Datasheet LT3570 (Analog Devices) - 9

LT3570
OPERATION
The LT3570 is a constant frequency, current mode, buck VC1 pin controls the current through the inductor to the converter and boost converter with an NPN LDO regula- output. The internal error amplifi er A1 regulates the output tor. Operation can be best understood by referring to the voltage by continually adjusting the VC1 pin voltage. The Block Diagram. threshold for switching on the VC1 pin is approximately 750mV and an active clamp of 1.15V limits the output If all of the SHDN pins are held low, the LT3570 is shut current. The soft-start capacitor C6A allows the part to down and draws zero quiescent current. When any of the slowly start up by ramping the internal reference. pins exceed 1.4V the internal bias circuits turn on. Each regulator will only begin regulating when its corresponding The driver for the buck regulator can operate from either SHDN pin is pulled high. VIN2 or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that Each switching regulator controls the output voltage in is higher than the input supply. This allows the driver to a similar manner. The operation of the switchers can be saturate the internal bipolar NPN power switch for effi cient understood by looking at the boost regulator. A pulse operation. The driver for the boost regulator is operated from the oscillator sets the RS fl ip-fl op A4 and turns on from V the internal NPN bipolar power switch Q1. Current in Q1 IN1. and the external inductor L1 begins to increase. When The BIAS pin allows the internal circuitry to draw its this current exceeds a level determined by the voltage at current from a lower voltage supply than the input. This VC1, comparator A3 resets A4, turning off Q1. The current reduces power dissipation and increases effi ciency. If the in L1 fl ows through the external Schottky diode D1 and voltage on the BIAS pin falls below 2.5V, then the LT3570 begins to decrease. The cycle begins again at the next quiescent current will fl ow from VIN2. pulse from the oscillator. In this way, the voltage on the 3570fb 9