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LT1108 Datasheet(PDF) 5 Page - Linear Technology |
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LT1108 Datasheet(HTML) 5 Page - Linear Technology |
5 / 12 page 5 LT1108 ATI 1 OPER negative input of A2 is the 1.245V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low battery detector application. A0 can sink 100 µA (use a 47k resis- tor pull-up to 5V). A resistor connected between the ILIM pin and VIN sets maximum switch current. When the switch current ex- ceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to VIN. Propagation delay through the current- limit circuitry is approximately 2 µs. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the induc- tor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The LT1108-5 and LT1108-12 are functionally identical to the LT1108. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output. No external resistors are needed. The LT1108 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1108 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.245V reference signal. When FB drops below 1.245V, A1 switches on the 19kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. The switch cycling action raises 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 output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 110 µA. The oscillator is set internally for 36 µs ON-time and 17µs OFF-time, allowing continuous mode operation in many cases such as 2V to 5V converters. Continuous mode greatly increases available output power. Gain block A2 can serve as a low battery detector. The I DAGRA BLOCK S LT1108 LT1108-5/LT1108-12 ILIM A2 A1 VIN GND FB SET A0 GAIN BLOCK/ ERROR AMP COMPARATOR DRIVER SW1 SW2 1.245V REFERENCE OSCILLATOR LT1108 • BD ILIM A2 A1 VIN GND SET A0 GAIN BLOCK/ ERROR AMP COMPARATOR DRIVER SW1 SW2 1.245V REFERENCE OSCILLATOR LT1108-5 • BD SENSE R1 R2 753k LT1108-5: R1 = 250k LT1108-12: R1 = 87.4k |
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