Datasheet LT3431 (Analog Devices) - 7

LT3431
W BLOCK DIAGRA
The LT3431 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 LT3431 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 equal to or higher than tor pulse which sets the RS flip-flop to turn the switch on. 3V, bias power will be drawn from the external source When switch current reaches a level set by the inverting (typically the regulated output voltage). This will improve input of the comparator, the flip-flop is reset and the efficiency if the BIAS pin voltage is lower than regulator switch turns off. Output voltage control is obtained by input voltage. using the output of the error amplifier to set the switch current trip point. This technique means that the error High switch efficiency is attained by using the BOOST pin amplifier commands current to be delivered to the output to provide a voltage to the switch driver which is higher rather than voltage. A voltage fed system will have low than the input voltage, allowing switch to be saturated. phase shift up to the resonant frequency of the inductor This boosted voltage is generated with an external and output capacitor, then an abrupt 180° shift will occur. capacitor and diode. Two comparators are connected to The current fed system will have 90° phase shift at a much the shutdown pin. One has a 2.38V threshold for under- lower frequency, but will not have the additional 90° shift voltage lockout and the second has a 0.4V threshold for until well beyond the LC resonant frequency. This makes complete shutdown. VIN 3, 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 SW SHDN 15 + 2, 5 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 3431 F01
Figure 1. LT3431 Block Diagram
sn3431 3431fs 7