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LT1578 Datasheet(PDF) 7 Page - Linear Technology |
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LT1578 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 28 page  7 LT1578/LT1578-2.5 BLOCK DIAGRAM and output capacitor, then an abrupt 180 ° shift will occur. The current fed system will have 90 ° phase shift at a much lower frequency, but will not have the additional 90 ° shift until well beyond the LC resonant frequency. This makes it much easier to frequency compensate the feedback loop and also gives much quicker transient response. High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher than the input voltage, allowing the switch to saturate. This boosted voltage is generated with an external capacitor and diode. Two comparators are connected to the shut- down pin. One has a 2.42V threshold for undervoltage lockout and the second has a 0.4V threshold for complete shutdown. The LT1578 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscilla- tor pulse which sets the RS flip-flop to turn the switch on. When switch current reaches a level set by the inverting input of the comparator, the flip-flop is reset and the switch turns off. Output voltage control is obtained by using the output of the error amplifier to set the switch current trip point. This technique means that the error amplifier commands current to be delivered to the output rather than voltage. A voltage fed system will have low phase shift up to the resonant frequency of the inductor – + – + Σ INPUT 2.9V BIAS REGULATOR 200kHz OSCILLATOR FREQUENCY SHIFT CIRCUIT VSW FB VC LOCKOUT COMPARATOR GND 1578 BD SLOPE COMP 0.025 Ω INTERNAL VCC CURRENT SENSE AMPLIFIER DC VOLTAGE GAIN = 35 SYNC SHDN SHUTDOWN COMPARATOR CURRENT COMPARATOR ERROR AMPLIFIER gm = 1000µMho FOLDBACK CURRENT LIMIT CLAMP BOOST RS FLIP-FLOP DRIVER CIRCUITRY S R 0.8V Q2 Q1 POWER SWITCH 1.21V 2.42V – + 0.4V 3.5 µA Figure 1. Block Diagram |
Similar Part No. - LT1578_15 |
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Similar Description - LT1578_15 |
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