LTC3561 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTC3561

1A, 4MHz, Synchronous Step-Down DC/DC Converter

Linear Technology 

BLOCK DIAGRA

SVIN

SGND

ITH

6

2

8

0.8V VOLTAGE

REFERENCE

VFB 7

+

–

ERROR

AMPLIFIER

–

+

VB

OSCILLATOR

ITH

LIMIT

PVIN

5

PMOS CURRENT

COMPARATOR

SLOPE

COMPENSATION

3 SW

LTC3561

LOGIC

NMOS

COMPARATOR

U

OPERATIO

1

SHDN/RT

REVERSE

COMPARATOR

4 PGND

3561 BD

The LTC3561 uses a constant frequency, current mode

architecture. The operating frequency is determined by

the value of the RT resistor.

The output voltage is set by an external divider returned to

the VFB pin. An error amplifier compares the divided

output voltage with a reference voltage of 0.8V and adjusts

the peak inductor current accordingly.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle

when the VFB voltage is below the reference voltage. The

current into the inductor and the load increases until the

current limit is reached. The switch turns off and energy

stored in the inductor flows through the bottom switch (N-

channel MOSFET) into the load until the next clock cycle.

The peak inductor current is controlled by the voltage on

the ITH pin, which is the output of the error amplifier. This

amplifier compares the VFB pin to the 0.8V reference.

When the load current increases, the VFB voltage de-

creases slightly below the reference. This decrease causes

the error amplifier to increase the ITH voltage until the

average inductor current matches the new load current. At

low load currents, the inductor current becomes discontinu-

ous, and pulses may be skipped to maintain regulation.

The main control loop is shut down by pulling the SHDN/RT

pin to SVIN. A digital soft-start is enabled after shutdown,

which will slowly ramp the peak inductor current up over

1024 clock cycles or until the output reaches regulation,

whichever is first. Soft-start can be lengthened by ramping

the voltage on the ITH pin (see Applications Information

section).

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases to 100% which is

the dropout condition. In dropout, the PMOS switch is

turned on continuously with the output voltage being

equal to the input voltage minus the voltage drops across

the internal P-channel MOSFET and the inductor.

Low Supply Operation

The LTC3561 incorporates an undervoltage lockout circuit

which shuts down the part when the input voltage drops

below about 2.5V to prevent unstable operation.

3561f

5

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