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LT1111CS8 Datasheet(PDF) 6 Page - Linear Technology |
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LT1111CS8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LT1111 Gain block A2 can serve as a low-battery detector. The negative input of A2 is the 1.25V 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. AO can sink 300 µA (use a 22k resistor 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 1 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 LT1111-5 and LT1111-12 are functionally identical to the LT1111. 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. LT11 ATI 11 OPER The LT1111 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 LT1111 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.25V reference signal. When FB drops below 1.25V, A1 switches on the 72kHz 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 300 µA. The oscillator is set internally for 7 µs ONtime and 7µs OFF time, optimizing the device for circuits where VOUT and VIN differ by roughly a factor of 2. Examples include a 3V to 5V step-up converter or a 9V to 5V step-down converter. I DAGRA BLOCK S LT1111 • BD01 IN V GND SET A0 A2 1.25V REFERENCE A1 OSCILLATOR DRIVER SW1 SW2 LIM I GAIN BLOCK/ ERROR AMP COMPARATOR + – + – FB LT1111-5/LT1111-12 LT1111 LT1111 • BD02 IN V GND SET A0 A2 1.25V REFERENCE A1 OSCILLATOR DRIVER R1 SW1 SW2 LIM I R2 220k SENSE LT1111-5: LT1111-12: R1 = 73.5k R1 = 25.5k GAIN BLOCK/ ERROR AMP COMPARATOR + – + – |
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