Datasheet LT3470A (Analog Devices) - 8

LT3470A
OPERATION
The LT3470A uses a hysteretic control scheme in conjunc- comparator trips and resets the latch causing the switch tion 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 one-shot expires, the latch turns the switch back on thus Operation can best be understood by studying the Block completing a full cycle. The hysteretic action of this control Diagram. An error amplifier measures the output voltage scheme results in a switching frequency that depends through an external resistor divider tied to the FB pin. If on inductor value, input and output voltage. Since the the FB voltage is higher than VREF, the error amplifier will switch only turns on when the catch diode current falls shut off all the high power circuitry, leaving the LT3470A below threshold, the part will automatically switch slower in its micropower state. As the FB voltage falls, the error to keep inductor current under control during start-up or amplifier will enable the power section, causing the chip short-circuit conditions. to begin switching, thus delivering charge to the output 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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