Datasheet LT3686 (Analog Devices) - 9

LT3686
operAtion
The LT3686 is a current mode step-down regulator. The decreases, less current is delivered. An active clamp (not EN/UVLO pin is used to place the LT3686 in shutdown. The shown) on the VC node provides current limit. 1.27V threshold on the EN/UVLO pin can be programmed The switch driver operates from either V by an external resistor divider (R4, R5) to disable the IN or from the BOOST pin. An external capacitor and the internal boost LT3686. When the EN/UVLO pin is driven above 1.27V, an diode are used to generate a voltage at the BOOST pin that internal regulator provides power to the control circuitry. is higher than the input supply. This allows the driver to This regulator includes both overvoltage and undervoltage fully saturate the internal bipolar NPN power switch for lockout to prevent switching when VIN is more than 37V efficient operation. or less than 3.6V. A comparator monitors the current flowing through the Tracking soft-start is implemented by providing constant catch diode via the DA pin and reduces the LT3686’s op- current via the SS pin to an external soft-start capacitor erating frequency when the DA pin current exceeds the (C4) to generate a voltage ramp. FB voltage is regulated to 1.7A valley current limit. This helps to control the output the voltage at the SS pin until it exceeds 0.8V; FB is then current in fault conditions such as shorted output with high regulated to the reference 0.8V. Soft-start also reduces input voltage. The DA comparator works in conjunction the oscillator frequency to avoid hitting current limit dur- with the switch peak current limit comparator to determine ing start-up. The SS capacitor is reset during fault events the maximum deliverable current of the LT3686. such as overvoltage, undervoltage, thermal shutdown and startup. The active load is enabled when MODE is tied above 0.8V and disabled when the MODE pin is below 0.4V. To use the An oscillator is programmed by resistor RT. The oscillator active load, the BD pin should be tied to V sets an RS flip-flop, turning on the internal 1.2A power OUT . The LT3686 will prevent pulse skipping at light loads by regulating the switch Q1. An amplifier and comparator monitor the cur- active load. The active load will assist startup by guarantee- rent flowing between the VIN and SW pins, turning the ing a minimum load to charge the boost capacitor. It also switch off when this current reaches a level determined by hastens the recharge of boost capacitor when operating the voltage at VC. An error amplifier measures the output beyond maximum duty cycle. voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output The active load works only when the BD pin is less than increases, more current is delivered to the output; if it 5.2V. 3686fc 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