Datasheet LTC3539, LTC3539-2 (Analog Devices) - 8

LTC3539/LTC3539-2
OPERATION
Zero Current Comparator: The zero current comparator be changed by adjusting the inductor value. Raising the monitors the inductor current to the output and shuts off inductor value will lower the load current at which Burst the synchronous rectifier once this current reduces to ap- Mode operation is entered. proximately 40mA. This prevents the inductor current from In Burst Mode operation, the LTC3539 still switches at a reversing in polarity, improving efficiency at light loads. fixed frequency, using the same error amplifier and loop Synchronous Rectifier: The P-channel MOSFET synchro- compensation for peak current mode control. This control nous rectifier is disabled when VOUT is much less than VIN. method eliminates any output transient when switching This is to control inrush current and to prevent inductor between modes. In Burst Mode operation, energy is deliv- current from running away ered to the output until it reaches the nominal regulation Anti-ringing Control: The anti-ringing control connects value, then the LTC3539 transitions to Sleep mode where a resistor across the inductor to prevent high frequency the outputs are off and the LTC3539 consumes only 10µA ringing on the SW pin during discontinuous current mode of quiescent current from VOUT. When the output voltage operation. The ringing of the resonant circuit formed by L droops slightly, switching resumes. This maximizes ef- and C ficiency at very light loads by minimizing switching and SW (capacitance on SW pin) is low energy, but can cause EMI radiation. quiescent current losses. Burst Mode output voltage ripple, which is application sensitive and typically 1% to Output Disconnect: The LTC3539 is designed to allow true 2% peak-to-peak if data sheet recommendations are fol- output disconnect by eliminating body diode conduction of lowed, can be reduced by using more output capacitance the internal P-channel MOSFET rectifier. This allows VOUT (47µF or greater). to go to zero volts during shutdown, drawing no current from the input source. It also provides inrush current As the load increases, the LTC3539 will automatically limiting at turn-on, minimizing surge currents seen by the leave Burst Mode operation. Once the LTC3539 has left input supply. Note that to obtain the advantages of output Burst Mode operation and returned to normal operation, disconnect, there must not be an external Schottky diode it will remain there until the output load is reduced below connected between the SW pin and V the burst threshold. OUT. The output disconnect feature also allows VOUT to be pulled high, Burst Mode operation is inhibited during start-up until without any reverse current into a battery on VIN. soft-start is complete and VOUT is at least 0.24V greater Thermal Shutdown: If the die temperature exceeds 160°C, than VIN. the device will go into thermal shutdown. All switches will When the MODE pin is below 0.3V, the LTC3539 features be turned off and the internal soft-start capacitor will be continuous PWM operation. In this mode, at very light discharged. The device will be enabled again when the die loads, the LTC3539 will exhibit pulse-skip operation. If temperature drops by about 15°C. the MODE pin voltage exceeds the greater of VIN or VOUT by 0.5V, the MODE pin will sink additional current.
Burst Mode Operation
Due to the high inductor current slew rate in 5V output When Burst Mode operation is enabled by bringing the applications, the LTC3539-2 may not enter Burst Mode MODE pin above 0.88V, the LTC3539 will automatically operation for input voltages less than 1.8V. For 5V applica- enter Burst Mode operation at light load current, then tions requiring Burst Mode operation, the 1MHz LTC3539 return to fixed frequency PWM mode when the load in- is recommended. Refer to the Typical Performance creases. Refer to the typical performance characteristics Characteristics curves for the Burst Mode thresholds for to see the output load Burst Mode threshold vs VIN. The different input and output voltages. load current at which Burst Mode operation is entered can 35392fc 8 For more information www.linear.com/3539/3539-2 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts