LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 block DiagraM The LT1767 is a constant frequency, current mode buck then an abrupt 180° shift will occur. The current fed sys- converter. This means that there is an internal clock and tem will have 90° phase shift at a much lower frequency, two feedback loops that control the duty cycle of the power but will not have the additional 90° shift until well beyond switch. In addition to the normal error amplifier, there is a the LC resonant frequency. This makes it much easier to current sense amplifier that monitors switch current on a frequency compensate the feedback loop and also gives cycle-by-cycle basis. A switch cycle starts with an oscillator much quicker transient response. pulse which sets the RS flip-flop to turn the switch on. When High switch efficiency is attained by using the BOOST pin switch current reaches a level set by the inverting input of to provide a voltage to the switch driver which is higher the comparator, the flip-flop is reset and the switch turns than the input voltage, allowing switch to be saturated. off. Output voltage control is obtained by using the output This boosted voltage is generated with an external capac- of the error amplifier to set the switch current trip point. itor and diode. A comparator connected to the shutdown This technique means that the error amplifier commands pin disables the internal regulator, reducing supply current to be delivered to the output rather than voltage. current. A voltage fed system will have low phase shift up to the resonant frequency of the inductor and output capacitor, 0.01Ω VIN 2 + – CURRENT 2.5V BIAS INTERNAL SENSE REGULATOR VCC AMPLIFIER VOLTAGE GAIN = 40 SLOPE COMP ∑ 1 BOOST 0.35V 1.25MHz SYNC 8 OSCILLATOR S Q1 CURRENT RS DRIVER POWER COMPARATOR FLIP-FLOP CIRCUITRY + SWITCH R SHUTDOWN – 3 VSW COMPARATOR 7µA + – PARASITIC DIODESDO NOTFORWARD BIAS 1.33V SHDN 5 – 6 FB 3µA gm + ERROR 7 VC AMPLIFIER g 1.2V m = 850µMho 4 GND 1767 F01 Figure 1. Block Diagram 1767fb For more information www.linear.com/LT1767 7 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Application
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