Datasheet LT1956, LT1956-5 (Analog Devices) - 7

LT1956/LT1956-5
W BLOCK DIAGRA
The LT1956 is a constant frequency, current mode buck it much easier to frequency compensate the feedback loop converter. This means that there is an internal clock and and also gives much quicker transient response. two feedback loops that control the duty cycle of the power Most of the circuitry of the LT1956 operates from an switch. In addition to the normal error amplifier, there is a internal 2.9V bias line. The bias regulator normally draws current sense amplifier that monitors switch current on a power from the regulator input pin, but if the BIAS pin is cycle-by-cycle basis. A switch cycle starts with an oscilla- connected to an external voltage higher than 3V, bias tor pulse which sets the RS flip-flop to turn the switch on. power will be drawn from the external source (typically the When switch current reaches a level set by the inverting regulated output voltage). This will improve efficiency if input of the comparator, the flip-flop is reset and the the BIAS pin voltage is lower than regulator input voltage. switch turns off. Output voltage control is obtained by using the output of the error amplifier to set the switch High switch efficiency is attained by using the BOOST pin current trip point. This technique means that the error to provide a voltage to the switch driver which is higher amplifier commands current to be delivered to the output than the input voltage, allowing switch to be saturated. rather than voltage. A voltage fed system will have low This boosted voltage is generated with an external capaci- phase shift up to the resonant frequency of the inductor tor and diode. Two comparators are connected to the and output capacitor, then an abrupt 180° shift will occur. shutdown pin. One has a 2.38V threshold for undervoltage The current fed system will have 90° phase shift at a much lockout and the second has a 0.4V threshold for complete lower frequency, but will not have the additional 90° shift shutdown. until well beyond the LC resonant frequency. This makes VIN 4 RLIMIT RSENSE 2.9V BIAS INTERNAL + – BIAS 10 REGULATOR VCC CURRENT COMPARATOR SLOPE COMP Σ SYNC 14 ANTISLOPE COMP BOOST 6 SHUTDOWN COMPARATOR + – 500kHz S OSCILLATOR Q1 RS DRIVER POWER FLIP-FLOP CIRCUITRY SWITCH R 0.4V 5.5µA 2 SW SHDN 15 + FREQUENCY – FOLDBACK LOCKOUT COMPARATOR ×1 Q2 FOLDBACK VC(MAX) Q3 CURRENT CLAMP LIMIT ERROR CLAMP AMPLIFIER – 12 FB gm = 2000µMho + 11 2.38V 1.22V VC GND 1, 8, 9, 16 1956 F01
Figure 1. LT1956 Block Diagram
1956f 7