3570 datasheet(9/20 Pages) LINER

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LT3570

3570fa

OPERATION

The LT3570 is a constant frequency, current mode, buck

converter and boost converter with an NPN LDO regula-

tor. Operation can be best understood by referring to the

Block Diagram.

If all of the SHDN pins are held low, the LT3570 is shut

down and draws zero quiescent current. When any of the

pins exceed 1.4V the internal bias circuits turn on. Each

regulator will only begin regulating when its corresponding

SHDN pin is pulled high.

Each switching regulator controls the output voltage in

a similar manner. The operation of the switchers can be

understood by looking at the boost regulator. A pulse

from the oscillator sets the RS ﬂip-ﬂop A4 and turns on

the internal NPN bipolar power switch Q1. Current in Q1

and the external inductor L1 begins to increase. When

this current exceeds a level determined by the voltage at

VC1, comparator A3 resets A4, turning off Q1. The current

in L1 ﬂows through the external Schottky diode D1 and

begins to decrease. The cycle begins again at the next

pulse from the oscillator. In this way, the voltage on the

VC1 pin controls the current through the inductor to the

output. The internal error ampliﬁer A1 regulates the output

voltage by continually adjusting the VC1 pin voltage. The

threshold for switching on the VC1 pin is approximately

750mV and an active clamp of 1.15V limits the output

current. The soft-start capacitor C6A allows the part to

slowly start up by ramping the internal reference.

The driver for the buck regulator can operate from either

VIN2 or from the BOOST pin. An external capacitor and

diode are used to generate a voltage at the BOOST pin that

is higher than the input supply. This allows the driver to

saturate the internal bipolar NPN power switch for efﬁcient

operation. The driver for the boost regulator is operated

from VIN1.

The BIAS pin allows the internal circuitry to draw its

current from a lower voltage supply than the input. This

reduces power dissipation and increases efﬁciency. If the

voltage on the BIAS pin falls below 2.5V, then the LT3570

quiescent current will ﬂow from VIN2.

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