Datasheet LT1110 (Linear Technology) - 6

LT1110
O U LT1110 OPERATI
The LT1110 is a gated oscillator switcher. This type A2 is a versatile gain block that can serve as a low battery architecture has very low supply current because the detector, a linear post regulator, or drive an under voltage switch is cycled only when the feedback pin voltage drops lockout circuit. The negative input of A2 is internally below the reference voltage. Circuit operation can best be connected to the 220mV reference. An external resistor understood by referring to the LT1110 block diagram divider from VIN to GND provides the trip point for A2. The above. Comparator A1 compares the FB pin voltage with AO output can sink 300µA (use a 47k resistor pull up to the 220mV reference signal. When FB drops below +5V). This line can signal a microcontroller that the battery 220mV, A1 switches on the 70kHz oscillator. The driver voltage has dropped below the preset level. To prevent the amplifier boosts the signal level to drive the output NPN gain block from operating in its linear region, a 2MΩ power switch Q1. An adaptive base drive circuit senses resistor can be connected from AO to SET. This provides switch current and provides just enough base drive to positive feedback. ensure switch saturation without overdriving the switch, A resistor connected between the I resulting in higher efficiency. The switch cycling action LIM pin and VIN adjusts maximum switch current. When the switch current ex- raises the output voltage and FB pin voltage. When the FB ceeds the set value, the switch is turned off. This feature voltage is sufficient to trip A1, the oscillator is gated off. A is especially useful when small inductance values are used small amount of hysteresis built into A1 ensures loop with high input voltages. If the internal current limit of 1.5A stability without external frequency compensation. When is desired, I the comparator is low the oscillator and all high current LIM should be tied directly to VIN. Propagation delay through the current limit circuitry is about 700ns. circuitry is turned off, lowering device quiescent current to just 300µA for the reference, A1 and A2. In step-up mode, SW2 is connected to ground and SW1 drives the inductor. In step-down mode, SW1 is con- The oscillator is set internally for 10µs ON time and 5µs nected to V OFF time, optimizing the device for step-up circuits where IN and SW2 drives the inductor. Output voltage is set by the following equation in either step-up or step- VOUT ≈ 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well down modes where R1 is connected from FB to GND and as step-down (buck) converters are possible at slight R2 is connected from V losses in maximum achievable power output. OUT to FB. R2  V m 220 V OUT = ( ) + 1 . 0 ( 1)  R1 
W O U LT1110-5, -12 BLOCK I D AGRA LT1110--5, -12 OPERATI
The LT1110-5 and LT1110-12 fixed output voltage ver- SET + sions have the gain setting resistors on-chip. Only three A2 AO external components are required to construct a 5V or 12V – V GAIN BLOCK/ERROR AMP IN output converter. 16µA flows through R1 and R2 in the I LT1110-5, and 39 LIM SW1 µA flows in the LT1110-12. This current represents a load and the converter must cycle from time 220mV REF to time to maintain the proper output voltage. Output A1 OSCILLATOR Q1 ripple, inherently present in gated oscillator designs, will DRIVER COMPARATOR typically run around 90mV for the LT1110-5 and 200mV SW2 R2 for the LT1110-12 with the proper inductor/capacitor R1 300kΩ SENSE LT1110-5: R1 = 13.8kΩ selection. This output ripple can be reduced considerably LT1110-12: R2 = 5.6kΩ by using the gain block amp as a pre-amplifier in front of GND LT1110 • BD02 the FB pin. See the Applications section for details. 6