Datasheet LT3990, LT3990-3.3, LT3990-5 (Analog Devices) - 9

LT3990/LT3990-3.3/LT3990-5
OPERATION
The LT3990 is a constant frequency, current mode step- If the EN/UVLO pin is low, the LT3990 is shut down and down regulator. An oscillator, with frequency set by RT, draws 0.7µA from the input. When the EN/UVLO pin ex- sets an RS flip-flop, turning on the internal power switch. ceeds 1.19V, the switching regulator will become active. An amplifier and comparator monitor the current flowing The switch driver operates from either V between the V IN or from the IN and SW pins, turning the switch off when BOOST pin. An external capacitor is used to generate a this current reaches a level determined by the voltage at voltage at the BOOST pin that is higher than the input VC (see Block Diagram). An error amplifier measures the supply. This allows the driver to fully saturate the internal output voltage through an external resistor divider tied to bipolar NPN power switch for efficient operation. the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; To further optimize efficiency, the LT3990 automatically if it decreases, less current is delivered. switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling Another comparator monitors the current flowing through the output switch is shut down reducing the input supply the catch diode and reduces the operating frequency when current to 1.8µA. the current exceeds the 450mA bottom current limit. This foldback in frequency helps to control the output current The LT3990 contains a power good comparator which in fault conditions such as a shorted output with high trips when the FB pin is at 90% of its regulated value. The input voltage. Maximum deliverable current to the output PG output is an open-drain transistor that is off when the is therefore limited by both switch current limit and catch output is in regulation, allowing an external resistor to pull diode current limit. the PG pin high. Power good is valid when the LT3990 is enabled and V An internal regulator provides power to the control cir- IN is above 4.2V. cuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. 3990fa 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 Typical Application Related Parts