Datasheet LT3504 (Analog Devices) - 9

LT3504
operaTion
A comparator starts the reference when the EN/UVLO pin comparator enforces a catch diode current limit to prevent rises above the 1.44V rising threshold. Other comparators inductor current runaway during transient conditions. An prevent switching when the input voltage is below 2.9V or error amplifier measures the output voltage through an the die temperature is above 175°C. When the EN/UVLO external resistor tied to the FB pin and servos the VC node. is above 1.44V, the input voltage is above 3.2V, and the If the error amplifier’s output increases, more current is temperature is below 175°C, the boost regulator begins delivered to the output; if it decreases, less current is switching and charges the SKY capacitor to 4.85V above delivered. A clamp on the VC pin provides switch current VIN. When the SKY voltage is less than 4.5V above VIN, limit. Each buck regulator switch driver operates by drawing the RUN/SS pins and VC nodes are actively pulled low to current from the SKY pin. Regulating the SKY pin to 4.85V prevent the buck regulators from switching. above the VIN pin voltage is necessary to fully saturate the The boost regulator (Channel 5) consists of an internal bipolar power switch for efficient operation. 0.4A power switch (Q5), an internal power Schottky diode Soft-start is implemented by generating a voltage ramp at (D5), and the necessary logic and other control circuitry the RUN/SS pin. An internal 1.3µA current source pulls the to drive the switch. The switch current is monitored to RUN/SS pin up to 2.1V. Connecting a capacitor from the enforce cycle-by-cycle current limit. The diode current RUN/SS pin to ground programs the rate of the voltage is monitored to prevent inductor current runaway during ramp on the RUN/SS pin. A voltage follower circuit with a transient conditions. An error amplifier servos the SKY 0.1V offset connected from the RUN/SS pin to the RAMP voltage to 4.85V above VIN. A comparator detects when node prevents switching until the voltage at the RUN/SS the SKY voltage is 4.5V above VIN and allows the buck pin increases above 0.1V. When the voltage at the RAMP regulators to begin switching. node is less than 0.9V, the error amplifier servos the FB The oscillator produces two antiphase clock signals running voltage to the RAMP node voltage. When the RAMP node at 50% duty cycle. Channels 1, 3 and 5 run antiphase to voltage increases above 0.9V, then the error amplifier ser- Channels 2 and 4. The oscillator can be programmed by vos the FB voltage to 0.8V. Additionally, a current amplifier connecting a single resistor from RT/SYNC to ground, or reduces the catch diode current limit when the FB voltage by applying an external clock signal to RT/SYNC. A sync is below 0.8V to limit the inductor current during startup. detect circuit distinguishes between the type of input. Each individual buck regulator can be placed in shutdown Tying a resistor to GND directly sets the bias current of by pulling the respective RUN/SS pin below 0.1V. The EN/ the oscillator. The sync signal is converted to a current to UVLO pin can be pulled low (below a VBE) to place the set the bias current of the oscillator. entire part in shutdown, disconnecting the outputs and The oscillator enables an R reducing the input current to less than 2µA. S flip-flop, turning on the internal 1.7A power switch Q1. An amplifier and compara- The LT3504 is pin compatible with the 28-lead QFN package tor monitor the current flowing between the VIN and SW LT3514. The LT3514 is a three channel step-down converter pins, turning the switch off when this current reaches a and has one channel (CH3) that outputs 2A instead of 1A. level determined by the voltage at the VC node. A second 3504fa For more information www.linear.com/LT3504 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 Package Description Revision History Typical Application Related Parts