Datasheet LT3470 (Analog Devices) - 8

LT3470
operaTion
The LT3470 uses a hysteretic control scheme in conjunction comparator trips and resets the latch causing the switch with Burst Mode operation to provide low output ripple to turn off. While the switch is off, the inductor current and low quiescent current while using a tiny inductor and ramps down through the catch diode. When both the bot- capacitors. tom current comparator trips and the minimum off-time Operation can best be understood by studying the Block one-shot expires, the latch turns the switch back on thus Diagram. An error amplifier measures the output voltage completing a full cycle. The hysteretic action of this control through an external resistor divider tied to the FB pin. If scheme results in a switching frequency that depends the FB voltage is higher than V on inductor value, input and output voltage. Since the REF, the error amplifier will shut off all the high power circuitry, leaving the LT3470 switch only turns on when the catch diode current falls in its micropower state. As the FB voltage falls, the error below threshold, the part will automatically switch slower amplifier will enable the power section, causing the chip to keep inductor current under control during start-up or to begin switching, thus delivering charge to the output short-circuit conditions. capacitor. If the load is light the part will alternate between The switch driver operates from either the input or from micropower and switching states to keep the output in the BOOST pin. An external capacitor and internal diode regulation (See Figure 1a). At higher loads the part will is used to generate a voltage at the BOOST pin that is switch continuously while the error amp servos the top higher than the input supply. This allows the driver to and bottom current limits to regulate the FB pin voltage fully saturate the internal bipolar NPN power switch for to 1.25V (See Figure 1b). efficient operation. The switching action is controlled by an RS latch and two If the SHDN pin is grounded, all internal circuits are turned current comparators as follows: The switch turns on, off and VIN current reduces to the device leakage current, and the current through it ramps up until the top current typically a few nA. NO LOAD 200mA LOAD VOUT VOUT 20mV/DIV 20mV/DIV IL 100mA/DIV IL 100mA/DIV 1ms/DIV 1µs/DIV 10mA LOAD 150mA LOAD VOUT VOUT 20mV/DIV 20mV/DIV IL IL 100mA/DIV 100mA/DIV 5µs/DIV 3470 F01a 1µs/DIV 3470 F1b
(1a) Burst Mode Operation (1b) Continuous Operation Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor
3470fd 8 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