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1956IFE Datasheet(PDF) 7 Page - Linear Technology |
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1956IFE Datasheet(HTML) 7 Page - Linear Technology |
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7 / 28 page  7 LT1956/LT1956-5 1956f it much easier to frequency compensate the feedback loop and also gives much quicker transient response. Most of the circuitry of the LT1956 operates from an internal 2.9V bias line. The bias regulator normally draws power from the regulator input pin, but if the BIAS pin is connected to an external voltage higher than 3V, bias power will be drawn from the external source (typically the regulated output voltage). This will improve efficiency if the BIAS pin voltage is lower than regulator input voltage. 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 switch to be saturated. This boosted voltage is generated with an external capaci- tor and diode. Two comparators are connected to the shutdown pin. One has a 2.38V threshold for undervoltage lockout and the second has a 0.4V threshold for complete shutdown. The LT1956 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 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 BLOCK DIAGRA Figure 1. LT1956 Block Diagram – + Σ VIN 2.9V BIAS REGULATOR 500kHz OSCILLATOR FREQUENCY FOLDBACK SW FB GND 1, 8, 9, 16 1956 F01 SLOPE COMP ANTISLOPE COMP BIAS INTERNAL VCC SYNC 0.4V 5.5 µA CURRENT COMPARATOR RLIMIT RSENSE ERROR AMPLIFIER gm = 2000µMho Q2 FOLDBACK CURRENT LIMIT CLAMP BOOST RS FLIP-FLOP DRIVER CIRCUITRY S R Q1 POWER SWITCH 1.22V 4 10 14 SHDN 15 6 2 12 11 VC LOCKOUT COMPARATOR SHUTDOWN COMPARATOR 2.38V ×1 Q3 VC(MAX) CLAMP |
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