Datasheet LT1107 (Analog Devices) - 6

LT1107
W BLOCK I D AGRA S LT1107 LT1107-5/LT1107-12
SET SET A2 AO A2 AO VIN VIN GAIN BLOCK/ GAIN BLOCK/ ERROR AMP I ERROR AMP LIM SW1 ILIM SW1 1.25V 1.25V REFERENCE REFERENCE A1 OSCILLATOR A1 OSCILLATOR DRIVER DRIVER COMPARATOR COMPARATOR R2 SW2 GND FB SW2 R1 220k LT1107-5: R1 = 73.5k 1107 BD01 GND SENSE LT1107-12: R1 = 25.5k 1107 BD02
U OPERATIO
The LT1107 is a gated oscillator switcher. This type Gain block A2 can serve as a low-battery detector. The architecture has very low supply current because the negative input of A2 is the 1.25V reference. A resistor switch is cycled when the feedback pin voltage drops divider from VIN to GND, with the mid-point connected to below the reference voltage. Circuit operation can best be the SET pin provides the trip voltage in a low-battery understood by referring to the LT1107 block diagram. detector application. AO can sink 300µA (use a 22k Comparator A1 compares the feedback (FB) pin voltage resistor pull-up to 5V). with the 1.25V reference signal. When FB drops below A resistor connected between the ILIM pin and VIN sets 1.25V, A1 switches on the 63kHz oscillator. The driver maximum switch current. When the switch current ex- amplifier boosts the signal level to drive the output NPN ceeds the set value, the switch cycle is prematurely power switch. The switch cycling action raises the output terminated. If current limit is not used, ILIM should be tied voltage and FB pin voltage. When the FB voltage is suffi- directly to VIN. Propagation delay through the current limit cient to trip A1, the oscillator is gated off. A small amount circuitry is approximately 1µs. of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator In step-up mode the switch emitter (SW2) is connected to output is low, the oscillator and all high current circuitry is ground and the switch collector (SW1) drives the induc- turned off, lowering device quiescent current to just 300µA. tor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The oscillator is set internally for 11µs ON time and 5µs OFF time in step-up mode, optimizing the device for The LT1107-5 and LT1107-12 are functionally identical to converters where V the LT1107. The -5 and -12 versions have on-chip voltage OUT ≈ 3VIN. The combination of high duty cycle and the current limit feature enables continuous setting resistors for fixed 5V or 12V outputs. Pin 8 on the mode operation in many applications, increasing available fixed versions should be connected to the output. No output power. external resistors are needed. 1107fa 6