LTC3774 查看數據表（PDF） - Linear Technology

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LTC3774

Dual, Multiphase Current Mode Synchronous Controller

Linear Technology 

LTC3774

OPERATION

Main Control Loop

The LTC3774 uses an LTC proprietary current sensing,

current mode step-down architecture. During normal

operation, the top MOSFET is turned on every cycle when

the oscillator sets the RS latch, and turned off when the

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

peak inductor current at which ICMP resets the RS latch

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

output of the error amplifier, EA. The remote sense amplifier

(diffamp) produces a signal equal to the differential voltage

sensed across the output capacitor divided down by the

feedback divider and re-references it to the local IC ground

reference. The error amplifier receives this feedback signal

and compares it to the internal 0.6V reference. When the

load current increases, it causes a slight decrease in the

VOSNS+ pin voltage relative to the 0.6V reference, which in

turn causes the ITH voltage to increase until the inductor’s

average current equals the new load current. After the top

MOSFET has turned off, the bottom MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by the reverse current comparator, IREV , or the

beginning of the next cycle.

The main control loop is shut down by pulling the RUN

pin low. Releasing RUN allows an internal 1.0µA current

source to pull up the RUN pin. When the RUN pin reaches

1.22V, the main control loop is enabled and the IC is

powered up. When the RUN pin is low, all functions are

kept in a controlled state.

Sensing Signal of Very Low DCR

The LTC3774 employs a unique architecture to enhance

the signal-to-noise ratio that enables it to operate with a

small sense signal of a very low value inductor DCR, 1mΩ

or less, to improve power efficiency, and reduce jitter due

to the switching noise which could corrupt the signal. The

LTC3774 can sense a DCR value as low as 0.2mΩ with

careful PCB layout.The LTC3774 comprises two positive

sense pins, SNSD+ and SNSA+, to acquire signals and

processes them internally to provide the response as with

a DCR sense signal that has a 14dB signal-to-noise ratio

improvement. In the meantime, the current limit threshold

is still a function of the inductor peak current and its DCR

value, and can be accurately set from 10mV to 30mV in

5mV steps with the ILIM pin. The filter time constant,

R1C1, of the SNSD+ should match the L/DCR of the output

inductor, while the filter at SNSA+ should have a bandwidth

of five times larger than SNSD+, R2•C2 equals R1•C1/5.

Internal Soft-Start

By default, the start-up of the output voltage is normally

controlled by an internal soft-start ramp. The internal soft-

start ramp represents a noninverting input to the error

amplifier. The VOSNS+ pin is regulated to the lower of the

error amplifier’s three noninverting inputs (the internal

soft-start ramp, the TK/SS pin or the internal 600mV ref-

erence). As the ramp voltage rises from 0V to 0.6V over

approximately 600µs, the output voltage rises smoothly

from its prebiased value to its final set value.

Certain applications can result in the start-up of the con-

verter into a non-zero load voltage, where residual charge

is stored on the output capacitor at the onset of converter

switching. In order to prevent the output from discharging

under these conditions, the bottom MOSFET is disabled

until soft-start is greater than VOSNS+.

Shutdown and Start-Up (RUN and TK/SS Pins)

The LTC3774 can be shut down using the RUN pin.

Pulling the RUN pin below 1.14V shuts down the main

control loop for the controller and most internal circuits,

including the INTVCC regulator. Releasing the RUN pin

allows an internal 1.0µA current to pull up the pin and

enable the controller. Alternatively, the RUN pin may be

externally pulled up or driven directly by logic. Be careful

not to exceed the absolute maximum rating of 6V on this

pin. The start-up of the controller’s output voltage, VOUT ,

is controlled by the voltage on the TK/SS pin. When the

voltage on the TK/SS pin is less than the 0.6V internal

reference, the LTC3774 regulates the VOSNS+ voltage to

the TK/SS pin voltage instead of the 0.6V reference. This

allows the TK/SS pin to be used to program a soft-start

by connecting an external capacitor from the TK/SS pin

to GND. An internal 1.25µA pull-up current charges this

capacitor, creating a voltage ramp on the TK/SS pin. As

the TK/SS voltage rises linearly from 0V to 0.6V (and

beyond), the output voltage, VOUT , rises smoothly from

zero to its final value. Alternatively, the TK/SS pin can be

3774f

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For more information www.linear.com/LTC3774

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