC3539_15 |
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