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OPERATIO
U LT1073
去 取The LT1073 is gated oscillator switcher. This type archi-tecture has very low supply current becau the switch is cycled only when the feedback pin voltage drops below the reference voltage. Circuit operation can best be under-stood by referring to the LT1073 Block Diagram. Com-parator A1 compares the FB pin voltage with the 212mV reference signal. When FB drops below 212mV, A1 switches on the 19kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch Q1. An adaptive ba drive circuit ns switch current and provides just enough ba drive to ensure switch satura-tion without overdriving the switch, resulting in higher efficiency. The switch cycling action rais the output voltage and FB pin voltage. When the FB voltage is suffi-cient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator is low the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 95µA for the reference, A1 and A2.
The oscillator is t internally for 38µs ON time and 15µs OFF time, optimizing the device for step-up circuits where V OUT≈ 3V IN, e.g., 1.5V to 5V. Other step-up ratios as well as step-down (buck) converters are possible at slight loss in maximum achievable power output.
A2 is a versatile gain block that can rve as a low-battery detector, a linear post-regulator, or drive an undervoltage lockout circuit. The negative input of A2 is internally connected to the 212mV reference. An external resistor divider from V IN to GND provides the trip point for A2. The AO output can sink 100µA (u a 56k resistor pull-up to 5V). This line can signal a microcontroller that the battery voltage has dropped below the pret level.A resistor connected between the I LIM pin and V IN adjusts maximum switch current. When the switch current ex-ceeds the t value, the switch is turned off. This feature is especially uful when small inductance values are ud with high input voltages. If the internal current limit of 1.5A is desired, I LIM should be tied directly to V IN. Propagation delay through the current-limit circuitry is about 2µs.
In step-up mode, SW2 is connected to ground and SW1 drives the inductor. In step-down mode, SW1 is con-nected to V IN and SW2 drives the inductor. Output voltage is t by the following equation in either step-up or step-down modes where R1 is connected from FB to GND and R2 is connected from V OUT to FB.
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LT1073-5 and LT1073-12
The LT1073-5 and LT1073-12 fixed output voltage ver-sions have the gain-tting resistor on-chip. Only three external components are required to construct a fixed-output converter. 5µA flows through R1 and R2 in the LT1073-5, and 12.3µA flows in the LT1073-12. This current reprents a load and t
he converter must cycle from time to time to maintain the proper output voltage. Output ripple, inherently prent in gated-oscillator de-signs, will typically run around 150mV for the LT1073-5 and 350mV for the LT1073-12 with the proper inductor/ capacitor lection. This output ripple can be reduced considerably by using the gain block amp as a preamplifier in front of the FB pin. See the Applications Information ction for details.
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