Datasheet LT3470A (Analog Devices) - 8

link to page 7 link to page 7 LT3470A
OPERATION
The LT3470A uses a hysteretic control scheme in con- comparator trips and resets the latch causing the switch junction with Burst Mode operation to provide low output to turn off. While the switch is off, the inductor current ripple and low quiescent current while using a tiny induc- ramps down through the catch diode. When both the bot- tor and 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 con- through an external resistor divider tied to the FB pin. If trol 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 LT3470A 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 If the SHDN pin is grounded, all internal circuits are turned two 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 100nA. 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 3470a F01a 1µs/DIV 3470a F1b
(1a) Burst Mode Operation (1b) Continuous Operation Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor
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