LT1765/LT1765-1.8/LT1765-2.5/ LT1765-3.3/LT1765-5 BLOCK DIAGRAM The LT1765 is a constant frequency, current mode buck resonant frequency of the inductor and output capacitor, converter. This means that there is an internal clock and then an abrupt 180° shift will occur. The current fed system two feedback loops that control the duty cycle of the power will have 90° phase shift at a much lower frequency, but switch. In addition to the normal error amplifi er, there is a will not have the additional 90° shift until well beyond current sense amplifi er that monitors switch current on a the LC resonant frequency. This makes it much easier to cycle-by-cycle basis. A switch cycle starts with an oscillator frequency compensate the feedback loop and also gives pulse which sets the RS fl ip-fl op to turn the switch on. When much quicker transient response. switch current reaches a level set by the inverting input of High switch effi ciency is attained by using the BOOST pin the comparator, the fl ip-fl op is reset and the switch turns to provide a voltage to the switch driver which is higher off. Output voltage control is obtained by using the output than the input voltage, allowing the switch to be saturated. of the error amplifi er to set the switch current trip point. This boosted voltage is generated with an external capacitor This technique means that the error amplifi er commands and diode. A comparator connected to the shutdown pin current to be delivered to the output rather than voltage. disables the internal regulator, reducing supply current. A voltage fed system will have low phase shift up to the 0.005Ω INPUT + – CURRENT 2.5V BIAS INTERNAL SENSE REGULATOR V AMPLIFIER CC VOLTAGE GAIN = 40 SLOPE COMP ∑ BOOST 0.4V 1.25MHz SYNC S OSCILLATOR Q1 CURRENT RS DRIVER POWER COMPARATOR FLIP-FLOP CIRCUITRY SWITCH + R SHUTDOWN – V PARASITIC DIODES SW COMPARATOR 7μA DO NOT FORWARD BIAS + – – FB 1.33V + ERROR VC AMPLIFIER 1.2V INTERNAL SHDN gm = 850μMho VCC GND 3μA 1765 F01 Figure 1. Block Diagram 1765fd 7
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