Datasheet LT3840 (Analog Devices) - 10

LT3840
operaTion Light Load Operation (Burst Mode Operation,
In pulse-skipping mode, during light loads, the supply
Pulse-Skipping Mode or Continuous Mode)
operates in discontinuous mode where the inductor cur- The LT3840 is capable of operating in Burst Mode, pulse- rent is not allowed to reverse direction. Output voltage skipping mode, or continuous mode. Connect the MODE regulation is maintained by skipping TG on pulses. At pin to GND for Burst Mode operation, to the FB pin for light loads pulse-skipping mode is more efficient than pulse-skipping mode, or to INTV forced continuous mode, but not as efficient as Burst CC for continuous mode. Mode operation. In Burst Mode operation the LT3840 forces a minimum peak inductor current via an internal clamp on the V In continuous operation the inductor current is allowed C pin. If the average inductor current is greater than the load to reverse direction at light loads or under large transient current the output voltage will begin to increase and the conditions. The reverse current comparator protects the error amplifier, EA, will attempt to decrease the V BG MOSFET by turning it off if the reverse current exceeds C volt- age. When the internal voltage clamp on V the maximum reverse current sense threshold voltage. C is engaged and the FB voltage increases slightly, the LT3840 goes
Constant Current Operation
into sleep mode. For applications requiring a regulated current source the In sleep mode, both external MOSFETs are turned off LT3840 has a control loop to accurately regulate the aver- and much of the internal circuitry is turned off, reducing age output current. A current monitor function provides the quiescent current. The load current is supplied by output current information for telemetry and diagnostics. the output capacitor. As the output voltage decreases, the LT3840 comes out of sleep mode and the controller The current through the sense resistor, RSENSE, produces resumes normal operation by turning on the TG MOSFET a voltage applied to the SENSE pins. The differential sense on the next cycle of the internal oscillator. The output voltage is amplified by 20x, buffered and output to the voltage increases and the controller goes back to sleep. IMON pin. The capacitor on the IMON pin filters the ripple This cycle repeats until the average load current is greater component to average the signal. than the minimum forced peak inductor current. The 20x amplified differential sense voltage is also applied When Burst Mode operation is selected, the inductor to an internal GM amplifier and compared against either current is not allowed to go negative. A reverse current 1V or ICTRL voltage, whichever is smaller. A voltage comparator turns off the BG MOSFET just before the applied to ICTRL reduces the maximum average current inductor current reaches zero, preventing it from revers- sense threshold. When the 20× amplified differential sense ing and going negative. Thus, the controller operates in voltage exceeds the 1V internal reference or the ICTRL discontinuous operation. voltage the ICOMP node is driven high and VC is pulled 3840fa 10 For more information www.linear.com/LT3840 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts