Datasheet LT3971A, LT3971A-5 (Analog Devices) - 9

LT3971A/LT3971A-5
BLOCK DIAGRAM
VIN VIN – C1 INTERNAL 1.19V REF + BD 1V + SWITCH SLOPE COMP Σ LATCH – SHDN BOOST EN R OSCILLATOR C3 Q 200kHz TO 2MHz RT S R L1 T VOUT SW Burst Mode SYNC DETECT D1 C2 PG ERROR AMP VC CLAMP + 1.09V + VC 1μA – – C5 SS SHDN R3 R2 R1 C4 GND FB VOUT LT3971A-5 ONLY R2 R1 3971A BD LT3971A LT3971A-5: R1 = 7.62M, R2 = 2.38M ONLY C5
OPERATION
The LT3971A 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 LT3971A 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 the FB pin and servos the VC node. If the error amplifier’s consumed 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 LT3971A’s operating frequency on the V when the voltage at the FB pin is low. This frequency fold- C node provides current limit. The VC node is also clamped by the voltage on the SS pin; soft-start is back helps to control the output current during start-up implemented by generating a voltage ramp at the SS pin and overload. using an external capacitor. The LT3971A contains a power good comparator which If the EN pin is low, the LT3971A 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 1V, the PG output is an open-drain transistor that is off when 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 LT3971A The switch driver operates from either VIN or from the is enabled and V BOOST pin. An external capacitor is used to generate a IN is above 4.3V. 3971af 9 Document Outline FEATURES DESCRIPTION APPLICATIONS 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 TYPICAL APPLICATION RELATED PARTS