LT3757 datasheet(11/36 Pages) LINER

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LTC3787

3787fc

BLOCK DIAGRAM

SLEEP

SWITCHING

LOGIC

AND

CHARGE

PUMP

–

4.8V

3.8V

VBIAS

VIN

CIN

INTVCC

PLLIN/

MODE

PGOOD

–

1.32V

1.08V

–

VFB

EXTVCC

5.4V

LDO

VCO

PFD

0.425V

SENS LO

BOOST

COUT

VOUT

CLKOUT

PGND

INTVCC

VFB

1.32V

1.2V

RSENSE

0.5μA/

4.5μA

10μA

11V

SHDN

–

SHDN

2.5V

–

SENS

ITH

CSS

CC2

0.7V

2.8V

SLOPE COMP

2mV

–

SENSE –

SENSE+

SHDN

CLK2

CLK1

RUN

SGND

INTVCC

FREQ

DUPLICATE FOR SECOND CONTROLLER CHANNEL

+ –

–

5.4V

LDO

20μA

100k

SYNC

DET

ILIM

PHASMD

3787 BD

CURRENT

LIMIT

ICMP

IREV

OPERATION

Main Control Loop

The LTC3787 uses a constant-frequency, current mode

step-up architecture with the two controller channels

operating out of phase. During normal operation, each

external bottom MOSFET is turned on when the clock for

that channel sets the RS latch, and is turned off when the

main current comparator, ICMP, resets the RS latch. The

peak inductor current at which ICMP trips and resets the

latch is controlled by the voltage on the ITH pin, which is

the output of the error amplifier EA. The error amplifier

compares the output voltage feedback signal at the VFB

pin (which is generated with an external resistor divider

connected across the output voltage, VOUT, to ground), to

the internal 1.200V reference voltage. In a boost converter,

the required inductor current is determined by the load

current, VIN and VOUT. When the load current increases,

it causes a slight decrease in VFB relative to the reference,

which causes the EA to increase the ITH voltage until the

average inductor current in each channel matches the new

requirement based on the new load current.

After the bottom MOSFET is turned off each cycle, the

top MOSFET is turned on until either the inductor current

starts to reverse, as indicated by the current comparator,

IR, or the beginning of the next clock cycle.

LT3757 Datasheet(PDF) 11 Page - Linear Technology

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