LT3844EFE(RevB) Просмотр технического описания (PDF) - Linear Technology

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LT3844EFE
(Rev.:RevB)

High Voltage, Current Mode Switching Regulator Controller with Programmable Operating Frequency

Linear Technology 

LT3844

OPERATION (Refer to Functional Diagram)

the CSS pin voltage then the current threshold set by the

DC control voltage, VC, is decreased and the inductor cur-

rent is lowered. This in turn decreases the output voltage

slew rate allowing the CSS pin voltage ramp to catch up

to the VFB pin voltage. An internal 100mV offset is added

to the VFB pin voltage relative to the to CSS pin voltage

so that at start-up the soft-start circuit will discharge the

VC pin voltage below the DC control voltage equivalent to

zero inductor current. This will reduce the input supply

inrush current. The soft-start circuit is disabled once the

CSS pin voltage has been charged to 200mV above the

internal reference of 1.231V.

During a VIN UVLO, VCC UVLO or SHDN UVLO event, the

CSS pin voltage is discharged with a 50μA current source.

In normal operation the CSS pin voltage is clamped to a

diode above the VFB pin voltage. Therefore, the value of

the CSS capacitor is relevant in how long of a fault event

will retrigger a soft-start. In other words, if any of the

above UVLO conditions occur, the CSS pin voltage will be

discharged with a 50μA current source. There is a diode

worth of voltage headroom to ride through the fault before

the CSS pin voltage enters its active region and the soft-

start function is enabled.

Also, since the CSS pin voltage is clamped to a diode above

the VFB pin voltage, during a short circuit the CSS pin volt-

age is pulled low because the VFB pin voltage is low. Once

the short has been removed the VFB pin voltage starts to

recover. The soft-start circuit takes control of the output

voltage slew rate once the VFB pin voltage has exceeded

the slowly ramping CSS pin voltage, reducing the output

voltage overshoot during a short-circuit recovery.

Slope/Antislope Compensation

The IC incorporates slope compensation to eliminate

potential subharmonic oscillations in the current control

loop. The IC’s slope compensation circuit imposes an

artificial ramp on the sensed current to increase the rising

slope as duty cycle increases.

Typically, this additional ramp affects the sensed current

value, thereby reducing the achievable current limit value

by the same amount as the added ramp represents. As

such, the current limit is typically reduced as the duty cycle

increases. The LT3844, however, contains antislope com-

pensation circuitry to eliminate the current limit reduction

associated with slope compensation. As the slope compen-

sation ramp is added to the sensed current, a similar ramp

is added to the current limit threshold. The end result is that

the current limit is not compromised so the LT3844 can

provide full power regardless of required duty cycle.

Shutdown

The LT3844 includes a shutdown mode where all the

internal IC functions are disabled and the VIN current is

reduced to less than 10μA. The shutdown pin can be used

for undervoltage lockout with hysteresis, micropower shut-

down or as a general purpose on/off control of the converter

output. The shutdown function has two thresholds. The

first threshold, a precision 1.23V threshold with 120mV

of hysteresis, disables the converter from switching. The

second threshold, approximately a 0.7V referenced to

SGND, completely disables all internal circuitry and reduces

the VIN current to less than 10μA. See the Application

Information section for more information.

APPLICATIONS INFORMATION

The basic LT3844 step-down (buck) application, shown

in the Typical Application on the front page, converts a

larger positive input voltage to a lower positive or negative

output voltage. This Application Information section assists

selection of external components for the requirements of

the power supply.

RSENSE Selection

The current sense resistor, RSENSE, monitors the inductor

current of the supply (See Typical Application on front

page). Its value is chosen based on the maximum required

output load current. The LT3844 current sense amplifier

has a maximum voltage threshold of, typically, 100mV.

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