Datasheet LT3991, LT3991-3.3, LT3991-5 (Analog Devices) - 9

LT3991/LT3991-3.3/LT3991-5
operaTion
The LT3991 is a constant frequency, current mode step- voltage at the BOOST pin that is higher than the input down regulator. An oscillator, with frequency set by RT, supply. This allows the driver to fully saturate the internal sets an RS flip-flop, turning on the internal power switch. bipolar NPN power switch for efficient operation. An amplifier and comparator monitor the current flowing To further optimize efficiency, the LT3991 automatically between the VIN and SW pins, turning the switch off when switches to Burst Mode operation in light load situations. this current reaches a level determined by the voltage at Between bursts, all circuitry associated with controlling VC (see Block Diagram). An error amplifier measures the the output switch is shut down, reducing the input supply output voltage through an external resistor divider tied to current to 1.7μA. In a typical application, 2.8μA will be con- the FB pin and servos the VC node. If the error amplifier’s sumed from the supply when regulating with no load. output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp The oscillator reduces the LT3991’s operating frequency on the VC node provides current limit. The VC node is when the voltage at the FB pin is low. This frequency also clamped by the voltage on the SS pin; soft-start is foldback helps to control the output current during start- implemented by generating a voltage ramp at the SS pin up and overload. using an external capacitor and resistor. The LT3991 contains a power good comparator which If the EN pin is low, the LT3991 is shut down and draws trips when the FB pin is at 91% of its regulated value. The 700nA from the input. When the EN pin exceeds 1.01V, PG output is an open-drain transistor that is off when the the switching regulator will become active. output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3991 is The switch driver operates from either VIN or from the enabled and V BOOST pin. An external capacitor is used to generate a IN is above 4.3V.
applicaTions inForMaTion Achieving Ultralow Quiescent Current
1000 VIN = 12V VOUT = 3.3V To enhance efficiency at light loads, the LT3991 operates 800 in low ripple Burst Mode, which keeps the output capacitor charged to the desired output voltage while minimizing 600 the input quiescent current. In Burst Mode operation the LT3991 delivers single pulses of current to the output ca- 400 pacitor followed by sleep periods where the output power is supplied by the output capacitor. When in sleep mode SWITCHING FREQUENCY (kHz) 200 the LT3991 consumes 1.7μA, but when it turns on all the circuitry to deliver a current pulse, the LT3991 consumes 0 0 20 40 60 80 100 120 1.5mA of input current in addition to the switch current. LOAD CURRENT (mA) 3991 F01 Therefore, the total quiescent current will be greater than 1.7μA when regulating.
Figure 1. Switching Frequency in Burst Mode Operation
As the output load decreases, the frequency of single cur- quiescent current performance at light loads, the current rent pulses decreases (see Figure 1) and the percentage in the feedback resistor divider and the reverse current of time the LT3991 is in sleep mode increases, resulting in the catch diode must be minimized, as these appear in much higher light load efficiency. By maximizing the to the output as load currents. Use the largest possible time between pulses, the converter quiescent current feedback resistors and a low leakage Schottky catch diode gets closer to the 1.7μA ideal. Therefore, to optimize the in applications utilizing the ultralow quiescent current 3991fa 9 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