Datasheet LT1073 (Analog Devices) - 5

LT1073
OPERATION LT1073
The LT1073 is gated oscillator switcher. This type archi- A resistor connected between the ILIM pin and VIN adjusts tecture has very low supply current because the switch is maximum switch current. When the switch current exceeds cycled only when the feedback pin voltage drops below the the set value, the switch is turned off. This feature is espe- reference voltage. Circuit operation can best be understood cially useful when small inductance values are used with by referring to the LT1073 Block Diagram. Comparator A1 high input voltages. If the internal current limit of 1.5A is compares the FB pin voltage with the 212mV reference desired, ILIM should be tied directly to VIN. Propagation signal. When FB drops below 212mV, A1 switches on the delay through the current-limit circuitry is about 2µs. 19kHz oscillator. The driver amplifier boosts the signal In step-up mode, SW2 is connected to ground and SW1 level to drive the output NPN power switch Q1. An adap- drives the inductor. In step-down mode, SW1 is connected tive base drive circuit senses switch current and provides to V just enough base drive to ensure switch saturation without IN and SW2 drives the inductor. Output voltage is set by the following equation in either step-up or step-down overdriving the switch, resulting in higher efficiency. The modes where R1 is connected from FB to GND and R2 is switch cycling action raises the output voltage and FB pin connected from V voltage. When the FB voltage is sufficient to trip A1, the OUT to FB. oscillator is gated off. A small amount of hysteresis built ( )R2   + into A1 ensures loop stability without external frequency VOUT = 212mV 1  R1  compensation. When the comparator is low the oscillator and all high current circuitry is turned off, lowering device
LT1073-5 and LT1073-12
quiescent current to just 95µA for the reference, A1 and A2. The LT1073-5 and LT1073-12 fixed output voltage versions The oscillator is set internally for 38µs ON time and 15µs have the gain-setting resistor on-chip. Only three external OFF time, optimizing the device for step-up circuits where components are required to construct a fixed-output con- VOUT ≈ 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well verter. 5µA flows through R1 and R2 in the LT1073-5, and as step-down (buck) converters are possible at slight 12.3µA flows in the LT1073-12. This current represents losses in maximum achievable power output. a load and the converter must cycle from time to time to A2 is a versatile gain block that can serve as a low-battery maintain the proper output voltage. Output ripple, inher- detector, a linear post-regulator, or drive an undervolt- ently present in gated-oscillator designs, will typically age lockout circuit. The negative input of A2 is internally run around 150mV for the LT1073-5 and 350mV for the connected to the 212mV reference. An external resistor LT1073-12 with the proper inductor/capacitor selection. divider from VIN to GND provides the trip point for A2. The This output ripple can be reduced considerably by using AO output can sink 100µA (use a 56k resistor pull-up to the gain block amp as a preamplifier in front of the FB 5V). This line can signal a microcontroller that the battery pin. See the Applications Information section for details. voltage has dropped below the preset level. Rev B 5 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagrams Operation Applications Information Typical Applications Package Description Typical Application Related Parts