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LT1107MJ8-5 Datasheet(PDF) 6 Page - Linear Technology |
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LT1107MJ8-5 Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LT1107 1107fa I DAGRA BLOCK S OPERATIO The LT1107 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 LT1107 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 63kHz 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 11 µs ON time and 5µs OFF time in step-up mode, optimizing the device for converters where VOUT ≈ 3VIN. The combination of high duty cycle and the current limit feature enables continuous mode operation in many applications, increasing available output power. OSCILLATOR 1.25V REFERENCE VIN GND SENSE COMPARATOR A2 A1 DRIVER ILIM SW1 SW2 AO GAIN BLOCK/ ERROR AMP SET 1107 BD02 R1 R2 220k LT1107-5: R1 = 73.5k LT1107-12: R1 = 25.5k OSCILLATOR 1.25V REFERENCE VIN GND FB COMPARATOR A2 A1 DRIVER ILIM SW1 SW2 AO GAIN BLOCK/ ERROR AMP SET 1107 BD01 LT1107 LT1107-5/LT1107-12 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 LT1107-5 and LT1107-12 are functionally identical to the LT1107. 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. |
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