Datasheet LT1961 (Linear Technology) - 6

LT1961
W BLOCK DIAGRAM
The LT1961 is a constant frequency, current-mode boost amplifier commands current to be delivered to the output converter. This means that there is an internal clock and rather than voltage. A voltage fed system will have low two feedback loops that control the duty cycle of the power phase shift up to the resonant frequency of the inductor switch. In addition to the normal error amplifier, there is a and output capacitor, then an abrupt 180° shift will occur. current sense amplifier that monitors switch current on a The current fed system will have 90° phase shift at a much cycle-by-cycle basis. A switch cycle starts with an oscilla- lower frequency, but will not have the additional 90° shift tor pulse which sets the R until well beyond the LC resonant frequency. This makes S flip-flop to turn the switch on. When switch current reaches a level set by the inverting it much easier to frequency compensate the feedback loop input of the comparator, the flip-flop is reset and the and also gives much quicker transient response. switch turns off. Output voltage control is obtained by A comparator connected to the shutdown pin disables the using the output of the error amplifier to set the switch internal regulator, reducing supply current. current trip point. This technique means that the error VIN 1 2.5V BIAS INTERNAL REGULATOR VCC SLOPE COMP Σ 0.3V 1.25MHz 2 SW SYNC 8 S OSCILLATOR Q1 CURRENT RS DRIVER POWER COMPARATOR FLIP-FLOP CIRCUITRY SWITCH + R SHUTDOWN – CURRENT SENSE AMPLIFIER VOLTAGE COMPARATOR GAIN = 40 7μA + – + 0.01Ω 1.35V – SHDN 5 – 6 FB 3μA + ERROR 3 GND 7 VC AMPLIFIER 1.2V gm = 850μMho 4 GND 1767 F01
Figure 1. Block Diagram
1961fa 6